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University Microfilms International 300 N. Zeeb Road Ann Arbor, Ml 48106 8424425 G ottschalk, Kurt William GROWTH, BIOMASS YIELD, CROWN DEVELOPMENT, AND GAS EXCHANGE OF FOUR INTENSIVELY-CULTURED POPULUS CLONES IN SOUTHERN MICHIGAN Michigan Stale University University Microfilms International 300 N. Zeeb Road, Ann Arbor, Ml 48106 PH.D. 1984 PLEASE NOTE: In all c a se s this material has been filmed in the best possible way from the available copy. Problems encountered with this docum ent have been identified here with a check mark 7 1. Glossy photographs or pages 2. Colored illustrations, paper or print_____ 3. Photographs with dark background 4. Illustrations are poor co p y _______ 5. P ages with black marks, not original copy______ 6. Print shows through a s there is text on bothsid es of p ag e______ 7. Indistinct, broken or small print on several p a g e s 8. Print exceeds margin requirem ents_____ _ 9. . S S Tightly bound copy with print lost in spine______ 10. 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O ther____________________________________________________________ _ University Microfilms International GROWTH, BIOMASS YIELD, CROWN DEVELOPMENT, AND GAS EXCHANGE OF FOUR INTENSIVELY-CULTURED POPULUS CLONES IN SOUTHERN MICHIGAN By Kurt William Gottschalk A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Forestry 1984 ABSTRACT GROWTH, BIOMASS YIELD, CROWN DEVELOPMENT, AND GAS EXCHANGE OF FOUR INTENSIVELY-CULTURED POPULUS CLONES IN SOUTHERN MICHIGAN By Kurt William Gottschalk A small plot research plantation was established in 1977 using cultural practices determined at that time to best approximate the most practical and productive system for industrial users. Measurements of the growth, biomass yield, crown development, and gas exchange (stomatal conductance and photosynthesis) of these four clonal stands were taken for two years in an effort to understand the process of woody biomass production. Growth, biomass yield, and crown development were obtained from nondestructive sampling and complete measurements of all trees at the end of the season. Destructive sampling of small numbers of trees was used to develop prediction equations for the nondestructive samples. Gas exchange was measured throughout the second year using a diffusion porometer for stomatal conductance and 14CC^-technique for photosynthesis. Measurements were taken at three crown levels. Environmental parameters were also monitored in the plots. Biomass production for the four clones varied from 1.6 to 2.5 and 6.8 to 9.2 t ha”* for the first two years. Second year mean annual increment was 3.4 to 4.6 t ha * yr Max­ imum leaf area index ranged from 1.4 to 2.5 and 3.2 to 4.2 m 2 -m 2 for the two years. . Clone NE 353 + 308 partitioned a larger proportion of its biomass into branches, while NE 48 had higher specific leaf weights than the other clones. Gas exchange varied significantly by clone and leaf position. Upper and mid-crown leaves had higher photosynthetic rates (maximum 39 mg CC^ dm mum 28 mg CC^ dm -2 hr — 2 -1 hr -1 ) than lower crown leaves (maxi). NE 48 had lower photosynthetic rates than the other clones. The major influence on all of the growth processes of the four clones was drought stress. Two minor drought peri­ ods in the first year caused only small reductions in growth however, two major drought periods in the second year result ed in large growth reductions and reduced gas exchange rates. NE 48 appeared to be more drought tolerant than the other clones used in this study. ACKNOWLEDGMENTS I wish to thank Dr. Donald Dickmann, Chairman of my graduate committee, for his many long hours of assistance, support, suggestions, and fellowship which made this study possible. In addition, I express sincere gratitude to my graduate committee members, Drs. J. W. Hanover, A. J. M. Smucker, and K. Pregitzer for their contributions to this dissertation. I also wish to express gratitude to Drs. J. B. Hart, R. H. Heiligmann, and D. E. Linvill, and to my colleagues John H. Bassman, John L. Crane Jr., and Donald A. Michael for their many helpful and thought-provoking discussions. At this time, I wish to acknowledge the excellent assistance of Antonio J. DeAraujo, N. Kaye Freeman, Steven Weber, John L. Crane Jr., and J. Michael O'Conner in helping to collect and analyze the data used in this dissertation and Shirley I. Smith and Helen A. Machesky for their typing assistance. Finally, I wish to thank my wife, Janice, for her excellent typing assistance and moral support. ii TABLE OF CONTENTS Page LIST OF TABLES......................................... iv LIST OF FIGURES........................................vi INTRODUCTION .......................................... 1 CHAPTER I. GROWTH AND BIOMASS YIELD ................... 6 Introduction .................................... 6 Materials and Methods ........................... 8 Results........................................... 16 Discussion....................... 41 CHAPTER II. CROWN DEVELOPMENT ......................... 50 Introduction .................................... 50 52 Materials and Methods ........................... Results....................... 56 Discussion........................................ 81 CHAPTER III. GAS EXCHANGE.............................. 93 Introduction .................................... 93 Materials and Methods ........................... 95 Results . . . . . . . . . . . . . . . ........... 103 Discussion....................................... 121 SUMMARY AND CONCLUSIONS ............................. 130 APPENDIX..............................................133 BIBLIOGRAPHY 141 LIST OF TABLES Table Page 1. Height, diameter, and survival of four hybrid poplar clones after one growing season ........... 18 2. First- and second-year height, diameter, and survival of four hybrid poplar clones ........... 21 3. Average height, diameter, stem weight and number of trees by crown class of four hybrid poplar clones after two growing seasons ................. 27 4. Regression and prediction equation parameters from D H versus dry weight regressions for four one-year-old hybrid poplar clones ............... 32 5. Regression and prediction equation parameters from D H versus dry weight regressions for four two-year-old hybrid poplar clones ............... 33 6. Actual., and estimated above-ground biomass yields (t ha” ) of four hybrid poplar clones after one growing season ................................... 34 7. Actual and estimated above-ground woody biomass yields (t ha” ) of four hybrid poplar clones after two growing seasons......................... 36 8. Partitioning of wood and bark biomass of four hybrid poplar clones after the first and second growing seasons ................................. 40 9. Distribution of wood, bark, and branch biomass between the major and minor stems present on each cutting for four hybrid poplar clones after the first and second growing seasons ................. 42 10. Distribution of branches by branch type of four hybrid poplar clones after the first and second growing seasons ................................. 66 11. Distribution of branches on major stems by crown class for four hybrid poplar clones after the second growing season ....................... 68 iv 12. Partitioning of wood, bark, and branch biomass into various branch types for four hybrid poplar clones after the second growing season ............ 70 13. Specific leaf weight (SLW) values by branch type for four hybrid poplar clones, early and late in the second growing season ....................... 77 14. Average area per leaf by branch type for four hybrid poplar clones late in the first and second growing seasons ........................... 86 15. Formula and constants used for calculation of photosynthetic r a t e .............................. 101 16. Analysis of variance for split-split plt design of stomatal conductance to water (cm s ) versus date, clone, and leaf position for four hybrid poplar clones measured throughout the second growing season ................................. 109 17. Analysis of variance for split-split plot design of photosynthetic rate (mg C02 dm- hr” ) versus date, clone, and leaf position for four hybrid poplar clones measured throughout the second growing season ................................. 112 18. Range of environmental variables on days when gas exchange measurements were taken during the second growing season ........................... 113 19. Stepwise multiple regression of stomatal conduc­ tance to water (cm s ) versus environmental var­ iables and the log^g, square, and reciprocal of these variables.................................. 114 20. Stepwise multiply regression of photosynthetic rate (mg CO„ dm- hr ) versus environmental and physiological variables and the log^g, square, and reciprocal of these variables ............... 120 21. Regression and prediction equation parameters from leaf length versus leaf area regressions for four one-year-old hybrid poplar clones .......... 133 22. Regression and prediction equation parameters from leaf length versus leaf area regressions for four two-year-old hybrid poplar clones .......... 134 v LIST OF FIGURES Page Figure 1. Plot layout of the four replications with four plots per replication ..................... 10 2. East-west row layout of a poplar plot 13 3. Seasonal height growth pattern of four hybrid poplar clones during the first growing season (Day 161 = June 10, NE 353+308 a , NE 48 + f NE 58 0 , Eugenei o ) ......................... 17 4. Seasonal height growth pattern of dominant major stems (A), codominant major stems (+), intermediate major stems (□), and suppressed major or any minor stems (o) of four hybrid poplar clones during the second growing season (Day 125 = May 5) . . . . 20 5. Seasonal diameter growth pattern of four hybrid poplar clones during the first growing season (Day 161 = June 10, NE 353+308 A , NE 48 +, NE 58 0, Eugenei o ) ........................................ 23 6. Seasonal diameter growth pattern of dominant major stems (A) codominant major stems (+ ), intermediate major stems (□), and suppressed major or any minor stems (o) of four hybrid poplar clones during the second growing season (Day 125 = May 5 ) .................................25 7. Clone NE 58 at the end of thefirstgrowing season............................................ 29 8. Clone NE 58 at the end of the second growing season............................................ 31 9. Seasonal change in percentage of total biomass present in leaves of four hybrid poplar clones during the first growing season (Day 172 = June 21, NE 353+308 A , NE 48 +, NE 58 a, Eugenei o) . . . vi 37 10. Seasonal change in percentage of total biomass present in woody stem (A), branches (+), leaves (□), and petioles (o) of four hybrid poplar clones during the second growing season (Day 151 = May 3 1 ) ....................... .. ............. 39 11. Schematic diagram ofa two-year-oldhybrid poplar tree showing the major stem (A), minor stem (B), first-year primary branch (C), cutting (D), primary and secondary branches of indeterminate long, determinate long, and short branch types, and height growth increments (HGI) of first and second y e a r s .......................... 58 12. Seasonal change in leaf area index (LAI) of four hybrid poplar clones during the first (Day 161 = June 10) and the second (Day 125 = May 5) growing seasons (NE 353+308 A , NE 48 +, NE 58 O , Eugenei o ) ............................................... 61 13. Seasonal change in total leaf area per stem of LAI sample trees for dominant major (A), codominant major (+), intermediate major (o), dominant minor (o), and codominant minor (x) stems of four hybrid poplar clones during the second growing season (Day 125 =May 5 ) ............. 64 14. Seasonal change in leaf area ratio (LAR) of four hybrid poplar clones during the first (Day 172 = June 21) and the second (Day 151 = May 31) growing seasons (NE 353+308 A , NE 48 +, NE 58 D , Eugenei o ) ............................................... 73 15. Seasonal change in specific leaf weight (SLW) of four hybrid poplar clones during the first (Dav 172 = June 21) and the second (Day 151 = May 31) growing seasons (NE 353+308 A , NE 48 +, NE 58 □ , Eugenei o ) ........................................75 16. Seasonal change in the percentage of total tree leaf area present on branches of four hybrid poplar clones during the first growing season (Day 172 = June 21, NE 353+308A , NE 48 +, NE 58D , Eugenei o ) ........................................ 78 17. Seasonal change in the percentage of total tree leaf area present on the main stem (A), indeter­ minate long (+), determinate long ( D ) , and short (o) branches, and all secondary and/or current lateral branches (x) of four hybrid poplar clones during the second growing season (Day 151 = May 3 1 ) .............................................. 80 vii Seasonal change in the average leaf area present on the main stem (A), indeterminate long (+ ), determinate long ( o ) , and short (o) branches, and all secondary and/or current lateral branches (x) of four hybrid poplar clones during the second growing season (Day 151 = May 31) ............... 83 Seasonal change in the average number of leaves present on the main stem ( A ) , indeterminate long (+), determinate long (□), and short (o) branches, and all secondary and/or current lateral branches (x) of four hybrid poplar clones during the 85 second growing season (Day 151 = May 31) ........ 14 Photosynthetic measurement using CO2 field apparatus.................................. . . 100 Seasonal changes in soil moisture tension for two soil types at two depths for the first (Day 161 = June 10) and second (Day 125 = May 5) growing seasons (Kalamazoo sandy loam, 0-30 cm A , 30-60 cm +, Hillsdale sandy loam, 0-30 cm 0, 30-60 cm o) 105 Seasonal change in stomatal conductance to water of leaves present in the upper (A), middle (+), and lower (o) crowns of four hybrid poplar clones dur­ ing the second growing season (Day 125 = May 5) . . 107 Seasonal change in photosynthetic rate of leaves present in the upper ( A ) , middle (+), and lower (o) crowns of four hybrid poplar clones during the second growing season (Day 125 = May 5) ......... 111 Boundary line plot of stomatal conductance to water versus photosynthetic photon flux density for the upper (A), middle (+), and lower (o) crown posi­ tions of four hybrid poplar clones during the second growing season ........................... 117 Boundary line plot of stomatal conductance to water versus vapor pressure deficit for the upper (A) , middle (+), and lower (o) crown positions of four hybrid poplar clones during the second growing 119 season ........................................ Boundary line plot of photosynthetic rate versus stomatal conductance to water for the upper (A), middle (+), and lower (o) crown positions of four hybrid poplar clones during the second growing season (A = upper and middle crown, B = lower crown) ........................................ viii 123 27. Boundary line plot of photosynthetic rate versus photosynthetic photon flux density for the upper ( A ) , middle (+ ), and lower (o) crown positions of four hybrid poplar clones during the second growing season (A = upper and middle crown, B = lower c r o w n )........................................... 125 28. Water retention curves for two soil types at two depths as determined with undisturbed soil cores (low tension) and screened soil (high tension) in a ceramic plate pressure apparatus (Kalamazoo sandy loam, 0-30 cm L , 30-60 cm +, Hillsdale sandy loam, 0-30 cm □, 30-60 cm o ) ...................... 135 29. Channels ratio versus counting efficiency calibra­ tion curve for C-labeled photosynthetic samples (Glass vials A , Glass vials +, Plastic vials o) . . 136 30. Map of study area showing block and plot arrangement and harvest schedule ........................... 138 31. Map of plot showing row layout, border area, large biomass and LAI subplot, and small growth analysis subplots......................................... 140 ix INTRODUCTION An experiment was begun in the mid-1960s to produce wood fiber on very short rotations. The concept called "silage sycamore" pulled together several cultural practices similar to those utilized by agronomists in agricultural production (McAlpine et al. 1966, Herrick and Brown 1967). The silage sycamore concept was based on the use of fast-growing hardwood species, high planting densities, intensive cultural methods, including fertilization and irrigation, rotations of 5 to 10 years or less, and coppice regrowth of harvested stands. These techniques were integrated into a comprehensive silvicultural system for intensive production of wood fiber in a short time interval. Larson and Gordon (1969a) theorized that manipulating the growth of the tree could increase wood yield. Based on these ideas, the USDA-Forest Service established a Maximum Yield Work Unit in 1971 at Rhinelander, WI to conduct research on techniques for maximizing wood fiber production (USDA-FS 1976, 1980). Supplemental funding from the Department of Energy was received beginning in 1977 (USDA-FS 1980, 1983), and in 1978 the Work Unit was expanded to a Research and Development Program. In the meantime work continued in the South on sycamore, cottonwood, and other species (Steinbeck et al. 1972, Saucier et al. 1972, Dutrow 1 et al. 1970, Dutrow and Saucier 1976), and in other areas of the country (Iowa State University 1975, 1976, Schreiner 1970) . This system of wood fiber production has been known by several names other than "silage sycamore": intensive culture, agroforestry, short rotation forestry, and short rotation intensive culture (SRIC). The latter term is gradually becoming the accepted name. The wide interest in the area was evident by the turnout at two conferences on intensive culture of forest crops sponsored by Iowa State University in 1974 and 1975 (Iowa State University 1975, 1976) and was fueled by predictions of shortages of wood fiber and also of energy due to the energy (oil) crisis that occured in 1974-1975. The potential use of wood to replace oil as an energy and chemical feedstock source prompted the expansion of the Forest Service program in 1976 and the involvement of the Department of Energy in 1977. The areas of research and development included in the program were: species selection and genetic improvement, stand establishment methods, fiber and energy yields in response to spacing and harvest cycles, yield response to nutrient and moisture regimes, effect of crown architecture and canopy density on yield, physiological factors influencing yield, raw material quality of intensive culture trees, diseases of intensive culture species, insect problems in intensive culture, design of mechanized equipment for intensive culture systems, and cost of producing wood for fuel and fiber (USDA-FS 1980) . Summaries of the first five and twelve years of research results from this program have been published (USDA-FS 1976, 1983). Some of the advantages of short-rotation intensive culture are: 1) an early return on invested capital due to the short time period of the rotation; 2) high production of wood fiber on small land areas close to the mills which allows increased mechanization and lower transportation costs; 3) use of the most productive non-agricultural land, allowing less productive forest land to be maintained in forest cover for other uses; and 4) it is less objectionable to the public to grow and harvest intensively on small areas in a manner similar to agriculture and decrease clearcutting and other objectionable practices on less productive forest land (Schreiner 1970) . However, there are some disadvantages to the system as well. They are: 1) a high initial cost is required to establish the plantations and acquire the necessary equipment; 2) the process is very fossil fuel intensive for both the equipment and fertilizer needs; 3) soil nutrients and organic matter can be depleted very rapidly due to the high production and whole tree utilization; 4) the wood fiber produced by the system may not be acceptable to industry for the processes they are now using. These advantages and disadvantages formed the foundation of the Forest Service research program. Dawson and others (1978,1980) discuss the basis for managing forests for maximum biomass production. Schreiner (1970) has suggested rotations of 2 to 5 years, 6 to 15 years, and 16 to 30 years for the production of fiber, boltwood, and sawtimber, respectively. Active programs in intensive culture of eucalypts in Brazil (Ayling and Martins 1981) and hybrid poplars in Michigan (Rose et al. 1981) are examples of how the system can be used. Economic analyses of short rotation intensive culture systems have suggested that the system is profitable only on the best sites and feasible only for large industrial owners (Dutrow and Saucier 1976, Rose et al. 1981). Rose and others (1981) showed that net energy production was feasible only if irrigation was not used. A survey of forest industries showed that they are anticipating large increases in cultural practices in the future primarily in the areas of genetics, site preparation, fertilization, and commercial thinning but little expansion of short rotation forestry was forseen (DeBell et al. 1977). Due to the economic analyses and industry projections, research emphasis on short rotation intensive culture systems in USDA-FS has waned in the last couple of years. One major research area in the SRIC system has been the interaction of environmental factors and silvicultural practices in their effect on the morphological and physiological processes in tree crowns (Isebrands 1982, Dickmann 1979). Understanding the seasonal and perennial patterns of these processes is important to a biological approach for increasing yields of tree stands (Isebrands 1982) . The objective of the Michigan State University research program is to understand the physiological and genetical bases for growth and development in intensively-cultured, short rotation forest stands (Dickmann et al. 1979) . This dissertation research project was conceived to begin to meet that objective. A small plot research plantation was established in 1977 using cultural practices (spacing, clones, fertilization, etc.) that were determined at that time to approximate the system that would be most practical and productive for an industrial user. Measurements of the growth and development of these stands were taken in conjunction with morphological, physiological, and environmental parameters in an attempt to understand the process of woody biomass production. CHAPTER I. GROWTH AND BIOMASS YIELD Introduction Among the species tested for short-rotation, intensive culture (SRIC) in the northern United States and elsewhere, species and hybrids of the genus Populus have been used the most. Their indeterminate growth patterns and fast juvenile growth produce high biomass yields because of the potential for complete utilization of the growing season (Larson et al. 1976, Dickmann 1975). Mean annual biomass increment (MAI) of hybrid poplars has ranged from 1.8 to 14.1 t ha * yr * depending upon species, site, and cultural practices, whereas native aspen stands produced MAI's of 0.6 to 5.3 t ha-* yr * (Pollard 1971, 1972a, Schlaegel 1975, Perala 1979, Heilman et al. 1972, Dawson et al. 1976, Switzer et al. 1976, Schreiner 1970, Cannell 1980). Biomass yield is affected by the cultural practices used. Fertilization usually increases yield but must be used in conjunction with other practices such as weed control to realize its full potential (Zsuffa et al. 1977, Harrington et al. 1979). Tree spacing affected yields but only in combination with rotation length. The shorter the rotation, the closer the spacing to maximize yields (Zavitkovski et al. 1976, Bowersox and Ward 1976). With longer rotations, spacing becomes neutral in its effect on 6 biomass yields, so it is better to invest less money and use wider spacings (Cannell 1980). Irrigation or naturally moist sites increased biomass yield of hybrid poplars over unirrigated or droughty sites (Cooley 1978, Zakhariev et al. 1975, Rawitz et al. 1966). The yield increases due to irrigation are because of the limiting effect water supply and drought have on poplar growth. Liphschitz and Waisel (1970a,b) showed that height and diameter growth patterns of P. euphratica were altered by the water supply available to the tree. Drought resulted in early budset, reduced cambial growth, and low biomass yields in other studies (Sucoff and Hiesey 1978, Smith and Gatherum 1974, Braun 1974). The partitioning of biomass into plant organs changes as a stand ages. Young stands (less than 10 years) partition up to 30% of their biomass into leaves while stands older than ten years partition I to 5% into leaves (Person et al. 1971, Gordon and Promnitz 1976, Switzer et al, 1976). As the proportion of biomass partitioned into leaves decreases, that partitioned into woody stems and branches increases. Thus, longer rotations will increase the proportion of woody biomass formed. Genetic differences in the proportion of biomass partitioned into wood occurs, so the potential to select genetic material that favors partitioning of dry weight into woody stems can be used to increase per hectare woody biomass production (Cannell and Willett 1976, Wray and Promnitz 1976, Gordon and Promnitz 1976). Wider spacings result in higher proportions of woody biomass partitioned into branches as opposed to stem (Heilman et al. 1972, Dawson et al. 1976) . Higher proportions of branches and bark reduces the yield and quality of the biomass for some utilization processes and products; however, these factors have been shown to be unimportant or within industry standards and tolerances for many processes and products (Crist 1983). Careful analysis of the height and diameter growth patterns in relation to climatic conditions, tree age, and cultural practices enable the forester to make decisions on how to best manage SRIC plantations to maximize biomass yield. Biomass yield and growth patterns during the first two growing seasons (1977, 1978) of a planned three-year rotation of poplar hybrids are presented here. Materials and Methods Experimental Design The general plot layout was a randomized complete block design with four replications (Figure 30 Appendix). Each replication contained three plots of one poplar hybrid (Figure 1) and one plot of corn (Zea mays) (data not reported here). Three poplar plots in each replication were harvested according to the following schedule: one plot was allowed one year of growth from planting, then cut, followed by two years of coppice regrowth; one plot grew for two years before cutting, followed by one year of coppice regrowth; the third plot grew for three yoars before cutting. This schedule allowed the productivity of various rotations of poplar during the initial three years following establishment to be compared with corn yields. Because of 9 Figure 1. Plot layout of the four replications with four plots per replication. 11 this plot design objective, the yields of the individual clones reported here cannot be compared statistically, since they were not replicated. The individual plots were 6 m wide (east-west) and 8 m long (north-south). Each plot consisted of nine east-west rows spaced 1 m apart (Figures 2 and 31 Appendix) with spacing within the rows of 0.33 m, giving a maximum plant density of 30,303 trees ha ^ (12,263 trees acre *). subplots were laid out. Two The growth analysis subplot consisted of rows 2 and 3 and was used for periodic growth analysis harvests. The biomass subplot consisted of rows 5 to 8 and was used for determination of height and diameter growth and biomass values. At least one border row and three border trees within a row (ca. 1 m) were located around the harvested subplots to minimize edge effects (Zavitkovski 1981a). Plots were separated by grass buffer strips of 6 m (north-south) and 5 m (east-west). The two-year initial, one-year coppice plot of clone NE 58 was suppressed by a large black cherry tree that interfered with a portion of the plot and biased that plot's data downward. Cultural Methods Unrooted, dormant cuttings of hybrid clones NE 353 — ^ (P . deltoides x nigra caudina), NE 308 (P^ nigra charkowiensis x P^ nigra cv. Incrassata), NE 48 [P . maximowiczii x (P_j_ x berolinensis)], NE 58 [(P^ x T7 USDA Forest Service, Northestern (NE) Forest Experiment Station numbers. 12 Figure 2. East-west row layout of a poplar plot. 13 Figure 14 rasumowskyana) x P\_ nigra cv. Incrassata] , and Eugenei (P^_ x euramericana cv. Eugenei) were planted in early May, 1977. Each replication contained a different clone, except that NE 353 and NE 308 were inadvertently planted together as a random mixture in one replication. Weed control in 1977 kg ha-'*' active was accomplished by spraying 2.8 ingredient (ai) of simazine wettable powder and 4.6 1 ha-'*' of amitrol-T two weeks prior to planting. Shortly after bud break in 1978, an additional 2.8 kg ha "*■ ai of simazinewettable powder All plots were was sprayed on the plots. fertilized with 224 kg ha * of 10-20-20 (N-P-K) granular fertilizer on May 17, 1977 and May 18, 1978. In addition, 75 kg ha * of ammonium nitrate (33-0-0) was applied on August 1, 1977 and July 7, 1978. was not used. Irrigation The soil types present were a well-drained, slightly eroded Kalamazoo sandy loam (a fine-loamy, mixed, mesic Hapludalf), with a 0-2% slope and a sandy clay loam to clay loam subsoil, and a well-drained, slightly eroded Hillsdale sandy loam (a coarse-loamy, mixed, mesic Hapludalf), with a 2-6% slope and a sandy clay loam or loam subsoil. Experimental Methods Height and Diameter Growth. At two-week intervals the first year, stem height and diameter at 5 cm were measured on a ten-tree permanent sample within the biomass subplot. During the second year, trees in the biomass subplot were divided into crown classes (dominant, codominant, intermediate, suppressed) based on their crown light 15 interception and height relative to neighboring trees. Height and diameter growth patterns were determined by crown class. Growth Analysis Harvests. A two-tree sample was selected at random within the growth analysis subplot and harvested every two weeks throughout the growing season. Stem height and diameter at 5 cm were measured prior to harvesting. Trees were partitioned into stem, branches, and leaves, oven-dried at 75 C, and weighted. Biomass partitioning was then calculated as percentage of total above-ground biomass. Biomass Harvests. After the onset of dormancy and leaf abscission in the fall, the biomass subplots were harvested. Stem height and diameter at 5 cm were measured prior to harvesting. weighed. Each stem was then oven-dried at 75 C and Above-ground biomass yield of wood and bark and branches was obtained by totaling the weights of all the stems, converting to tonnes, and dividing by the plot area. Total above-ground biomass was then calculated by using the ratios of leaf weight to total above-ground biomass from the last growth analysis harvest prior to fall leaf abscission to estimate the leaf biomass produced. Individual stem height (H) and diameter (D), and oven-dry weight (DW) values were used to develop a least 2 squares regression of the mensurational variable D H versus dry weight for each poplar clone, for each year (Zavitkovski 1971, 1976, Zavitkovski et al. 1974, Bowersox and Murphy 1975). The allometric equation (Y=aX^) was transformed to 16 its logarithmic form [In Y = In a + b(ln X)] to compute the regressions. 0.89. All regressions had r 2 values greater than Bartlett's test for homogeneity of variance values was significant,so it was not possible to test for clonal differences in slope of the regressions. A correction factor for bias due to the transformation was used for each clone (Baskerville 1972) . Biomass yield of wood and bark in unharvested plots was calculated via the every tree summation method (Baskerville 1965, Attiwell and Ovington 1968) using the prediction equations developed for each clone. Total above-ground biomass was then calculated in the same manner as in the harvested plots. Results Height Growth First-year height growth began shortly after planting and continued until terminal bud formation occurred in late August. Clone NE 48, however, did not set bud until late September (Figure 3). Average height of the four clones ranged from 1.2 to 1.3 m (Table 1). Second-year height growth began with bud break in early May, with clone NE 48 one week ahead of the other clones. Terminal bud formation occurred in mid-July for intermedi­ ate, suppressed, and minor stems of all clones and for domi­ nant and codominant stems of clones NE 58 and Eugenei (Fig­ ure 4). Dominant stems of clones NE 353 + 308 set bud in late August. Dominant and codominant stems of clone NE 48 did not set bud until late September. Average second-year height of the four clones ranged from 2.0 to 2.2 m (Table 2). 17 CO tr LU h" LU 2 168 196 224 252 280 DAYS Figure 3. Seasonal height growth pattern of four hybrid poplar clones during the first growing season (Day 161 = June 10, NE 353+308 A, NE 48 +, NE 58 □ , Eugenei o). Table 1. Height, diameter, and survival of four hybrid poplar clones after one growing season. Harvested Plot Clone Number Height (m) Diameter*5 (cm) Average of all plotsa Survival (%) Height (m) Diameter (cm) Survival (%) NE 353+308 1.27 1.3 100 1.34 1.2 99 NE 48 1.29 1.4 98 1.18 1.2 98 NE 58 1.32 1.2 99 1.24 1.2 99 Eugenei 1.18 1.2 95 1.25 1.2 95 AVERAGE 1.26 1.3 98 1.26 1.2 98 Average of one-year-old harvested plot and two one-year-old unharvested plots. b Diameter was measured 5 cm above ground level or emergence from cutting. 19 Figure 4.. Seasonal height growth pattern of dominant major stems (A) , codominant major stems (+), intermediate major stems (□), and suppressed major or any minor stems (o) of four hybrid poplar clones during the second growing season (Day 125 = May 5). 20 NE 353 +3 08 NE 48 Q-L | CO [ Q CO in cm Q CM in ■ o O O 0 -0 © o o ■o in METERS o § J I I I I I I 1 I L NE 58 EUGENEI J 120 148 176 2 0 4 232 260 I I 1 I I 1 I L 120 148 176 2 04 232 260 DAYS Figure 4. L Table 2. First- and second-year height, diameter, and survival of four hybrid poplar clones Average of both Plotsa Harvested Plot First Year Second Year b First Year Second Year Ht. (m) Dia. (cm) Ht. (m) Dia. (cm) 96 1.38 1.2 2.18 1.8 95 1.8 98 1.13 1.2 1.96 1.7 96 1.67 1.5 100 1.20 1.2 1.98 1.6 99 1.1 2.10 1.8 94 1.28 1.2 2.02 1.7 96 1.1 1.97 1.7 97 1.25 1.2 2.04 1.7 97 Ht. (m) Dia. (cm) 1.2 2.07 1.7 1.33 1.3 2.03 NE 58 1.04 1.0 Eugenei 1.24 AVERAGE 1.21 Clone Number Ht. (m) Dia. (cm) NE 353+308 1.33 NE 48 Surv. (%) a Average of two-year-old harvested plot and of one two-year-old unharvested plot. b Diameter was measured 5 cm above ground level or emergence from cutting. Surv. (%) 22 Diameter Growth First-year diameter growth also began shortly after planting and continued until the beginning of September. Clone NE 48, however, continued diameter growth until late September (Figure 5). Average diameter of the four clones was 1.2 cm (Table 1). A mild drought in mid-August caused a decline in diameter growth which was most noticeable in clone NE 48 followed by NE 353 + 308 (Figure 5). Diameter growth then recovered when rainfall replenished the soil moisture (Figure 21 Chapter III). Second-vear diameter growth began shortly after bud break in early May and ceased in late June for intermediate, suppressed, and minor stems of all the clones (Figure 6). Codominant stems of all four clones ceased diameter growth in mid-July (Figure 6) when the first major drought occurred (Figure 21 Chapter III). Some of the NE 48 codominant stems recovered and grew with rains in August. Dominant stems of NE 58 and Eugenei also ceased diameter growth in mid-July; however the dominant stems of NE 353 + 308 and NE 48 maintained diameter growth until early September when a second major drought occurred. Average second-year diameter of the four clones ranged from 1.6 to 1.8 cm (Table 2). Segregation into Crown Classes During the first growing season, intraclonal competition within the plots began to segregate the trees into crown classes. With the onset of growth in the second growing season, segregation intensified, with some trees becoming dominants, the majority remaining codominant, some 23 o 168 196 224 252 280 DAYS Figure 5. Seasonal diameter growth pattern of four hybrid poplar clones during the first growing season (Day 161 = June 10, NE 353+308 a, NE 48 +, NE 58 □ , Eugenei o). 24 Figure 6. Seasonal diameter growth pattern of dominant major stems (A) , codominant major stems (+), intermediate major stems (o), and suppressed major or any minor stems (o) of four hybrid poplar clones during the second growing season (Day 125 = May 5) . 25 NE 353+308 NE 48 P. *cf in co o co CM P CM in♦ Q to d j i i 2 O NE 58 EUGENEI o to CO o CO to CM o CM* to T“ ^ g - - e - e - e- o to d od d L 120 148 176 2 0 4 232 26 0 120 148 176 204 232 260 DAYS F ig u re 5. 26 becoming intermediates and a few becoming suppressed. Distribution of trees into crown classes is shown for the four clones in Table 3 and is surprisingly uniform between clones. Average height, diameter, and stem weight is given for all trees and each crown class (Table 3). As crown class moved from suppressed to dominant, height increased 2 to 3-fold, diameter 2 to 4-fold, and stem weight 10 to 20-fold depending upon clone. Clone NE 58 at the end of the first and second growing seasons is shown in Figures 7 and 8. Biomass Yield Stem Dry Weight Prediction Equations. Tables 4 and 5 give the regression results and prediction equations for estimating stem dry weight from the mensurational variable D 2 H. All of the regressions were significant and had r 2 values greater than 0.88 for one-year-old trees and greater than 0.99 for two-year-old trees. The parameters for one-year-old trees had a larger range than the ones for two-year-old trees. Actual and Estimated Biomass Yields. Actual biomass yield of wood and bark in the harvested plots ranged from 1.6 to 2.9 t ha-1 (Table 6). With the leaves added, biomass yields increased to 2.4 to 4.6 t ha ^. Estimates of biomass yields using the prediction equations for one-year-old trees in unharvested plots ranged from 1.5 to 2.3 t ha * for wood _i and bark and 2.2 to 3.7 t ha for total above-ground biomass. 27 Table 3. Average height, diameter, stem weight and number of trees by crown class of four hybrid poplar clones after two growing seasons. Clone Number Crown Class Major Stems Number Height (m) Diameter (cm) Stem . Weight (g) NE 353 + 308 All 50 Dominant 4 33 Codominant Intermediate 7 6 Suppressed 2.74 3.46 2.92 2.38 1.64 2.3 3.0 2.5 1.7 1.2 259 452 305 116 48 NE 48 51 All Dominant 7 Codominant 30 Intermediate 9 5 Suppressed 2.72 3.58 2.92 2.16 1.32 2.5 3.3 2.7 1.9 1.1 301 552 334 138 40 NE 58 All 52 Dominant 5 Codominant 25 Intermediate 16 6 Suppressed 2.09 2.73 2.42 1.82 0.90 1.9 2.5 2.2 1.6 0.8 155 305 204 84 16 Eugenei All 49 Dominant 9 Codominant 26 Intermediate 11 Suppressed 3 2.54 3.10 2.68 2.15 1.09 2.3 3.3 2.4 1.7 0.8 234 465 235 101 19 a Diameter was measured 5 cm above ground level or emergence from cutting, b Stem weight includes branches and bark. 28 Figure 7. season. Clone NE 58 at the end of the first growing 2 .? Figure. 7. 30 Figure 8. season. Clone NE 58 at the end of the second growing 31 Figure S. Regression and prediction equationcl parameters from D2H versus dry weight regressions for four one-year-old hybrid poplar clones. Table 4. Regression Significance of F-ratio Clone Number Coefficients revalue Intercept (b ) 0 Variable 1 (b ) l Significance of t value Correction factor NE 343+308 ** 0.89 2.83 0.867 ** 0.0468 NE 48 ** 0.99 2.70 0.913 ** 0.00297 NE 58 ** 0.99 2.81 0.840 ** 0.00188 Eugenei ** 0.92 2.76 0.845 ** 0.0159 a Prediction Equation: Regression Equation: A (b + b (Ln D2H) + CF} A 2 b DW = e 0 1 or DW = a(D H) 1 2 Ln DW = b + b (Ln D H) which is the transformed form of the 0 1 Where: DW D H D2H Ln e CF = = = = = = = a = 2 b equation, DW = a(D H) 1 dry weight ofwood + bark in grams. diameter at 5cm above ground level oremergence from cutting in cm. height in m. 2 mensurationalvariable, diameter squared timesheight in cm -m. natural logarithm (base e). exponential function. o2 correction factor due to bias of transformation = /2. (b + CF) constant which equals e 0 b ** = significant at a = 0.01 level. Table 5. ci o Regression and prediction equation parameters from DII versus dry weight regressions for four two-year-old hybrid poplar clones Regression Significance revalue of F ratio Clone Number NE 353+308 it it NE 48 Intercept (b ) 0 Coefficients Variable 1 Significancg (b ) of t value l Correction factor 0.99 2.96 0.926 * it 0.00968 ** 0.99 2.94 0.927 itic 0.00808 NE 58 ** 0.99 2.99 0.922 it it 0.00788 Eugenei ** 0.99 2.98 0.893 it it 0.00495 a Prediction Equation: Regression Equation: 2 ^ (b + b (Ln D H) + CF} a 2 b DW = e 0 1 or DW = a(D H) 1 2 Ln DW = b + b (Ln D H) which is the transformed form of the 0 Where DW D H = D H Ln e = CF = a = 1 2 b equation, DW = a(D H) 1 = dry weight of wood + bark in grams. = diameter at 5 cm above ground leveloremergence from cutting in cm. height in m. 2 = mensurational variable, diameter squaredtimesheight incm-m. = natural logarithm (base e). exponential function. o2 correction factor due to bias of transformation = /2. (b + CF) constant which equals e 0 b ** = significant at a = 0.01 level. Table 6. Actual and estimated above-ground biomass yields (t ha.1) of four hybrid poplar clones after one growing season Harvested Plot Clone Number Wood and bark Average of Estimated Plots Wood, bark and leaves Wood and bark Wood, bark and leaves NE 353+308 2.9 4.6 2.3 3.7 NE 48 2.6 3.7 1.6 2.3 NE 58 1.9 2.8 1.5 2.2 Eugenei 1.6 2.4 1.6 2.4 AVERAGE 2.2 3.4 1.8 2.6 35 Actual biomass yields of wood and bark in the harvested plots at the end of the second year ranged from 5.3 to 9.8 t ha-*, a 4 to 5-fold increase over the first year estimates for the same plots (Table 7). Estimates of biomass yields using the prediction equations for two-year-old trees ranged from 6.0 to 10.4 t ha *, a 3 to 7-fold increase over the first year estimates for the same plots (Table 7). The MAI (mean annual increment) of the two-year-old plots ranged from 3.4 to 4.6 t ha-* yr-*, with an average of 3.9 t ha-* yr -1 . Biomass Partitioning Partitioning Between Organs. Early in the first year 65 to 80% of the total above-ground biomass was present as leaves and the remaining 20 to 35% as stem. The percentage in leaves dropped steadily throughout the growing season to 25 to 40% prior to leaf fall (Figure 9), whereas the percentage in stem wood increased to 60 to 75%. During the second growing season, the percentage of total above-ground biomass in leaves declined from 30 to 40% in June to 10 to 20% in September (Figure 10). The percentage in petioles declined also from 3 to 4% down to 1 to 2%. At the end of the second growing season, the woody stem comprised 65 to 75% and branches 15 to 20% of the total above-ground biomass. Table 8 gives the percentages of the total wood and bark biomass partitioned into current year stem, branches, and two-year-old stem. In the first year, 13% of the wood and bark of clone NE 353 + 308 was comprised of sylleptic branches, whereas in the other three clones, Table 7. Actual and estimated above-ground woody biomass yields (t ha“^) of four hybrid poplar clones after two growing seasons. Harvested Plot Clone Number Wood, bark & branches Average of Both Plots Estimated Plot Wood, bark & branches First year estimate Wood, bark & branches Second year estimate Wood, bark & branches First year estimate Wood, bark & branches Wood, bark & branches First year estimate NE 353+308 8.1 2.0 10.4 2.6 9.2 2.3 NE 48 9.8 2.1 6.6 1.0 8.2 1.6 NE 58 5.3 1.2 8.2 1.8 6.8 1.5 Eugenei 7.7 1.5 6.0 1.8 6.8 1.6 AVERAGE 7.7 1.7 7.8 1.8 7.8 1.8 37 LU o cc LU Q. 172 200 226 256 DAYS Figure 9. Seasonal change in percentage of total biomass present in leaves of four hybrid poplar clones during the first growing season (Day 172 = June 21, NE 353+308 A, NE 48 + , NE 58 □ , Eugenei o). 38 Figure 10. Seasonal change in percentage of total biomass present in woody stem (A), branches (+), leaves (□), and petioles (o) of four hybrid poplar clones during the second growing season (Day 151 = May 31). 39 NE 353+308 NE 48 PERCENT 1 O _ O _ o— EUGENEI NE 58 O O tD O IT) O o r ^ . CO o 04 ’©— ©— «=»— o ! 11- o— © o— ©_ -© i'iiIiI 151 172 193 214 235 257 Figure 10. —© 151 172 193 214 235 257 DAYS Table 8. Partitioning of wood and bark biomass of four hybrid poplar clones after the first and second growing seasons. Percentage of Total Wood + Bark Biomass First Year Clone Number Current Stem Second Year Current Lateral Branches Current Stem Two-year-old Stem All Branches NE 353+308 87 13 9 70 21 NE 48 98 2 9 70 21 NE 58 98 2 11 71 18 Eugenei 100 0 12 67 21 AVERAGE 96 10 70 20 41 these branches comprised only 0.3 to 2.5%. Differences in percentage of wood and bark in branches in the second growing season were not large (all approximately 20%), while current stem wood was 9 to 12% and two-year-old wood was 67 to 70% of the total wood and bark biomass. Partitioning Between Stems. Most of the planted cuttings produced more than one shoot (Table 9). In the first year, major stems accounted for 78 to 88% of the woody biomass while minor stems accounted for 12 to 22%. During the second year, major stems became more dominant and minor stems grew little. As a result, major stems comprised 86 to 93% of the woody biomass, while only 7 to 14% was in minor stems. These minor stems are obviously on their way out of the stand. Discussion The high survival rates obtained in this study were due to a combination of good quality planting stock, excellent weed control, and favorable environmental conditions in the period immediately following planting. Whereas the growth potential of poplars is high, they are "prima donnas" and must be planted and cared for according to a strict silviclutural regimen (Dickmann and Stuart 1983). Anything less will produce mediocre results. Clone NE 48 differed from the other poplar clones used in this study in timing and seasonal duration of growth; it broke bud earlier, grew faster late in the season, and set bud later. These differences in budbreak and budset are in part due to genetic differences in response to photoperiod Table 9. Distribution of wood, bark, and branch biomass between the major and minor stems present on each cutting for four hybrid poplar clones after the first and second growing seasons. Second Year First Year Clone Number Number of stems per cutting Percentage of woody biomass __________________ _______ Major Stems Minor Stems Number of stems per cutting Percentage of woody biomass _________________________ Major Stems Minor Stems NE 353+308 1.9 88 12 1.8 93 7 NE 48 2.0 80 20 1.9 92 8 NE 58 1.8 78 22 1.7 89 11 Eugenei 1.8 78 22 1.7 86 14 AVERAGE 1.9 81 19 1.8 90 10 43 and temperature. Pauley and Perry (1954) demonstrated that duration of shoot growth in clones of several Populus species was sensitive to daylength and inversely correlated with the latitude of origin of each clone. In terms of tree improvement, if genotypes that can utilize the entire growing season, like NE 48, can be combined with clones having other favorable characteristics, improved growth and biomass productivity could result, although increased risk of frost damage must be considered also. During the first growing season, a mild drought occurred that decreased diameter growth in clones NE 48 and NE 353 + 308, but more favorable conditions were created with additional rainfall. Clones NE 58 and Eugenei may have also suffered from the drought, but the lack of resumption of diameter growth with rainfall made it difficult to determine if that was the case. The severe drought periods which occurred during the second growing season reduced height and diameter growth significantly over what would have been attainable with more favorable moisture conditions. Intraclonal competition during these drought periods was intense. Reductions in growth of the smaller stems was at least partially due to competition for light as well as water. Height growth stopped first in intermediate, suppressed, and minor stems, whereas codominant and especially dominant stems kept growing until the second major drought period occurred. The dominant stems of NE 353 + 308 and NE 48 maintained growth better than Eugenei and NE 58. Clone NE 48 again responded to early September rainfall 44 with a period of renewed height and diameter growth. It is very likely that these differences in phenology among crown classes were due to a better exploitation of the available soil volume by the larger trees (Faulkner and Fayle 1979), although no root distribution data were collected. Irrigation would have increased height growth, especially during the second growing season. Diameter growth would have responded even more than the height growth to irrigation since diameter growth is more sensitive to water stress (Dickmann 1979, Bonner 1967). Blake (1981) has shown that Tacamahaca poplars (like NE 48) have a higher water use efficiency than Aigeiros poplars, which could explain the greater drought tolerance of NE 48. Unfortunately, resistance to the canker fungus Septoria musiva is not among the genetic characteristics of clone NE 48. Incipient cankering observed during the second growing season killed almost all of the trees during years 3 and 4. Nonetheless, much potential apparently exists within the species P_;_ maximowiczii (Japanese poplar) and it should be considered a prime candidate for poplar improvement programs. A review of the poplar literature showed a wide range of height and diameter growth rates. However, growth values very similar to those achieved by dominant and codominant stems in this study were reported by Dawson and others (1976), Bowersox and Ward (1976,1977), Cannell (1980), and Merritt and Bramble (1966). Most of these studies thinned to one stem per cutting which concentrated subsequent growth 45 on the major stem. In the present study, the inclusion of both major and minor stems in the average height and diameter values tended to reduce these average values below those of thinned trees. The practice of thinning to one stem per cutting is impractical for field application and consequently was not done in this study. Cannell (1980) and Cannell and Smith (1980) discuss the possibility of a yield ceiling for biomass production of intensivelv-cultured temperate hardwoods of 10 to 12 t ha * yr ^ 4 to 5 years after planting and 10 to 30% greater yields after coppicing for 4 to 5 years. In a review of published results, Cannell and Smith (1980) show that the two major factors determining yield are age and spacing (or plant density), with species, site, and error being minor factors. The above-ground biomass yields obtained in this study agree closely with yields reported elsewhere in the literature. Larger first-year yields in the present study than those reported by Bowersox and Ward (19 76) are due to their practice of thinning to one stem per cutting shortly after planting while our plots had from 1.7 to 2.0 stems per cutting. However, in the second year these minor stems became less important contributors to stand yield and eventually died. Cannell (1980) reported similar first-year yields of 2 t ha * for P_;_ trichocarpa planted at 30,000 trees ha ^ as unrooted cuttings and thinned to one stem per cutting two months after planting. were 10 t ha \ But second-year yields higher than values reported here. Apparently yields in this study were decreased due to 46 drought in the second year and, to a smaller extent, in the first year. MAI was lower than the proposed yield ceiling due to drought and because the data represented only two years growth instead of 4 to 5 years growth. High LWRs (leaf weight ratios) of poplar cuttings early in the season were a significant factor in their establishment and early growth. The high partitioning of weight into leaves, 50 to 80% depending upon clone, provided a large photosynthetic surface upon which further growth was based. In fact, the rapid growth of young hybrid poplars and cottonwood can be largely attributed to this large initial investment in leaves. As the season progressed and the plants became established in the soil, a larger proportion of biomass was partitioned into the stem and the proportion in leaves dropped to 10 to 40%, depending upon clone. During the second year, the initial investment in leaves was lower than in the first year (30 to 40%) and again decreased over the season (down to 10 to 20%). These results are confounded, however, by the large amount of leaf abscission which occurred due to drought during the season (see Chapter II). The large differences in branch formation between NE 353 + 308 and the other three clones during the first season appears to be genetically related. (Branching patterns will be discussed in greater depth in Chapter II.) First-year branch biomass has also been reported for cottonwood (30%, Baker and Blackmon 1977), 30%, Cannell trichocarpa in Britain (28 to 1980), and for coppice growth of two hybrid 47 poplars in Wisconsin (7 to 23%f Crist and Dawson 1975). Second year branch biomass was much more uniform (17.8 to 21.4%) in the present study and was similar to values reported elsewhere for two- and three-vear-old stands (10 to 35%, Anderson and Zsuffa 1975, Heilman et al. 1972, Dawson et al. 1976, Cannell 1980). Apparently the relative branchiness of a poplar clone during the first year cannot be used as a predictor of branchiness in older trees. The development of crown class segregation very early in the life of the stands in this study has not often been reported. Merritt and Bramble (1966) found that after five years the individual heights of Eugenei and P_;_ deltoides trees in a widely-spaced plantation both followed normal bell-shaped distributions, with a tail to the intermediate and suppressed end. Codominant trees comprised the major portion of their curve. Johnson and Burkhardt (1976) found that three-year-old natural stands of already segregated into crown classes. deltoides had Dominant trees were only slightly taller than codominant trees, but had diameters 1/3 to 1/2 larger, wider crowns, and higher live crown length to total height ratios than codominant trees. Although heights of live crown were not measured in the present study, dominant trees generally had less abscission of leaves and branches and larger crowns (see Chapter II). The end result of segregation into crown classes will be similar to the effects of loss of minor stems. Suppressed and intermediate stems will gradually die (without thinning) and their biomass will be lost. However, the lost biomass may be more than recouped by the increased growth and quality of the surrounding trees. Wider spacings and thinnings could be utilized to prevent some of this biomass loss and to save high establishment costs for trees that will never be harvested. Very short rotations (one to two years) will prevent much biomass loss from occurring but will also decrease MAIs, while longer rotations will compound the biomass losses and necessitate intermediate thinnings to recover biomass before it is lost. Heilman and Peabody (1981) found maximum MAIs for black cottonwood at several spacings occurred at rotations of 8 years rather than 2 and 4 years. Cultural practices such as fertilization and irrigation initially decrease intraclonal competition and allow trees to survive longer. However, the increased stand growth magnifies the competition level of the stand and when shortages develop in light, water, or nutrients, use of these cultural practices hastens the loss of trees that could not successfully compete. Another result of the segregation into crown classes is a change in the biomass distribution within trees of different classes. Dominant trees have a lower proportion of biomass in the stem and a higher proportion in leaves while the proportion in branches is about the same as in codominant trees; codominant trees show the same differences over intermediate and suppressed trees (Carter and White 1971, Zavitkovski 1971). From the standpoint of maximizing per hectare yields over longer rotations, it would be best to have a uniform stand created by frequent thinnings than 49 to have larger dominant trees with high branch components along with dying suppressed trees. CHAPTER II. CROWN DEVELOPMENT Introduction The production of biomass by plants is dependent upon the carbohydrates available for growth. These carbohydrates are a product of net assimilation rate (NAR) and the leaf area available for light interception and photosynthesis (Watson 1952). The literature shows that dry matter yields depend more on variation in leaf area than in NAR (Larson and Isebrands 1972, Zavitkovski et al. 1974, Larson et al. 1976, Watson 1947, 1958). In this sense, leaf area duration (LAD), the integral of leaf area index (LAI, the ratio of leaf area to ground area) over time becomes the important determinant of biomass yield (Madgwick and Olson 1974, Watson 1952, 1956). Most cultural practices affect leaf area more than NAR, although the two processes can both change independently; cultural practices that increase yield do so wholly or mainly by influencing leaf growth and/or retention (Watson 1952, 1956). The study of crown development must take into account the production of new leaves. Larson and Isebrands (1971) have shown that leaf production, growth (leaf length, area, and dry weight), and maturation of physiological function are all correlated and related to the leaf plastochron index (LPI) in cottonwood. Pieters (1974), in an indepth study of 50 51 P . x euramericana cv. Robusta, supports these findings and shows that under constant conditions the rate of leaf production steadily increases with time until steady state growth is achieved (plastochron interval decreases to a constant value). Light intensity, temperature, moisture and nutrient supply all affect the plastochron interval. The annual cycle of height growth, leaf and branch development results in a regular ordering of internodes and branches. Isebrands and others (1977) have used the term "height growth increment" (HGI) to represent the main-stern growth produced in any given year. HGI's are numbered consecutively from the base of the tree upward. Branches which originate on an HGI segment are given the same designation as the HGI. Branches, within an HGI, are classified by their place of origin. Primary branches are those which originate from lateral buds on the current leader; secondary branches originate from lateral buds on primary branches and increasing orders can be present up to the age of the HGI (Maini 1966a,b, Nelson et al. 1980, 1981, Wilson 1970). Branches can further be classified by their growth into long and short shoots (Wilson 1970, Kozlowski and Clausen 1966, Maini 1966a, Pollard 1970, Gregory 1980). By the time the fourth or fifth order is reached, long shoots usually do not bear lateral long shoots; they bear only short shoots, many of which do not have any laterals. So only 5 to 6 orders of branches are usually present in a tree crown (Wilson 1970) , although there is much variabililty among species. Branch development also is 52 influenced by the environment and development of other branches on the tree (Richards and Larson 1981) . Development of both leaf and branch components of poplar crowns was measured in the same experimental plots described in Chapter I. The purpose of this study was to relate crown developmental patterns to biomass production in the plots. Materials and Methods The experimental design and cultural methods are the same as described in Chapter I. The growth analysis subplots were also used for some measurements related to crown development. Additional plots for measurement of LAI were also used and are described below. Leaf Area Index First Year. LAI determinations were made on a ten-tree permanent sample within the biomass subplot. At two-week intervals, stem height, diameter at 5 cm, and lengths of all leaves present on the trees were measured. A leaf marking system was employed so that only newly matured and immature leaves had to be measured each time. Old leaves were accounted for by counting and recording their presence or absence. Second Year. Due to the large increase in tree size and complexity in the second year, a sampling technique was used to determine LAI. Within the biomass subplot, trees were stratified by crown class and a stratified random sample of ten trees was made at two-week intervals. Thus, the structure of the sample approximated the structure of 53 the population. At each sampling time, a new random selection of ten trees was made (some trees were sampled more than once throughout the growing season). Height and diameter at 5 cm were measured on each tree. The ten-tree (large) sample was approximately 20% of the population. In addition, a small sample of three trees (6% of the population), one each of the dominant, codominant, and intermediate crown classes, was selected and permanently marked for repeated measurements. These small sample trees were excluded when selections were made for the large sample. At the same sampling times as the large sample, leaf lengths were measured on all current (main) stem leaves and on branch leaves of selected sample branches on the small sample trees. Branches were divided into three classes and were of two orders. Five random samples of each branch type (order and class combination) were made and permanently marked. In some cases five or fewer branches of a type were present in which case they were all measured. The total number of branches of each type was recorded along with the height and diameter of the stem at 5 cm. A leaf marking system similar to the first year was used. Least squares regressions of leaf length versus leaf area were developed for two-year-old leaves and were used to calculate leaf areas from the leaf length measurements. current stem leaf area was totaled. The Leaf areas of the sample branches were totaled and average leaf area calculated for each branch type. The total tree leaf area was then calculated by multiplying the total number of 54 branches of each type by the average area of each type and totaling all of the branch types plus the current stem. Least squares regressions of total leaf area (TLA) per 2 tree versus the mensurational variable D H were developed for each clone at each sampling time from the small sample trees [TLA = b^ + b^(D 2H)]. D2 H values of the large sample trees were then used to calculate total leaf area per tree using the prediction equation for that clone. Leaf areas of the ten large sample trees were totaled and divided by the ground area occupied by the ten trees to obtain LAI. Biomass Harvests The biomass subplots described in the first chapter were also used to determine the number and weight of branches by type. Each harvested tree was divided into its components (current leader, two-year-old stem, two-vear-old branch, and each current branch type) and their dry weights determined. In addition, the number of branches of each type were counted and recorded. The June determinations of number of branches of each type were determined from counts made on the large sample trees of the first LAI sampling period. Growth Analysis Harvests The growth analysis subplots described in the first chapter were used to measure the leaf area on each branch type and on the current leader throughout the growing season. In addition, the number and dry weight of leaves of each type were also recorded. From this data, the leaf area ratio (LAR, the ratio of leaf area to above-ground dry 55 weight of the tree) an indicator of plant leafiness and specific leaf weight (SLW, the ratio of leaf weight to leaf area - the inverse of specific leaf area, SLA), a measure of the thickness or density of the leaf per unit area were calculated (Ledig 1974). The average area per leaf, distribution of leaf dry weight, numbers, and area by branch type were also determined. Leaves of all size classes and branch types were sampled for measurement of leaf length and area for use in the development of length versus area prediction equations (Larson and Isebrands 1972). Branching Patterns To investigate branching patterns it was necessary to have a classification system for the different types of branches. A two-division system was developed using the height growth increment (HGI) concept of Isebrands and others (1977) . Orders of branching formed the first division. The orders were designated primary, secondary, tertiary, etc., with the limitation that no HGI could have more orders of branches than its age. Primary branches were branches which arose from lateral buds on the main stem. If these primary branches originated in the same year as the HGI on which they occurred (sylleptic branches) they were called current laterals. Secondary branches originated from lateral buds on primary branches, tertiary branches from lateral buds on secondary branches, etc. The highest order of branching to occur on the two-year-old trees was secondary. 56 The second division of branches was into three classes: 1. Short branches (true short shoots) were less than 10 cm long and had determinate growth with little or no internode elongation. Only preformed leaves were present in the bud and terminal bud formation occurred several weeks after budbreak. 2. Determinate branches had more than 10 cm of internode elongation, displayed only early leaves that were preformed in the buds, and set a terminal bud several weeks after budbreak. There was not a clear distinction between short and determinate branches, so 10 cm was arbitrarily selected. 3. Long shoots, here called indeterminate branches, showed indeterminate (or heterophyllous) growth similar to the main leader and did not form a terminal bud until later in the summer. Gregory (1980) independently developed the same divisions of branches for sugar maple. Each branch type was associated with a general position in the HGI. Indeterminate branches were located in the upper parts of the crown and graded into determinate branches lower down. Determinate branches graded into short branches at the bottom of the HGI. Figure 11 is a schematic diagram of a two-year-old tree with major and minor stem and all branch types illustrated. Results Leaf Area Index Prediction equations for the least squares regressions of leaf length versus leaf area of individual leaves for the first and second year are presented in the Appendix (Tables 21 and 22). All of the regressions were highly significant, 57 Figure 11. Schematic diagram of a two-vear-old hybrid poplar tree showing the major stem (A), minor stem (B), first-year primary branch (C), cutting (D), primary and secondary branches of indeterminate long, determinate long, and short branch types, and height growth increments (HGI) of first and second years. 58 HQI-2 / H Q 1-1 HQI-2 H Q 1-1 Figure 11. 59 having values of 0.97 to 0.99. Clone NE 48 had an elliptical leaf shape, whereas leaves of the other three clones were deltoid. NE 48's regression coefficients seemed to reflect this difference in leaf shape. Either no significant clonal differences in the coefficients was found or a lack of homogeneity of variance would not allow testing for differences. No significant differences were found between the coefficients of leaves present on different branch types, so all leaves were pooled for each clone. The regressions of total leaf area (TLA) per tree 2 versus D H that were used changed significantly from sampling period to sampling period because changes in leaf area due to abscission masked some of the increases due to growth. The r 2 values ranged from 0.93 to 0.99 for the individual sampling periods, whereas the pooled regressions 2 for the entire season had r values of 0.66 to 0.77. Thus, higher accuracy was obtained through the use of different equations for each sampling period. Had the plots been irrigated and leaf abscission minor, pooled equations for the season might have been sufficient. Leaf area index (LAI) during the first and second years is shown in Figure 12. During the first year, LAI increased steadily as the young trees grew in size, reaching a plateau shortly after budset in late August-early September. LAI of NE 48, however, continued to increase until late September due to its later budset. LAI increased despite several periods of abscission of lower leaves following water stress (Figure 21 Chapter III), but the leaf area increase due to 60 Figure 12. Seasonal change in leaf area index(LAI) of four hybrid poplar clones during the first (Day 161 = June 10) and the second (Day 125 = May 5) growing seasons (NE 353+308 A, NE 48 +, NE 58 □ , Eugenei o) . Figure YE A R 1 YE A R 2 in in * M- o O •- 12. * in m co co o 0 co co CMi in in oi 01 CM o CM* 0 01 in to in d in 2 cn d o d i i i i i i i i i » 168 196 224 252 X 280 §- J I 120 148 DAYS 1 L J 1 I 176 204 232 I L 260 + 62 new leaves was greater than that abscised. Maximum LAI values attained after one year were 2.5, 1.4, 1.9, and 1.6 for NE 353 + 308, NE 48, NE 58, and Eugenei, respectively. The second year initial LAI levels were almost double the first year maximum levels, due to growth of many primary and some secondary branches on the first HGI. The trends of LAI during the second year were confounded by sampling variation from the random selection of the large sample trees at each sampling period instead of measuring the same trees all season. The general trend included a peak at or shortly after the maturation of the initial budbreak flush and then a decline throughout the season. The declines in LAI were due to major abscission of leaves and even whole branches in response to severe water stress (Figure 21 Chapter III). Maximum LAI levels obtained were 4.2, 3.5, 3.2, and 3.3 for NE 353 + 308, NE 48, NE 58, and Eugenei, respectively. A clearer picture of the trend in leaf area is shown in Figure 13, which is a plot of the leaf area per tree of the repeatedly sampled small sample trees over the growing season. The clones can be divided into two groups. Clones NE 58 and, especially, Eugenei increased in leaf area until early July, then decreased due to water stress-induced leaf abscission in mid-July. Leaf area then remained constant until early September due to early terminal bud formation. In early September, leaf abscission caused by severe water stress again started, lowering LAI to below first-year levels. The second group, NE 353 + 308 and NE 48, started 63 Figure 13. Seasonal change in total leaf area per stem of LAI sample trees for dominant major (A), codominant major (+), intermediate major (0) , dominant minor (o) and codominant minor (x) stems of four hybrid poplar clones during the second growing season (Day 125 = May 5). 64 NE 353+306 NE 40 CM s EUGENEI NE 58 CM 120 148 176 204 232 260 120 148 176 204 232 260 DAYS Figure 13. 65 out similarly to the first group, but after the first drought period in July these clones regained some leaf area through production of new leaves on the main stem and indeterminate branches. When the second drought period was at its peak in early September, terminal bud formation occurred and leaf abscission began, resulting in lowering of LAI to just above or below first-year levels. Branching Patterns During the first year, a distinct difference in formation of current lateral branches was found. Clone NE 353 + 308 had an average of 9 branches per stem while the other three clones averaged less than one branch per stem (Table 10). In the second year, the two drought periods not only resulted in leaf abscission, but also in the abscission of entire branches after they had lost all of their leaves. This loss of branches is shown in Table 10 by comparing the June measurements (maximum number of branches present several weeks after budbreak) with the late September measurements (minimum number of branches present after two periods of abscission). Abscission started first with the primary and secondary short branches, then moved to the determinate branches, whereas none of the indeterminate branches abscissed. The reductions in total number of branches were 35%, 54%, 69% and 61%, while reductions for determinate and short branches were 42% and 49%, 56% and 84%, 67% and 88%, and 65% and 80%, respectively for NE 353 + 308, NE 48, NE 58, and Eugenei. Thus, loss rates varied by clone, with NE 353 + 308 losing fewer branches than the Table 10. Distribution of branches by branch type of four hybrid poplar clones after the first and second qrowing seasons. Average number of branches per tree Second Year First Year June September Clone Number Current3 Laterals Indet. Det. September Short Current Laterals Total Indet. Det. Short Current Laterals Total 21.5 6.2 27.7 NE 353+308 Primary Secondary Total 9.0 7.5 0.3 7.8 15.0 2.3 17.3 7.5 10.1 17.6 8.5 0.6 9.1 11.2 1.3 12.5 13.3 1.6 14.9 0.3 5.0 ... 0 .1 0.3 5.1 8.5 0.2 8.7 18.5 0.7 19.2 0.1 7.0 0.5 7.5 10.0 0.8 10.8 16.0 2.8 18.8 •.• 9.0 • •• 30.0 12.7 42.7 6.8 0.3 6.9 8.2 1.9 10.1 4.9 4.0 8.9 1.6 33.0 3.5 36.5 8.2 0.6 8.8 4.8 0.7 5.5 1.6 0.8 2.4 0.2 32.0 1.0 33.0 4.8 0.1 4.9 2.8 0.1 2.9 2.1 0.3 2.4 • •• 33.0 4.1 37.1 6.6 0.5 7.1 3.4 0.4 3.8 2.8 0.9 3.7 • •• 1.6 NE 48 Primary Secondary Total 0.6 • •• 0.6 • •• • •• .. . 0.2 14.8 2.0 16.8 NE 58 Primary Secondary Total •. . 9.7 0.5 10.2 Eugenei Primary Secondary Total ... 0.1 ... . .. Indet. = Indeterminate growth branches Det. = Determinate growth branches with internode elongation Short = Determinate growth branches with little or no internode elongation Current laterals = branches originating on the current year's growth of main stem. b Primary branches = branches which arise from axillary bud on the main stem. Secondary branches = branches which arise from axillary buds on the primary branches. . •• • •• 12.8 1.8 14.6 67 other three clones. Statistical differences in numbers of branches between clones were not significant because of high variance and lack of homogeneity of variance. The high variances resulted from dominant and major stems having many branches and suppressed and minor stems few branches. However, some trends are apparent from the second-year branch data in Table 10. The number of secondary branches is directly related to the number of current laterals produced in the first year, explaining the three-fold or higher number of secondary branches produced by NE 353 + 308 as compared to the other clones. The number of primary branches produced per tree in the second year by the four clones was almost identical: 30, 33, 32, and 33, with the differences in total branches influenced by the difference in secondary branches. The production of indeterminate branches was similar for all four clones, but production of determinate and short branches was not. NE 353 + 308 produced more determinate and fewer short branches, whereas NE 58 produced fewer determinate and more short branches. The production of current laterals in the second year was zero to few for NE 48, NE 58, and Eugenei, but 1.6 branches per stem for NE 353 + 308, much smaller than its first-year production. If the branch distribution on only the major stems by crown class is examined, a better picture of the pattern of branching is obtained. Minor stems produced few branches and all of the secondary branches were present on dominant and codominant stems. Table 11 shows branch distribution by Table 11. Distribution of branches on major stems by crown class for four hybrid poplar clones after the second growing season. Crown Class Number of Primary Branches per Stem Indet*? Det. Short Subtotal All 50 4 Dominant Codominan t 33 Intermediate 7 Suppressed 6 11.2 13.8 12.4 7.4 3.5 9.4 9.0 10.9 8.1 3.2 3.3 2.2 3.1 3.6 4.8 23.9 25.0 26.4 19.1 11.5 NE 48 All 51 7 Dominant Codominant 30 Intermediate 9 5 Suppressed 12.7 18.4 13.6 9.4 5.4 6.3 6.8 6.4 6.4 4.6 0.7 • •• 0.2 1.2 2.8 NE 58 All 52 Dominant 5 Codominant 25 Intermediate 16 Suppressed 6 6.6 8.2 8.2 5.4 1.0 2.7 4.2 2.7 2.6 2.3 Eugenei All 49 9 Cominant Codominant 26 Intermediate 11 3 Suppressed 8.6 13.6 9.0 5.5 1.3 3.6 4 .2 3.5 3.7 1.7 Number ME 353+ 308 Number a Number of Secondary Branches per Stem Det. Short 0.6 ••• 0.8 *• • 3.4 6.8 4.3 ••• 7.3 11.2 9.8 19.7 25.2 20.2 17.0 12.8 1.1 1.3 1.6 •• • 1.1 1.8 1.4 • •• 0.9 ••• 0.3 1.9 1.2 10.2 12.4 11.2 9.9 4.5 0.1 1.0 •• • • •• 1.0 0.3 0.8 1.9 1.3 13.2 18.1 13.3 11.1 4.3 0.7 2.4 0. 5 • ■• • •* Indet. Subtotal Total 11.3 18.0 14.9 •• • « --- 35.2 43.0 41.3 19.1 11.5 1.3 3.6 1.4 • •• 3.5 6.7 4.4 •• • 23.2 31.9 24.6 17.0 12.8 0.2 2.2 •• • • •• 0.6 6.2 ••• ... 0.9 9.4 ••• ... **• 11.1 21.8 11.2 9.9 4.5 0.6 1.4 0.6 • •• 1.3 3.9 1.2 2.7 7.7 2.3 15.9 25.8 15.6 11.1 4.3 ... ... a Primary branches = branches which arise from axillary buds on the main stem. Secondary branches = branches which arise from axillary buds on the primary branches. b Indet. = Indeterminate growth branches. Det. = Determinate growth branches with internode elongation. Short = Determinate growth branches with little or no internode elongation. Current laterals = branches originating on the current year's growth of main stem. ... 69 clone and crown class for the major stems at the end of the second year. General trends included a decrease in the number of primary branches with a decrease in crown class, although in many cases dominant and codominant stems were similar. Since the indeterminate branches were not affected by abscission, their numbers reflect both initial and final differences during the growing season. The number of indeterminate branches decreased steadily with crown class, with some differences between clones. The determinate and short-branch values are final values after abscission. As crown class decreased, the loss of branches appeared to decrease, especially for short branches. Since lower crown class trees had fewer branches (becasue of fewer lateral buds) and stopped growing earlier, they retained a higher percentage of their short and determinate branches than higher crown class trees which sacrificed the short and determinate branches to maintain growth on the current leader and indeterminate branches. Again the total number of branches per tree is highly inflenced by the number of secondary branches and decreases with crown class. The distribution of woody biomass into branch types is shown in Table 12. The total amount of biomass partitioned followed very closely the total number of branches in Table 10, while the biomass partitioned into each branch type followed closely the numbers of branches of each type in Table 11. Most of the branch biomass was in indeterminate branches. One of the major differences was in two-year-old portions of secondary branches (the first year current Table 12. Partitioning of ; wood, bark, and branch biomass into various branch types for four hybrid poplar clones after the second growing season. Percentage of woody biomass Secondary Branches Primary Branchesa Indet.k Det. Short Current Laterals Two--year-old woodc One-year-old wood NE 353+308 14.8 1.7 0.3 0.8 3.2 0.6 21.4 NE 48 16.9 1.6 0.1 0.2 1.6 0.4 20.8 NE 58 15.1 1.6 0.2 • • • 0.7 0.2 17.8 Eugenei 17.1 1.3 0.1 • • • 1.9 0.5 20.9 AVERAGE 16.0 1.6 0.2 0.2 1.8 0.4 20.2 Clone Number Total a Primary branches = branches which arise from axillary buds on the main stem. Secondary branches = branches which arise from axillary buds on the primary branches. b Indet. = Indeterminate growth branches Det. = Determinate growth branches with internode elongation Short = Determinate growth branches with little or no internode elongation Current laterals = branches originating on the current year's growth of main stem One-year-old wood = the secondary branches which emerged from the previous season's primary branches. Two-year-old wood = the previous season's primary branches with their second year increment. 71 laterals that bore the buds for the secondary branches); NE 353 + 308 had approximately twice the biomass in this branch component than the other three clones. As a result of the flushing of lateral buds during the first year, NE 353 + 308 had a slightly smaller amount of woody biomass present in indeterminate branches. The lower biomass and fewer number of branches on NE 58 was partly an artifact caused by the decreased growth of that particular plot induced by the large black cherry tree which bordered it. Leaf Characteristics and Distribution Growth Analysis Parameters. The leaf area ratio decreased steadily throughout both growing seasons (Figure 14) . The first season LAR values began at 105 to 115 cm 2 g-1 and decreased to 30 to 50 cm 2 -g 1 , while in the second season, the values began at 65 to 85 cm 2 g-1 and decreased to 15 to 25 cm 2 g-1 at the end of the season. Thus, leafiness of the trees decreased with increasing tree size. These decreases correspond to decreases in leaf weight ratio (LWR) or the amount of biomass partitioned into leaves shown in Chapter I (Figures 9 and 10). Specific leaf weight, in general, increased throughout both growing seasons (Figure 15). The first season SLW _2 values varied from 5.5 to 7.2 mg cm early m the season to 7.5 to 8.7 mg cm- 2 late in the season. . The major exception to the pattern was clone NE 58 in which SLW remained low and then increased late in the season. Clone NE 48, which maintained the highest SLW values for most of the season, also appeared to have thicker, more leathery leaves. The 72 Figure 14. Seasonal change in leaf area ratio (LAR) of four hybrid poplar clones during the first (Day 172 = June 21) and the second (Day 151 = May 31) growing seasons (NE 353 + 308 A , NE 48 +, NE 58 D , Eugenei o). Figure Y EAR 1 YEAR 2 O O CM- CM-H 14. O O oT“ h oT“ o o o o o CM o o 2 § O O) 0) 00 00 O N CO o o 10 IO 9 o o O (0 o CO o _ CM CM o__ I 172 » I I 200 I I I I 228 I I I I 256 I I II 151 DAYS 172 I 1 » I I I I I I I 193 214 235 + 257 74 Figure 15. Seasonal change in specific leaf weight (SLW) of four hybrid poplar clones during the first (Day 172 = June 21) and the second (Day 151 = Mav 31) growing seasons (NE 353+308 A, NE 48 +, NE 58 0, Eugenei o). Figure YEAR 1 YEAR 2 o O 15 oT 0) s- 00 o o 00 CO in * * o in S K s O K in id IA g o o s« * § in id IS o in s in in o •— o * j - i - 172 200 i i i i ... l. 228 256 151 DAYS i 172 i i j 193 l i 214 i i 235 i i 257 76 second year, SLW values were lower at the beginning of the -2 growing season than the first year, from 4.0 to 5.3 mg cm but ended up at similar levels of 7.0 to 8.5 mg cm _2 . Again NE 48 had the highest values for the entire season. The position of leaves in the crown affected SLW (Table 13). Upper crown leaves of current leader and indeterminate branches had higher SLW values and generally had a larger increase in SLW over the season than lower crown leaves borne by determinate and short branches. Leaf Distribution. Figure 16 shows the percentage of total tree leaf area present on branches in the first year. In accordance with the number of branches produced in the first year, the percentages are very low ( 5%) for all clones except NE 353 + 308, which had 40% of its leaf area present on branches. In the second year, patterns differed (Figure 17): all four clones began with 5 to 10% of total leaf area on the current leader, which increased over the season to 25 to 30% of the total leaf area. The remainder of the leaf area was distributed among the branch types, with the majority on indeterminate branches. In general, indeterminate branches carried from 45 to 70% of the leaf area throughout the season. Current leader and indeterminate branches had little loss of leaf area due to abscission. In general, both determinate, short, and any secondary branches decreased in leaf area over the season, beginning with percentages of 20 to 40 and decreasing to values less than 10 (Figure 17). These decreases were due to both abscission of leaves from these branches and Table 13. Specific leaf weight (SLW) values by branch type for four hybrid poplar clones, early and late in the second growing season. Specific Leaf Weight, mg cm-2 NE 353 + 308 NE 48 NE 58 Eugenei Branch Type June 13 August 22 June 13 August 22 June 13 August 22 June 13 August 22 Current Leader 5.49 8.72 7.33 9.81 6.10 8.53 5.66 7.66 Indet. Branches 5.39 7.46 6.63 9.04 5.58 7.18 5.07 6.40 Det. Branches 5.04 6.10 5.46 5.97 5.06 5.97 4.38 5.73 Short Branches 4.40 6.39 5.01 5.21 4.86 5.41 3.78 5.40 a Current Leader = current year's growth of main stem. Indet. = Indeterminate growth branches. Det. = Determinate growth branches with internode elongation. Short = Determinate growth branches with little or no internode elongation. Current laterals = branches originating on the current year's growth of main stem. 78 o ___ co H Z LU O o w CC S !P o _ 172 200 228 256 DAYS Figure 16. Seasonal change in the percentage of total tree leaf area present on branches of four hybrid poplar clones during the first growing season (Day 172 = June 21, LIE 353+ 308 A, NE 48 +, NE 58 □ , Eugenei o) . 79 Figure 17. Seasonal change in the percentage of total tree leaf area present on the main stem (A), indeterminate long (+), determinate long (0) , and short (o) branches, and all secondary and/or current lateral branches (x) of four hybrid poplar clones during the second growing season (Day 151 = May 31). 80 NE 353+308 NE 48 O CO o PERCENT CM EUOENEI NE 58 CO O-i- 151 172 193 214 235 257 151 172 193 214 235 257 DAYS Figure 17. 81 increases in leaf area on the current leader and indeterminate branches. The average leaf area and numbers present on the current leader increased throughout the second growing season before decreasing at the end of the season (Figures 18 and 19). The average leaf area and numbers present on indeterminate branches increased slightly to a plateau and remained there for most of the season. The average areas and numbers of determinate and short branches decreased slowly and steadily all season due to leaf abscission. In general, current leader leaves were large and succulent when young, indeterminate branch leaves were smaller but still relatively large and succulent when young, whereas determinate, short, and secondary branch leaves were smallest (Table 14). Clonal differences present, with clones NE 58 and Eugenei 308. in leaf sizewere NE 48 NE 353 + NE 353 + 308 had very small leaves on its short, secondary, and current lateral branches. Discussion Marshall (1968) discussed the various methods used to determine leaf area. One of the oldest and nondestructive methods is the estimation best of leaf areafrom linear measurements of leaves, a technique used since 1924 on a variety of species (Boynton and Harris 1950, Ackley et al.1958, Kubicek 1971, Fordham and Holgate 1972) including Populus (Pollard 1970, Larson and Isebrands 1972). Use of Larson and Isebrands' (1972) equation in the present study yielded even higher R 2 values than they had obtained, while 82 Figure 18. Seasonal change in the average leaf area present on the main stem (A), indeterminate long (+), determinate long (0) , and short (o) branches, and all secondary and/or current lateral branches (x) of four hybrid poplar clones during the second growing season (Day 151 = May 31). 83 NE 353+308 NE 48 CM CM CM CM EUGENEI NE 58 CM r— 2 concentration, soil fertility, wind , and canopy position (sun or shade leaf) (Larcher 1969). A number of these influences will be discussed in this paper, but the most significant one is the influence of drought on gas exchange. Drought tolerance of poplars varies between clones, species, and sites (Regehr et al. 1975, McGee et al. 1981, Tobiessen and Kana 1974, Ceulemans et al. 1978a,b, Kelliher and Tauer 1980, Luukkanen and Kozlowski 1972). Some of the drought tolerance mechanisms 93 94 shown by poplars are: closure of stomata to reduce transpiration, yellowing and abscission of older leaves, maintenance of growth and turgor in young leaves, exploitation of larger soil volumes by the root system, and acclimation of enzyme functions to lower plant water potentials (Neuwirth and Polster 1960, Furukawa 1972, Pieters and Zima 1975, Smith and Gatherum 1974, Kelliher et al. 1980, Domingo and Gordon 1974, Faulkner and Fayle 1979). The recovery of function following water stress was dependent on the duration and magnitude of the stress (Sivakumaran and Hall 1979, Regehr et al. 1975) . The loss of leaf area is most damaging to the plant once the water stress is removed, because leaf area cannot be quickly replaced. However, the impact of leaf area loss is minimized because older, shaded, less productive leaves are abscissed rather than young, well exposed, highly productive leaves (Neuwirth and Polster 1960, Tazaki and Ushijima 1963, Bonner 1967). Responses of trees to environmental variables, while often discussed separately, are all interrelated. The variables often change together and produce an interaction effect which the plant then integrates into its response (Ceulemans and Impens 1981, Pallardy and Kozlowski 1979b, Larcher 1969) . Stomatal conductance and photosynthesis are usually directly correlated but this relationship also is dependent on the specific factor that is limiting photosynthesis (Ceulemans and Impens 1980, 1981, Ceulemans et al. 1980a, Regehr et al. 1975) . 95 Measurements of stomatal conductance, photosynthetic rate, and several of the environmental variables that influence these processes were taken in the same experimental plots as in Chapters I and II during the tree's second year of growth. Materials and Methods Field Sampling Procedure In the biomass subplots described in Chapter I, weekly field measurements of photosynthesis, stomatal conductance, and environmental parameters were made on trees during their second growing season (late May to late September, 1978). Three trees each of the four clones were selected randomly from the plot interior and measured on each date. On each measurement date, leaves at three positions on the tree were sampled: upper crown - the first mature leaf below the terminal shoot apex (LPI 8 to 12); mid-crown - the first mature leaf below the the shoot apex of an indeterminate long branch located in the upper mid-crown (LPI 7 to 12); and lower crown - the first mature leaf below the shoot apex of a short or determinate long branch located in the lower portion of the crown (LPI 1 to 6) and not showing any visible signs of senescence. Leaves were always measured in sequence from upper to lower crown on each tree before moving to the next tree. All three trees of a clone were measured in a randomized order before measurement of the next clone began. Clones were measured in randomized order between 9:30 and 14:30 (Mean Solar Time) on sunny or partly sunny days. 96 Notes on the condition of the trees and their growth and leaf production were reported in Chapters I and II. Environmental Parameters Environmental parameters were monitored during the course of the study. Maximum and minimum temperatures, relative humidity, and total solar irradiance were extracted from data published by the U.S. Weather Bureau in Lansing, Michigan and collected at a weather station on the Michigan State University campus. Precipitation was recorded from a standard rain gauge at the study site. Undisturbed 7.6 cm diameter soil core samples of the two soil types present on the study site were collected from 0 to 30 cm and 30 to 60 cm depths. Water retention curves were developed for the two soil types and two depths (Figure 28 Appendix) using a ceramic pressure plate apparatus. Undisturbed soil cores were used for the low tension range and screened soil samples for the high tension range. Field soil moisture samples 2 cm in diameter were collected in the same manner as above 24 hours after precipitation events and periodically inbetween them. Water content was determined gravimetrically and converted to soil moisture tension using the water retention curves. Seasonal patterns of soil moisture were developed by integrating the soil moisture samples with precipitation and evaporation data. Measurements of wet and dry bulb temperatures were taken with an aspirated pistol-type psychrometer at 20 to 30 minute intervals during the periods when photosynthesis and stomatal conductance were measured. The dry bulb 97 temperature was used as the air temperature. Vapor pressure deficit (VPD) was calculated by converting the wet and dry bulb temperatures to millibars of water vapor at saturation and then taking the difference of the two values and converting to kiloPascals (kPa). Photosynthetic photon flux density (PPFD) (400 to 700 nm) was measured at the time of the photosynthetic measurements. PPFD was measured at the adaxial leaf surface in a horizontal plane at the time of 14C02 application using a quantum flux sensor and meter (Li-Cor, U.S.A.). Physiological Parameters Abaxial leaf diffusive resistance to water vapor (s cm"1) was measured immediately prior to photosynthesis on 14 the half of the leaf lamina not exposed to CC>2 with a diffusive resistance meter and horizontal sensor (Li-Cor, U.S.A.) that was shaded just prior to and during the measurement (Kanemasu et al. 1969, Morrow and Slatyer 1971). Leaf conductance to water vapor was obtained by taking the reciprocal of leaf diffusion resistance to water vapor. The porometer cup temperature was measured with a built-in thermistor and used as an estimate of the leaf temperature. Calibration of the porometer sensor was done at the beginning and end of the growing season. The photosynthetic rate of individual attached leaves was measured using a labeled carbon dioxide (14C02) method modified from that described by McWilliam and others (1973) and others (Incoll 1977, Strebeyko 1967, Shimshi 1969, Naylor and Teare 1975, Austin and Longden 1967). Adaxial 2 and abaxial surfaces of a small disc (0.5 cm ) of the intact leaf lamina midway between the leaf tip and base were exposed to 14CC^-labeled air (335 cm 3 -m 3 ) of known specific activity (5.3/«.Ci 1 * gas mixture) for 20 seconds at a 3-1 14 constant flow rate of 1.33 cm s (Figure 20). After C treatment, the exposed leaf area was removed immediately with a sharp cork borer. After removal, the leaf disc was 3 placed in 1.5 cm of 0.6 N NCS solubilizer (Amersham, 3 U.S.A.) in a 20 cm scintillation vial and digested for 24 hours in a 50 C oven. After the digestion period, the 3 sample was bleached with 0.5 cm of 30% hydrogen peroxide and 18 cm 3 of scintillation cocktail [60 cm 3 Spectrafluor PP0-P0P0P (Amersham, U.S.A.) and 400 cm^ of ethylene glycol 3 monomethyl ether (methyl cellosolve) in 1000 cm toluene] were added to the vials. The vials were placed in a darkened scintillation counter (Packard Tri-Carb) for several hours to reduce the effects of chemiluminescence. The samples were counted for 10 minutes each on wide and narrow channels. The sample count rates were corrected for background and counting efficiency determined from a channels ratio/counting efficiency curve developed from known samples (Figure 29 Appendix). Sample activities (dpm) were calculated by dividing the the sample count rates by the counting efficiency (expressed as a decimal). Photosynthetic rates were calculated using the reduced formula in Table 15. The conversion of CC^ volume to weight was derived from Catsky and others (1971, p. 164). discrimination factor to account for diffusive and The 99 Figure 20. apparatus. Photosynthetic measurement using field 2 ure Table 15. Formula and constants used for calculation of photosynthetic rate. Unreduced formula P = 3600 sec hr______________ (DF)_______________ (S) ______ (SA)___________________ (A) (-0.0026(LT)+0. 6587 7 j (T) Reduced formula P = 0.0717 Where: (S) (-0.00206 T (LT) + 0.65877) - 2, P = photosynthetic rate, mg C02dm_ hr“^ 3600 sec/hr = conversion from seconds to hours. 14 DF = discrimination factor for C02 versus 12 C02 = 1.1775. 101 S = sample activity, dpm. . . 14 SA = specific activity of CO 2 in gas mixture = 11.766 X 10 6 dpm/1 C02 (-0.00206 mg CC>2 (LT) + 0.65877 mg C02 = conversion factor for volume to weight for CO 2 as affected by leaf temperature (LT) c. 1 C02-C A = area of sample exposed to 14 CC>2 - 0.0050265 dm 2 T = time of exposure to ^CC> 2 , sec. 0.0717 = constant that includes time conversion, DF, SA, and A and has the units sec-1 CO^ 102 biochemical discrimination within leaves against 14 CC>2 (Van Norman and Brown 1952, Austin and Longden 1967, Incoll 1977, Yemm and Bidwell 1969, Bykov 1970, Voznesenskii et al. 1971) was calculated according to Van Norman and Brown (1952). The use and problems encountered in the 14CO^ method have been discussed (Austin and Longden 1967, Shimshi 1969, Strebeyko 1967, McWilliam et al. 1973, Karlsson and Sveinbjornsson 1981) and summarized (Incoll 1977). Photosynthetic rates measured with this technique are greater than net photosynthesis (Incoll 1977, Michael 1984) and may approximate gross photosynthesis in some cases. Comparisons between species utilizing this technique can be 14 risky due to differences in CC^ uptake with different genotypes (Karlsson and Sveinbjornsson 1981) but relative comparisons within a clone or crop species are very sensitive since 14CO 2 uptake characteristics are similar within the genotype. Data _ _ Analvsis - ^Analysis of variance for both photosynthesis and stomatal conductance was calculated using a split-split plot design with date of measurement the main plot, clone the first split plot and leaf position the second split plot. Stepwise multiple regression for both photosynthesis and stomatal conductance versus environmental and physiological parameters were calculated in an effort to partition the variation present. Transformations of the variables were performed using logarithms (base^), reciprocals, and squares of all of the parameters in order to find the 103 relationship that best partitioned the variation. The high variability of field data due to the large number of factors that can be limiting makes strict statistical tests very difficult to interpret (Incoll 1977). Webb (1972) developed a technique called boundary line analysis that can be used to analyze data from studies where interacting variables cannot be controlled or, in many cases, even identified (Hinckley et al. 1978a, Incoll 1977, Jarvis 1976, Watts 1977). The boundary line represents the limiting effect of the independent variable on the dependent variable; it is assumed that all values below this line result from independent variables or interactions of variables being limiting (Hinckley et al. 1978a, Webb 1972). Results Physiological Parameters Patterns of stomatal conductance for three canopy positions are presented in Figure 22 for the second growing season. Lower crown leaves had lower stomatal conductance values (range 0.01 to 0.49 cm s *) than mid- and upper crown leaves (range 0.01 to 0.99 cm s *), which were generally similar. The major influences on the stomatal conductance patterns were the drought periods of mid-July and early September (Figure 21) which resulted in stomatal closure. The lower crown leaves reacted first and the response moved up the tree's crown. As soil moisture tensions approached -1200 to -1500 kPa, stomata were totally closed. Significant differences in stomatal conductance values were found due to date of measurement, clone, and leaf position 104 Figure 21. Seasonal changes in soil moisture tension for two soil types at two depths for the first (Day 161 = June 10) and second (Day 125 = May 5) growing seasons (Kalamazoo sandy loam, 0-30 cm A , 30-60 cm + , Hillsdale sandy loam, 0-30 cmQ, 30-60 cm o). Figure YERR 2 YERR 1 Q. 21. o co O' co 2 ‘o ' ? 105 CM lO i i i i i i i i i i i i i J 8 196 224 252 + 280 DAYS 120 148 176 204 232 260 106 Figure 22. Seasonal change in stomatal conductance to water of leaves present in the upper (A), middle (+), and lower (o) crowns of four hybrid poplar clones during the second growing season (Day 125 = May 5). 107 NE 353+308 NE 48 Q-i. oo o> co o oo in co co o o in CM o• o o o o TV ° o CO o t i l l I I I I I i L i I I I I I I I L EUCENEI NE 58 oo r o> co o oo m co co o b d in CM o d r 9 O r o— • o I I 1 I I J 1-----1-----L 120 148 176 204 232 260 l 1-----1-----1-----L 120 148 176 204 232 260 DAYS Figure 22. I 108 (Table 16). The significance of the interaction terms indicates that the clones and leaf positions did not react in the same manner with changes in environmental conditions. Seasonal patterns of photosynthesis for the three canopy positions were similar to stomatal conductance patterns (Figure 23). Lower crown leaves had the lowest photosynthetic rates (range 0.6 to 28 mg CC>2 dm -2 hr -1 ) while mid- and upper crown leaves were similar and usually _2 much higher than lower leaves (range 1.7 to 39 mg CC^ dm hr ■*■). The depressing influence of the two drought periods is again evident. Significant differences in photosynthetic rates were found due to date of measurement, clone, and leaf position (Table 17). Like stomatal conductance, significance of the interaction terms means differences in response of the clones and leaf positions occurred due to changes in environmental conditions. Environmental Parameters A wide range of values for environmental parameters were encountered during the second growing season (Table 18). These values reflect the variation in weather during a typical Michigan summer and the microclimate within the poplar stands. For stomatal conductance, 60 to 73% of the variation within a clone could be explained by combinations of environmental parameters using stepwise multiple regression (Table 19). The first variable to enter the equation for three of the clones was PPFD in logarithmic form and VPD in logarithmic form for the remaining clone. The second Table 16. Analysis of variance for split-split plot design of stomatal conductance to water (cm s-l) date, clone, and leaf position for four hybrid poplar clones measured throughout the second growing season. Source of Variation Degrees of Freedom Mean Square Significance of F-ratio 0.00289 ns ** Rep. 2 Date 15 0.203 Error 1 30 0.0156 3 0.0918 ** Date x Clone 45 0.0550 ** Error 2 96 0.0138 2 0.568 ** 30 0.0805 ** 6 0.0203 * 90 0.0144 ** Residual Error 256 0.00725 Total 575 Clone Leaf Position Date x Leaf Position Clone x Leaf Position Date x Clone x Leaf Position a ns not significant at a = 0.05 level. * significant at a = 0.05 level. ** significant at a = 0.01 level. 110 Figure 23. Seasonal change in photosvnthetic rate of leaves present in the upper (A), middle (+), and lower (o) crowns of four hybrid poplar clones during the second growing season (Day 125 = May 5). Ill NE 353+308 NE 48 EUOENEI NE 58 CM O O * O 8 CM 2 W CO CM CM O CM CO T“ CM CO 'cr u l_i L J I I 1-- 1 -- 1-- L 120 148 176 204 232 260 Figure 23. 120 148 176 204 232 260 DAYS Table 17.-Analysis oE variance for split-split plot design of photosynthetic rate (mg CO dm hr- ) versus date, clone, and leaf position for four hybrid poplar clones 2 measured throughout the second growing season. Source of Variation Degrees of Freedom Mean Square Significance of F-ratioa Rep. 2 3.28 ns Date 15 1297.79 ** Error 1 30 75.16 3 850.05 ** Date x Clone 45 113.94 ** Error 2 96 46.07 2 12756.85 ** 30 152.71 *★ 6 114.12 ** 90 29.01 ns Residual Error 256 27.72 Total 575 Clone Leaf Position Date x Leaf Position Clone x Leaf Position Date x clone x Leaf Position a ns not significant at ot = 0.05 level * significant at rt = 0.05 level ** significant at a = 0.01 level Table 18. Range of environmental variables on days when gas exchange measurements were taken during the second growing season. Environmental Variable Range Air temperature (AT) 18 to 32.5 C Porometer leaf temperature (LT) 20 to 40 C Vapor pressure deficit (VPD) 0.03 to 2.0 kPa Photosynthetic photon flux density (PPFD) 20 to 2300 pmol m Soil moisture tension, 0-30 cm (SMTA) -45 to -720 kPa Soil moisture tension, 30-60 cm (SMTB) -35 to -450 kPa -2 sec -1 Table 19. Stepwise multiple regression of stomatal conductance to water (cm sec— ) versus environmental variables and the log^g, square, and reciprocal of these variables9 . Regression Clone Number NE 353+308 Significance of F-ratiob ** Dependent Variables 2 r value 0.73 NE 48 ** 0.71 NE 58 ** 0.66 ** Coefficient Significance of t-valueb ** ** ★★ ★ log10VPDC SMTA PPFD (PPFD) -0.116 2.60 x 1.48 x -5.68 x l°g10PPFD SMTA (SMTB) 0.105 1.01 x 2.23 x ** ** ** (SMTB) 8.42 x 6.47 x 10-4 1.04 x ** ** ** 5.90 x 3.36 x 10-3 3.28 x 10 ** ** * 0.61 _1 (VPD) iS-3 IS- a The regressions were forced through the origin and had a significance level of u = 0.05 for the partial Fratio used for entry and deletion of variables. The regression was done independently on eacti of four hybrid poplar clones measured throughout the second growing season. b ** significant at a = 0.01 level. * significant at n = 0.05 level. PPFD = photosynthetic photon flux density nmol m VPD = vapor pressure deficit, kPa AT = air temperature, C SMTA = soil moisture tension, 0-30 cm, kPa SMTB = soil moisture tension, 30-60 cm, kPa -2 -1 s 114 Eugenei Variables which entered regression 115 variable was soil moisture tension at 0 to 30 cm depth or its square. Other variables entering some of the equations were PPFD untransformed and squared, the reciprocal of VPD, and soil moisture tension at 30 to 60 cm depth squared. From these regressions it is apparent that the major influences on stomatal conductance were light, soil moisture tension, and VPD. Boundary line plots of stomatal conductance versus PPFD and VPD were done to further illustrate the influence of these variables. Figure 24 shows that the relationship between PPFD and stomatal conductance is similar for all four clones. A light threshold occurred at approximately 100 to 150^cmol m stomatal opening. -2 -1 s , above which light no longer limited VPD had an inverse threshold relationship with stomatal conductance (Figure 25). Below VPD values of 1.1 to 1.3 kPa, stomatal opening was not influenced by VPD. However, at VPD values above this threshold, stomatal closure reduced conductance rapidly. Both of these relationships appeared to be independent of any leaf position effects. Stepwise multiple regressions showed that from 84 to 90% of the variation in photosynthetic rate within a clone could be explained by some combination of environmental parameters and stomatal conductance (Table 20). The first variable to enter the regression for all clones was PPFD in logarithmic form. For three clones, the second variable was stomatal conductance also in logarithmic form and soil moisture tension at 0 to 30 cm depth for the remaining 116 Figure 24. Boundary line plot of stomatal conductance to water versus photosynthetic photon flux density for the upper (A), middle (+), and lower (o), crown positions of four hybrid poplar clones during the second growing season. 117 NE 353+308 NE 48 H h I 8 d a CO 8 A t o + 3p £ + A+A + + o + * af + * 5 +_i_ J.A+'I -■ A A 5 n+*_+ —.O+A' + '4 c^1*-A^ ° -^C A +.^A + A A + afA * V AA -P O *o+ ,%++A+ A+ +A-jk + § -h£ * d ++ A + Icpo o A A + J CO Z o L J I I J L I J L L EUGENEI NE 58 o T" A - (i CO + +* o> CO d +A * > A 3 - $° QO++^ O ' TA 5 CO I + O °£-f C& o + AA? 8 , Xi-zw , "A .HO .Aj.a xO^ O ^ © . A Z * P “ KT -tA A A Pi o % } + U ^ A ^ A ^4n#+ jQ. # + a a O Of $ A+ A A ± . A 8 + s A o H0 J L J I I J L 5 0 0 1000 1500 2000 I I I L 5 0 0 1000 1500 2 000 juMOL M *2 S*1 Figure 24. L 113 Figure 25. Boundary line plot of stomatal conductance to water versus vapor pressure deficit for the upper (A), middle (+), and lower (o) crown positions of four hybrid poplar clones during the second growing season. 119 NE 353*308 NE 48 A 00 o> CO o 00 II) CO CO IA 8* I O O © § £ + o + n ~ & ,+ > m c/i U ) tn w ♦ 0 (j O CD CD * +0° ° 6bo * H N> U1 126 thresholds for stomatal opening, but these same light levels were limiting to photosynthesis in these leaves (Aubuchon et al. 1978, Lemeur and Impens 1981, Pollard 1970b, Nelson and Michael 1982). Upper and mid-crown leaves were not limited by light, but high leaf temperatures and mid-day stomatal closure did limit photosynthesis (Aubuchon et al. 1978). The lower photosynthetic rates and stomatal conductance of NE 48 are probably related to its horizontal leaf orientation, thick cuticle, and heavy wax deposits on the lower leaf surface (Isebrands 1982, Nelson and Michael 1982). The influences of PPFD, VPD, and soil moisture tension on stomatal conductance and photosynthesis were very similar to responses found by others. Regressions by Pallardy and Kozlowski (1979a,b) showed PPFD to be the most important variable followed by VPD, but there was also an interaction between the two variables. The response to VPD was independent of soil and plant moisture tensions. The shape of the boundary line curves for stomatal conductance in this study are similar to curves reported elsewhere (Pospisilova and Solarova 1980, Jarvis 1976, Ceulemans and Impens 1980, 1981, Ceulemans et al. 1978b, McGee et al. 1981). PPFD had a major influence on photosynthesis (Lemeur and Impens 1981, Isebrands 1982, Michael 1984), and along, with stomatal conductance was highly correlated with it (Ceulemans et al. 1980a, Ceulemans and Impens 1980, 1981). The shape of the boundary line curves for photosynthesis are similar to curves reported elsewhere (Furakawa 1972, Larcher 127 1969, Pieters 1960, 1974, Fasehun 1978, Smith and Gatherum 1974, Domingo and Gordon 1974, Regehr et al. 1975, McGee et al. 1981). NE 48 had a lower light saturation point for photosynthesis than the other three clones. In addition, NE 48 reacted to VPD declines by reducing stomatal conductance and photosynthesis sooner than the other clones. This response is one mechanism to reduce water use that helps to make NE 48 more drought tolerant than the other clones and is consistent with the higher water use efficiency and drought tolerance of Tacamahaca poplars over Aigeiros poplars shown by Blake (1981). While soil moisture tensions were an important determining factor in both stomatal conductance and photosynthesis regressions, the influence of the two drought, periods on these processes is not fully evident from the regressions. Drought stress produced major effects on the growth of the poplar clones. As mentioned in Chapters I and II, these effects included reductions in biomass yield, height and diameter growth, and premature abscission of leaves and branches. The literature discusses a number of techniques utilized by woody plants to tolerate drought stress. Stomatal control was a technique used by many trees (Hinckley et al. 1979). As drought stress developed, photosynthetic rates were depressed to near the compensation point, but upper crown photosynthetic rates were reduced much more than lower crown rates (Aubuchon et al. 1978, Hinckley et al. 1979). Both of these patterns seem to fit 128 the responses shown bv poplar clones in my study. Federer (1977, 1980) and Pospisilova and Solarova (1980) showed that the water potential at which different species and clones reach a stress situation varies widely. One species or clone may be undergoing severe stress while another shows no sign of stress (Hinckley et al. 1978a). The abscission of leaves early is one mechanism that conserves water during drought periods so that conductance and photosynthesis in remaining leaves can be maintained at higher levels for longer time periods (Ginter-Whitehouse et al. 1983) . Kaufmann (1982) found increased conductance in senescing aspen branches two months proir to any visual symptoms of senescence. A similar phenomenon probably occurred in my study, as many short shoots in the lower crowns, which eventually senesced and abscised, showed higher conductance rates than leaves in the upper canopy just as each of the drought periods began. The loss of stomatal function due to senescence processes is the probable cause of this increased water loss prior to visible leaf decline. Poplars cope with drought through stomatal closure, leaf and branch abscission, preconditioning of processes to stress, and exploitation of larger soil volumes. Preconditioning is evident by comparing stomatal conductance and photosynthesis values of the more severe second drought period to those of the first. In both cases, reductions in these processes were either no lower in the second drought or did not decrease until soil moisture levels reached higher potentials than the first drought. It is highly 129 likely that the precondititioning is due to increses in osmotic potential during the first drought period (osmotic adjustment) (Hinckley et al. 1978b, Parker et al. 1982). The interactive effects of environmental variables on gas exchange measured in the field make direct correlations and interpretations difficult, especially when one or several factors may be limiting gas exchange at any given moment. This difficulty has prompted many people to use modelling approaches or more sophisticated statistical approaches such as multivariate analysis in an attempt to predict gas exchange and biomass productivity. No siginificant correlations were found between eight gas exchange variables and field productivity of eight poplar clones but some promise was shown for separating the more highly productive clones (Ceulemans et al. 1980a). Further analysis using multivariate analysis techniques showed that field productivity of poplar clones could be predicted from gas exchange parameters at their maximal values (optimal leaf age)(Ceulemans and Impens 1982). In yet another experiment, Ceulemans and Impens (1983) showed a very good correlation between gas exchange rates at their maximal values and first and second year height and biomass yield. It is only through the integration of the growth processes by the plant that the true interaction effects can be seen. Simulation models may provide one means of obtaining the same information without having to do large field experiments for many years. The data gathered in this study could be used to help formulate such a model. SUMMARY AND CONCLUSIONS The interrelationships of growth and biomass yield to crown development and gas exchange of four hybrid poplar clones has been presented in Chapters I, II, and III. Ceulemans and Impens (1982, 1983) have shown potential for using gas exchange parameters at maximal values (optimal leaf age) to predict field productivity of poplar clones. However, it is obvious that intra- and interplant physiological relationships are necessary in order to accurately predict productivity (Watson 1952). The results of this study support this view as looking at only one parameter or process would not allow you to accurately predict biomass yield. The four clones showed differences in growth pattern, biomass yield, LAI, branching pattern, biomass partitioning, gas exchange, and response to drought stress. Increases in wood yield may be obtained by manipulating crown shape and structure, leaf display, and photosynthetic efficiency, the latter either through higher photosynthetic rates or lower respiration rates (Larson and Gordon 1969a). Photosynthetic efficiencies of short rotation intensively-cultured poplar systems varied from 1.88 to 3.72% as spacing decreased (Lemeur and Impens 1981) which was higher than the efficiencies of many crop plants. 130 This 131 highly efficient conversion of solar energy into biomass is one of the major advantages of using short-rotation, intensive culture to grow trees. Some of the physiologically-based management recommendations that can be made have been summarized by Isebrands (1982): 1. Clones should be selected to match the spacing and rotation length, the length of the growing season, site requirements, and desirable root characteristics. 2. Height growth is controlled by current leader leaves and factors which reduce their number, size, and growth will reduce height growth. 3. Factors that affect lateral branch leaves will not affect height growth, but will affect either lateral branch growth if prior to budset or stem diameter and root growth if after budset. The higher the branch on the tree, the greater the effect on stem diameter and root growth. 4. Pruning is not recommended until branches are older than three years. 5. Season of harvest will affect coppicing with late summer being the worst time to harvest. The results from this study provide additional support to several of these points. The following conclusions can be drawn from this study: 1. The NE 353 + 308 clonal mixture had the highest LAI, biomass yield, and MAI; had the highest amount of biomass partitioned into branches and lowest into minor stems; and had among the highest photosynthetic rates. 132 2. NE 48 had the second highest LAI, biomass yield, and MAI; was intermediate in the proportion of biomass partitioned into branches and second lowest into minor stems; had the highest SLW, thickest leaves, and least amount of leaf abscission; and had the lowest photosynthetic rates. 3. NE 58 and Eugenei had the lowest LAIs, biomass yields, and MAIs; had the most biomass partitioned into minor stems and least into branches; and had among the highest photosynthetic rates. 4. Drought stress had a major impact by reducing growth and biomass yield of all the clones. Leaf and branch abscission due to this stress was less severe on NE 48 than the other clones. Gas exchange was also reduced significantly by drought stress, although NE 48 seemed to show more preconditioning to drought than the other clones. 5. NE 48 appeared to be more drought tolerant than the other three clones. APPENDIX Table 21. Regression and prediction equation'3 parameters from leaf length versus leaf area regressions for four one-year-old hybrid poplar clones. Regression Clone Number Significance of F-ratio Coefficients 2 r value Intercept (b ) 0 Variable 1 (b ) 1 Siyni ficance of t value^ Variable 2 (b ) Signi ficanci of t value 2 NE 343+308 ** 0.97 -1.48 -0.0542 ** 2.65 ** NE 48 ** 0.98 -1.04 0.0594 ** 1.87 ** NE 58 ** 0.98 -1.72 -0.0291 * 2.64 ** Eugenei ** 0.99 -1.37 -0.00096 ns 2. 36 ** a Prediction Equation: LA = e (b + b (LL) t- b (Ln LL) + CF) b b (LL) 1 or LA = a(LL) 7e 1 Where: LA LL Ln e CF = = = = = a = Ln LA = b + b (LL) + b (LL) which is the transformed form of the equation, 0 1 b b (LL) LA = a(LL) 2e 1 ! 2 leaf area m- cm l e a f l e n g t h in cm natural logarithm exponential function o2 correction factor due to bias of transformation = /2 (b + CF) constant which equals e 0 b ns not significant at i = 0.05 level. * significant at u = 0.05 level. ** significant at n = 0.01 level. 133 Regression Equation: Table 22. Regression and prediction equation3 parameters from leaf length versus leaf area regressions for four two-year-old hybrid poplar clones. Coef ficients Regression Significance of F-ratio Clone Number , r 2value Intercept (b ) M Variable 1 (b ) l Significancg of t value Variable 2 (b ) Significance of t value 7 NE 343+308 ** 0.96 -0.978 0.028 ** 2.07 ** NE 48 ** 0.96 -1.121 0. 069 ** 1.87 ** NE 58 ** 0.96 -0.996 0.051 ** 1.95 ** Eugenei ** 0.97 -0.986 0.066 ** 1.88 ** a Prediction Equation: LA = e (b + b (I,L) + b (Ln LL) + CFl b b (LL) 0 1 2 or LA = a(LL) ?e 1 Where: LA LL Ln e CF = = = = = a = Ln LA = b + b (LL) + b (Ln LL) which is the transformed form of the equation, 0 1 r b b (LL) 2 LA = a(LL) 2e 1 leaf area in cm leaf length in cm natural logarithm exponential function o2 correction factor due to bias of transformation = /2 (b + CF) constant which equals e 0 b ** significant at a = 0.01 level. 134 Regression Equation: 135 O — O X < CL 10 20 25 30 PERCENT Figure 28. Water retention curves for two soil types at two depths as determined with undisturbed soil cores (low tension) and screened soil (high tension) in a ceramic plate pressure apparatus (Kalamazoo sandy loam, 0-30 cm A , 30-60 cm +, Hillsdale sandy loam, 0-30 cm □, 30-60 cm o). 136 < 2 Q. O 0 20 40 60 80 PERCENT Figure 29. Channels ratio versus counting efficiency calibration curve for l^C-labeled photosynthetic samples (Glass vials A , Glass vials □, Plastic vials o). 100 137 Figure 30. Map of study area showing block and plot arrangement and harvest schedule. 138 CLONE * N ■■■■■■■ ■ ■ ■ ■ ■ 2 -1 : NE 353+308 ■■■■■ m m :Cs NE 48 1-2 ■■■■■■■ ■ ■ ■ ■ NE 58 > o 1 5 3 ■ ■ ■ ■ ■ EUGENEI 52-15 ■ ■ 1977 1978 1979 R W W W W W n W W W W X f c i ' M ' * " * ' 1* 1'* " ■ ■ ■ ■ ■ ■ ■ 1-2 2-1 3 C * CUTTING G R O W T H : COPPICE GROWTH £■<><><>oo*3oo<>oo<>^ m* • • • Figure 30. 139 Figure 31. Map of plot showing row layout, border area, large biomass and LAI subplot, and small growth analysis subplots. 140 IN — l._ .j GROWTH ANALYSIS SUBPLOTS ! ■ ■ r--i i m i i ■ t ■ i■ i• i i L__ ■ i ■ :• i f--i i » ■ i L--- BIOMASS AND LA! SUBPLOT Figure 31. ■ • • m * i BIBLIOGRAPHY BIBLIOGRAPHY Ackley, W. B.; P. C. Crandall; and T. S. Russell. 1958. The use of linear measurements in estimating leaf areas. Proc. Am. Soc. Hort. Sci. 72:326-330. Anderson, H. W. 1979. Biomass production of hybrid poplar grown in mini-rotation. In: Poplar Research, Management, and Utilization in Canada. Proc. N.A. Poplar Council Annu. Meeting. Fayle, D. C. F.; L. Zsuffa; and H. W. Anderson. eds. Brockville, Ont., Sept. 6-7, 1977, Ontario Min. Nat. Resour., For. Res. Info. Pap. #102, pp. 11-1 to 11-13. Anderson, H. W. and L. Zsuffa. 1975. Yield and wood quality of hybrid cottonwood grown in two-year rotation. Ontario Min. Nat. Resour., For. Res. Rep. #101, 35 pp. Attiwell, P. M. and J. D. Ovington. 1968. forest biomass. For. Sci. 14:13-15. Determination of Aubuchon, R. R.; D. R. Thompson; and T. M. Hinckley. 1978. Environmental influences on photosynthesis within the crown of a white oak. Oecologia (Berl.) 35:295-306. Austin, R. B. and P. C. Longden. 1967. A rapid metho^.for the measurement of rates of photosynthesis using Ann. Bot. 31:245-253. Ayling, R. D. and P. J. Martins. 1981. The growing of eucalypts on short rotation in Brazil. For. Chron. 57:9-16. Baker, J. B. and B. G. Blackmon. 1977. Biomass and nutrient accumulation in a cottonwood plantation - the first growing season. Soil Sci. Soc. Am. J. 41:632-636. Barden, J. A. 1978. Apple leaves, their morphology and photosynthetic potential. Hort. Sci. 13:644-646. Baskerville, G. L. 1965. Estimation of dry weight oftree components and total standing crop in conifer stands. Ecology 46:867-869. 141 142 Baskerville, G. L. 1972. Use of logarithmic regression in the estimation of plant biomass. Can. J. For. Res. 2:49-53. Blake, T. J. 1981. Water use efficiency studies in poplar clones and hybrids. Fac. of For., Univ. Toronto, Toronto, Ontario, Canada. tl Bohlmann, D. 1971. [The physiology of abscission of branches of Quercus, Populus, and Tilia]. Allg. Forst-u. J.-Zt. 142:208-211. Bonner, F. T. 1967. Responses of 1-year-old cottonwood to increasing soil moisture tension. USDA-FS, Res. Note SO-56, 3 pp. Bowersox, T. W. and W. K. Murphey. 1975. Tree weight estimates for small-sized trees. Tappi 58(1):130-131. Bowersox, T. W. and W. W. Ward. 1976. Growth and yield of close-spaced, young hybrid poplars. For. Sci. 22:449-454. Bowersox, T. W. and W. W. Ward. 1977. Soil fertility, growth, and yield of young hybrid poplar plantations in central Pennsylvania. For. Sci. 23:463-469. Boynton, D. and R. W. Harris. 1950. Relationships between leaf dimensions, leaf area, and shoot length in the McIntosh apple, Elberta Peach, and Italian prune. Proc. Am. Soc. Hort. Sci. 55:16-20. Braun, H. J. 1974. [Growth reaction of poplar to optimum or reduced water supply]. Allg. Forst-u. J.-Ztg. 145:112-113. Bray, J. R. and L. A. Dudkiewicz. 1963. The composition, biomass, and productivity of two Populus forests. Bull. Torrey Bot. Club. 90:298-308. Bykov, 0. D. 1970. The method of calculation of isotopic effect in photosynthesis and its value. Photosynthetica 4:195-201. Cannell, M. G. R. 1980. Productivity of closelv-spaced young poplar on agricultural soils in Britain. Forestry 53:1-21. Cannell, M. G. R. and R. I. Smith. 1980. Yields of minirotation closely-spaced hardwoods in temperate regions: review and appraisal. For. Sci. 26:415-428. 143 Cannell, M. G. R. and S. C. Willett. 1976. Shoot growth phenology, dry matter distribution and root:shoot ratios of provenances of Populus trichocarpa, Picea sitchensis, and Pinus contorta growing in Scotland. Silvae Gen. 25:49-59. Carter, M. C. and E. H. White. 1971. The necessity for intensive cultural treatment in cottonwood plantations. Auburn Agric. Exp. Sta. Circ. #189, 11 pp. Catsky, J.; J. Janac; and P. G. Jarvis. 1971. General principles of using IRGA for measuring CC>2 exchange rates. In: Plant Photosynthetic Production. Manual of Methods. Sestak, Z.; J. Catsky; and P. G. Jarvis, eds. Dr. W. Junk, N.V. Publ., The Hague. pp. 161-166. Ceulemans, R. and I. Impens. 1979. Study of C02 exchange processes, resistances to carbon dioxide ana chlorophyll content during leaf ontogenesis in poplar. Biologia Plant. 21:302-306. Ceulemans, R. and I. Impens. 1980. Leaf gas exchange processes and related characteristics of seven poplar clones under laboratory conditions. Can. J. For. Res. 10:429-435. Ceulemans, R. and I. Impens. 1981. Effects of physical environmental parameters on photosynthesis and related gas exchange characteristics of cuttings of Populus clones under laboratory conditions, In: Photosynthesis VI. Photosynthesis and Productivity, Photosynthesis and Environment. Akovunoglou, G. ed. Balaban International Science Services, Philadelphia. pp. 203-207. Ceulemans, R. and I. Impens. 1982. ECOPASS - A multivariate model used as an index of growth performance of poplar clones. For. Sci. 28:862-867. Ceulemans, R. and I. Impens. 1983. Net CO- exchange rate and shoot growth of young poplar (Populus) clones. J. Exp. Bot. 34:866-870. Ceulemans, R.; I. Impens; F. Hebrant; and R. Moermans. 1980a. Evaluation of field productivity for several poplar clones based on their gas exchange variables determined under laboratory conditions. Photosynthetica 14:355-362. 144 Ceulemans, R.; I. Impens; R. Lemeur; R. Moermans; and Z. Samsuddin. 1978a. Water movement in the soil-poplar-atmosphere system. I. Comparative study of stomatal morphology and anatomy, and the influence of stomatal density and dimensions on the leaf diffusion characteristics in different poplar clones. Oecol. Plant. 13:1-12. Ceulemans, R.; I. Impens; R. Lemeur; R. Moermans; and Z. Samsuddin. 1978b. Water movement in the soil-poplar-atmosphere system. II. Comparative study of the transpiration regulation during water stress situations in four different poplar clones. Oecol. Plant. 13:139-146. Ceulemans, R.; I. Impens; and R. Moermans. 1980b. The response of C0? exchange rate to photosvnthetic photon flux density f5r several Populus clones under laboratory conditions. Photosynthesis Res. 1:137-142. Cooley, J. H. 1978. Survival and early growth of selected trees on waste water application sites. USDA-FS, Res. Note NC-231. 4 pp. Crist, J. B. 1983. Synopsis of utilization research on SRIC raw materials. In: Intensive Plantation Culture: 12 Years Research. Hanson, E. A. comp. USDA-FS, Gen. Tech. Rep. NC-91, 155 pp., pp. 115-118. Crist, J. B. and D. H. Dawson. 1975. Anatomy and dry weight yields of two Populus clones grown under intensive culture. USDA-FS, Res. Pap. NC-113. 6 pp. Dawson, D. H.; J. G. Isebrands; and J. C. Gordon. 1976. Growth, dry weight yields, and specific gravity of 3-year-old Populus grown under intensive culture. USDA-FS, Res. Pap. NC-122. 7 pp. Dawson, D. H.; J. Zavitkovski; and J. G. Isebrands. 1978. Managing forests for maximum biomass production. In: Proc. 2nd Annual Symp. On Fuels From Biomass. June 20-22, 1978, Troy, NY. pp. 151-167. Dawson, D. H.; J. Zavitkovski; and J. G. Isebrands. 1980. Managing forests for maximum biomass production. In: The Role of Chemical Engineering in Utilizing the National Forest Resources. Lightsey, G. R. ed. AICHE Symp. Series No 195, Vol. 76:36-42. DeBell, D. S. 1975. Short-rotation culture of hardwoods in the Pacific Northwest. Ia. St. J. Res. 49:345-352. DeBell, D. S.; A. P. Brunette; and D. L. Schweitzer. 1977. Expectations from intensive culture on industrial forest lands. J. For. 75:10-13. 145 Dickmann, D. I. 1971a. Photosynthesis and respiration by developing leaves of cottonwood (Populus deltoides Bartr.). Bot. Gaz. 132:253-259. Dickmann, D. I. 1971b. Chlorophyll, ribulose-1, 5-diphosphate carboxylase, and Hill reaction activity in developing leaves of Populus deltoides. Plant Physiol. 48:143-145. Dickmann, D. I. 1975. Plant materials appropriate for intensive culture of wood-fiber in the North Central region. Ia. St. J. Res. 49:281-286. Dickmann, D. I. 1979. Physiological determinants of poplar growth under intensive culture. In: Poplar Research Management, and Utilization in Canada. Proc. N .A. Poplar Council Annual Meeting. Fayle, D. C. F.; L. Zsuffa; and A. W. Anderson. eds. Brockville, Ont., Sept. 6-9, 1977, Ontario Min. Nat. Resour., For. Res. Info. Pap. #102. pp. 12-1 to 12-12. Dickmann, D. I.; D. H. Gjerstad; and J. C. Gordan. 1975. Developmental patterns of C02 exchange, diffusion resistance, and protein synthesis in leaves of Populus x euramericana. In: Environmental and Biological Control of Photosynthesis. Marcelle, R., ed. Dr. W. Junk, N. V. Publ.", The Hague. pp. 171-181. Dickm^n, D. I. and J. C. Gordon. 1975. Incorporation of C-photosynthate into protein during leaf development in young Populus plants. Plant Physiol. 56:23-27. Dickmann, D. I.; K. W. Gottschalk; and J. H. Bassmann. 1979. Physiological studies of growth and development of young hyubrid poplars. In: Proc. N .A . Poplar Council Annu. Meeting. Thompsonville, MI., Aug. 14-17, 1979. pp. 123-132. Dickmann, D. I. and K. W. Stuart. 1983. The culture of poplars in eastern North America. Mich. State Univ., East Lansing. 168 pp. Domingo, I. L. and J. C. Gordon. 1974. Physiological responses of an aspen-poplar hybrid to air temperature and soil moisture. Bot. Gaz. 135:184-192. Dutrow, G. F.; J. S. McNight; and S. Guttenberg. 1970. Investment guide for cottonwood planters. USDA-FS, Res. Pap. SO-59. 15 pp. Dutrow, G. F. and J. R. Saucier. 1976. Economics of short-rotation sycamore. USDA-FS, Res. Pap. SO-114. 16 pp. 146 Faulkner, H. and D. C. F. Fayle. 1979. Root development of three 5-year-old Euramerican poplar clones at two plantation sites. In: Poplar Research, Management, and Utilization in Canada. Proc. N .A. Poplar Council Annu. Meeting. Fayle, D. C. F.; L. Zsuffa; and H. W. Anderson, eds. Brockville, Ont., Sept. 6-9, 1977, Ontario Min. Nat. Res., For. Res. Info. Rep. #102. pp. 8-1 to 8-10. Fasehun, F. E. 1978. Effect of irradiance on growth and photosynthesis of Populus x euramericana clones. Can. J. For. Res. 8:94-99. Federer, C. A. 1977. Leaf resistance and xylem potential differ among broadleafed species. For. Sci. 23:411-419. Federer, C. A. 1980. Paper birch and white oak saplings differ in responses to drought. For. Sci. 26:313-324. Fordham, R. and M. E. Holgate. 1972. Estimation of leaf area of tea (Camellia sinensis L.) from linear measurements. J. Hortl Sci. 47:131-135. Furukawa, A. 1972. Photosynthesis and respiration in poplar plant. J. Jap. For. Sci. 54(3):80-84. Furukawa, A. 1973a. Carbon dioxide compensation points in poplar plant. J. Jap. For. Soc. 55:95-99. Furukawa, A. 1973b. Photosynthesis and respiration in poplar plant in relation to leaf development. J. Jap. For. Soc. 55:119-123. Ginter-Whitehouse, D. L,; T. M. Hinckley; and S. G. Pallardy. 1983. Spatial and temporal aspects of water relations of three tree species with different vascular anatomy. For. Sci. 29:317-329. Gordon, J. C. and L. C. Promnitz. 1976. A physiological approach to cottonwood yield improvement. In: Proc. Symp. on Eastern Cottonwood and Related Species. Thielges, B. A. and S. B. Land, Jr. eds. Greenville, MS. Sept. 28-Oct. 2, 1976, LSU. pp. 66-88. Gregory, R. A. 1980. Annual cycle of shoot development in sugar maple. Can. J. For. Res. 10:316-326. Harrington, C. S.; D. S. DeBell; and R. F. Strand. 1979. An experiment in biomass production: results from three consecutive harvests of cottonwood and alder. In: Proc. Solar79 Northwest, Seattle, WA. August 1979, pp. 363-366. 147 Heilman, P. and D. V. Peabody, Jr. 1981. Effect of harvest cycle and spacing on productivity of black cottonwood in intensive culture. Can. J. For. Res. 11:118-123. Heilman, P. E.; D. V. Peabody, Jr.; D. S. DeBell; and R. F. Strand. 1972. A test of close-spaced, short-rotation culture of black cottonwood. Can. J. For. Res. 2:456-459. Herrick, A. M. and C. L. Brown. 1967. A new concept in cellulose production - silage sycamore. Agric. Sci. Rev. 5:8-13. Hinckley, T. M.; R. G. Aslin; R. R. Aubuchon; C. L. Metcalf; and J. E. Roberts. 1978a. Leaf conductance and photosynthesis in four species of the oak-hickory forest type. For. Sci. 24:73-84. Hinckley, T. M.; P. M. Dougherty; J. P. Lassoie; J. E. Roberts; and R. 0. Teskey. 1979. A severe drought: Impact on tree growth, phenology, net photosynthetic rate, and water relations. The Am. Midi. Nat. 102:307-316. Hinckley, T. M.; J. P. Lassoie; and S. W. Running. 1978b. Temporal and spatial variations in the water status of forest trees. For. Sci. Monogr. #20, 72 pp. Incoll, L. D. 1977. ^|ield studies of Photosynthesis: monitoring with C0_. In: Environmental Effects on Crop Physiology. Landsberg, J. J. and C. V. Cutting, eds. Academic Press, London. Sect. II, Chapter 2, pp. 137-155, Discussion pp. 191-192. Iowa State University. 1975. Conference on intensive culture of forest crops. Des Moines, IA. March 5-6, 1974. Ia. St. J. Res. 49:263-352. Iowa State University. 1976. Conference on intensive culture of forest crops. Des Moines, IA. March 4-5, 1975. Ia. St. J. Res. 50:267-315. Isebrands, J. G. 1983. Toward a physiological basis of intenstive culture of poplar. In: Proc. 1982 TAPPI Res. and Develop. Div. Conf. Asheville, NC, Aug. 29-Sept. 1, 1982. pp. 81-90. Isebrands, J. G. and N. D. Nelson. 1978. Distribution of number of leaves and leaf area in short rotation intensive cultured Populus 'Tristis', (Abstr.) In: Proc. Fifth N.A. For. Biol. Workshop. Hollis, C. A. and A. E. Squillace. eds. Gainesville, Fla. March 13-15, 1978. p. 426. 148 Isebrands, J. G. and N. D. Nelson. 1982. Crown architecture of short-rotation, intensively cultured Populus II. Branch morphology and distribution of leaves within the crown of Populus 'Tristis' as related to biomass production. Can. J. For. Res. 12:853-864. Isebrands, J. G.; L. C. Promnitz; and D. H. Dawson. 1977. Leaf area development in short rotation intensive cultured Populus plots. In: TAPPI Conf. Papers/Forest Biology-Wood Chemistrv Conf. Madison, WI. June 20-22, 1977. pp. 201-210. Jarvis, P. G. 1976. The interpretation of the variations in leaf water potential and stomatal conductance found in canopies in the field. Phil. Trans. R. Soc. Lond. Ser. B. 273:593-610. Jarvis, P. G. and M. S. Jarvis. 1964. Growth rates of woody plants. Physiol. Plant. 17:654-666. Johnson, R. L. and E. C. Burkhardt. 1976. Natural cottonwood stands— past management and implications for plantations. In: Proc. Symp. on Eastern Cottonwood and Related Species. Thielges, B.A. and S. B. Land, Jr. eds. Greenville, MS. Sept. 28-Oct. 2, 1976. LSU. pp. 20-30. Kallis, A. and H. Tooming. 1974. Estimation of the influence of leaf photosynthetic parameters, specific leaf weight, and growth functions on yield. Photosynthetica 8:91-103. Kanemasu, E. T.; G. W. Thurtell; and C. B. Tanner. 1969. Design, calibration, and field use of a stomatal diffusion porometer. Plant Physiol, 44:881-885. Karlsson, S. and B. Sveinbjornsson. 1981. Methodological comparison of photosynthetic rates measured by the C0„ technique or infra-red gas analysis. Photosynthetica 15:447-452. Kaufmann, M. R. 1982. Leaf conductance during the final season of a senescing aspen branch. Plant Phvsiol. 70:655-657. Kelliher, F. M.; M. B. Kirkham; and C. G. Tauer. 1980. Stomatal resistance, transpiration, and growth of drought-stressed eastern cottonwood. Can. J. For. Res. 10:447-451. Kelliher, F. M. and C. G. Tauer. 1980. Stomatal resistance and growth of drought-stressed eastern cottonwood from a wet and dry site. Silvae Gen. 29:166-171. 149 Koster, R. 1976. Observations on Populus deltoides provinances grown in Holland. In: Proc. Symp. on Eastern Cottonwood and Related Species. Thielges, B. A. and S. B. Land, Jr. eds. Greenville, MS. Sept. 28-Oct. 2, 1976. LSU. pp. 126-133. Kozlowski, T. T. and J. J. Clausen. 1966. Shoot growth characteristics of heterophyllous woody plants. Can. J. Bot. 44:827-843. Kubicek, F. 1971. Leaf area estimation in Quercus petraea using linear regression equations. Photosynthetica 5:426-428. Larcher, W. 1969. The effect of environmental and physiological variables on the carbon dioxide gas exchange of trees. Photosynthetica 3:167-198. Larson, P. R.; R. E. Dickson; and J. G. Isebrands. 1976. Some physiological applications for intensive culture. In: Intensive Plantation Culture: Five Years Research. USDA-FS, Gen. Tech. Rep. NC-21. 117 pp. pp. 10-18. Larson, P. R. and J. C. Gordon. 1969a. Photosynthesis and wood yield. Agric. Sci. Rev. 7:7-14. Larson, P. R. and J. C. Gojc^on. 1969b. Leaf development, photosynthesis, and C distribution in Populus deltoides seedlings, Am. J. Bot. 56:1058-1066. Larson, P. R. and J. G. Isebrands. 1971. The plastochron index as applied to developmental studies of cottonwood. Can. J. For. Res. 1:1-11. Larson, P. R. and J. G. Isebrnds. 1972. The relation between leaf production and wood weight in first-year root sprouts of two Populus clones. Can. J. For. Res. 2:98-104. Larson, P. R.; J. G. Isebrands; and R. E. Dickson. 1972. Fixation patterns of C within developing leaves of eastern cottonwood. Planta 107:301-314. Ledig, F. T. 1974. Concepts of growth analysis. In: Proc. 3rd N.A. For. Biol. Workshop. Reid, C. P. P. and G. H. Fechner. eds. Fort Collins, CO. Sept. 9-12, 1974. Colorado State Univ. pp. 166-182. Lemeur, R. J. P. and I. I. Impens. 1981. Stand architecture, biomass production, energy flow, and production efficiency in a man-made poplar ecosystem. In: Proc. XVII IUFRO World Congress. Kyoto, Japan. Sept. 6-17, 1981. pp. 205-225. 150 Liphschitz, N. and Y. Waisel. 1970a. The effect of water stresses on radial growth of Populus euphratica Oliv. La-Yaaran 20:53-61. English summary 80-84. Liphschitz, N. and Y. Waisel. 1970b. Effects of environment on relations between extension and cambial growth of Populus euphratica Oliv. New Phytol. 69:1059-1064. Loach, K. 1970. Shade tolerance in tree seedlings II. Growth analysis of plants raised under artificial shade. New Phytol. 69:273-286. Luukkanen, O. and T. T. Kozlowski. 1972. Gas exchange in six Populus clones. Silvae Gen. 21:220-229. Madgwick, H. A. and D. F. Olson. 1974. Leaf area index and volume growth in thinned stands of Liriodendron tulipifera. J. Appl. Ecol. 11:575-579. Maini, J. S. 1966a. Apical growth of Populus spp. I. Sequential pattern of internode, bud, and branch length of young individuals. Can. J. Bot. 44:615-622. Maini, J. S. 1966b. Apical growth of Populus spp. II. Relative growth potential of apical and lateral buds. Can. J. Bot. 44:1581-1590. Marshall, J. K. 1968. Methods for leaf area measurement of large and small leaf samples. Photosynthetica 2:41-47. McAlpine, R. C.; C. L. Brown; A. M. Herrick; and H. E. Ruark. 1966. "Silage" svcamore. For. Farmer 26 (1) :6-7, 16. McGee, A. B.; M. R. Schmierback; and F. A. Bazzaz. 1981. Photosynthesis and growth in populations of Populus deltoides from contrasting habitats. Am. Midi. Nat. 105:305-311. McWilliams, J. R.; P. J. Phillips; and R. R. Parkes. 1973. Measurement of photosynthetic rate using labelled carbon dioxide. CSIRO, Aust. Div. PI. Ind. Tech. Pap. #31. 12 pp. Merritt, C. and W. C. Bramble. 1966. Poplar plantation research in Indiana - a five-year report. Purdue Univ., Agric. Exp. Sta. Res. Bull. #818. 15 pp. Michael, D. A. 1984. Growth and photosynthesis of two field-grown Populus clones during the establishment year. Michigan State University, PHD Dissertation, 187 pp. 151 Morrow, P. A. and R. 0. Slatyer. 1971. Leaf resistance measurements with diffusion porometers: precautions in calibration and use. Agric. Meteorol. 8:223-233. Naylor, D. G. and I. D. Teare. 1975. An improve^ rapid, field method to measure photosynthesis with C0-. Agron. J. 67:404-406. Nelson, N. D.; T. Burk; and J. G. Isebrands. 1981. Crown architecture of short-rotation intensively cultured Populus I. Effects of clone and spacing on first-order branch characteristics. Can. J. For. Res. 11:73-81. Nelson, N. D.; T. Burk; J. G. Isebrands; and A. R. Ek. 1980. Clone and spacing effects on branch characteristics of short-rotation intensively-cultured Populus. In: Proc. 1st. N.C. Tree Improvement Conf. Madison, WI. Aug. 21-23, 1979. pp. 74-85. Nelson, N. D. and D. A. Michael. 1982. Photosynthesis, leaf conductance, and specific leaf weight in long and short shoots of Populus 'Tristis #1' grown under intensive culture. For. Sci. 28:737-744. Neuwirth, G. and H. Polster. 1960. [Water consumption and dry matter production in Populus nigra and aspen under the strain of drought]. Arch. Forstw. 9:789-810. Pallardy, S. G. and T. T. Kozlowski. 1979a. Stomatal response of Populus clones to light intensity and vapor pressure deficit. Plant Physiol. 64:112-114. Pallardy, S. G. and T. T. Kozlowski. 1979b. Relationships of leaf diffusion resistance of Populus clones to leaf water potential and environment. Oecologia 40:371-380. Parker, W. C.; S. G. Pallardy; T. M. Hinkley; and R. O. Teskey. 1982. Seasonal changes in tissue water relations of three woody species of the Quercus-Carya forest type. Ecology 63:1259-1267. Pauley, S. S. and T. 0. Perry. 1954. Ecotypic variation of the photoperiodic response in Populus. J. Arnold Arbor. 35:167-188. Perala, D. A. 1979. Regeneration and productivity of aspen grown on repeated short rotations. USDA-FS, Res. Pap. NC-176. 7 pp. Person, R. A.; A. R. Hallgren; and J. W. Hubbard. 1971. Yields from short-rotation aspen suckers. Minn. For. Res. Notes #224. 4 pp. 152 Peterson, E. B.; Y. H. Chan; and J. B. Cragg. 1970. Aboveground standing crop, leaf area, and caloric value in an aspen clone near Calgary, Alberta. Can. J. Bot. 48:1459-1469. Pieters, G. A. 1960. On the relation between the maximum rate of photosynthesis and the thickness of the mesophyll in sun and shade leaves of Acer psuedoplatanus L. Meded. Landbouwhogeschool, Wageningen 60:1-6. Pieters, G. A. 1974. The growth of sun and shade leaves of Populus x euramericana 'Robusta' in relation to age, light intensity, and temperature. Meded. Landbouwhogeschool, Wageningen #74-11. 106 pp. Pieters, G. A. and M. Zima. 1975. desiccating leaves of poplar. 34:56-61. Photosynthesis of Physiol. Plant. Pollard, D. F. W. 1970a. Leaf area development on different shoot types in a young aspen stand and its effect upon production. Can. J. Bot. 48:1801-1804. Pollard, D. F. W. 1970b. The effect of rapidly changing light on the rate of photosynthesis in largetooth aspen (Populus grandidentata). Can. J. Bot. 48:823-829. Pollard, D. F. W. 1971. Mortality and annual changes in distribution of above-ground biomass in an aspen sucker stand. Can. J. For. Res. 1:262-266. Pollard, D. F. W. 1972a. Above-ground dry matter production in three stands of trembling aspen. For. Res. 2:27-33. Can. J. Pospisilova, J. and J. Solarova. 1980. Environmental and biological control of diffusive conductances of adaxial and abaxial leaf epidermes. Photosynthetica 14:90-127. Rawitz, E.; R. Karschon; and K. Mitrani. 1966. Growth and consumptive water use of two poplar clones under different irrigation regimes. Israel J. Agric. Res. 16:77-88. Regehr, D. L.; F. A. Bazzaz; and W. R. Boggess. 1975. Photosynthesis, transpiration, and leaf conductance of Populus deltoides in relation to flooding and drought. Photosynthetica 9:52-61. Richards, J. H. and P. R. Larson. 1981. Morphology and development of Populus deltoides branches in different environments. Bot. Gaz. 142:382-393. 153 Rose, D.; K. Ferguson; D. C. Lothner; and J. Zavitkovski. 1981. An economic and energy analysis of poplar intensive cultures in the Lake States. USDA-FS, Res. Pap. NC-196. 44 pp. Saucier, J. R.; A. Clark III; and R. G. McAlpine. 1972. Above ground biomass yields of short-rotation sycamore. Wood Sci. 5:1-6. Schier, G. A. 1979. Shoot development in young aspen. USDA-FS, Res. Note INT-275. 4 pp. Schlaegel, B. E. 1975. Yields of four 40-year-old conifers and aspen in adjacent stands. Can. J. For. Res. 5:278-280. Schmidt, P. 1975. [Growth and water consumption of three Poplar clones]. Allg. Forst-u. J.-Ztg. 146:38-42. Schreiner, E. J. 1970. Mini-rotation forestry. Res. Pap. NE-174. 32 pp. USDA-FS, Shimshi, D. 1969. A rapid field method for measuring photosynthesis with labelled carbon dioxide. J. Exp. Bot. 20:381-401. Sivakumaran, S. and M. A. Hall. 1979. Hormones in relation to stress recovery in Populus robusta cuttings. J. Exp. Bot. 30:53-63. Siwecki, R. and T. T. Kozlowski. 1973. Leaf anatomy and water relations of excised leaves of six Populus clones. Arboretum Kornickie 18:83-106. Smith, D. W. and G. E. Gatherum. 1974. Effects of moisture and clone on photosynthesis and growth of an aspen-poplar hybrid. Bot. Gaz. 135:293-296. Steinbeck, K.; R. G. McAlpine; and J. T. May. 1972. rotation culture of sycamore: a status report. For. 70:210-213. Short J. Strebeyko, P. 1967. Rapid method for measuring ohotosynthetic rate using C0~. Photosynthetica 1:45-49. Sucoff, E. and D. Hiesey. 1978. Preliminary evalution of leaf water potential for irrigating hybrid poplar. In: Proc. 5th N .A. For. Biol. Workshop. Hollis , C. A. and A. E. Squillace. eds. Gainesville, Fla. March 13-15, 1978. pp. 187-195. 154 Switzer, G. L.; L. E. Nelson; and J. B. Baker. 1976. Accumulation and distribution of dry matter and nutrients in Aiqeiros poplar plantations. In: Proc. Symp. on Eastern Cottonwood and Related Species. Thielges, B. A. and S. B. Lund, Jr. eds. Greenville, MS. Sept. 28-Oct. 2, 1976. LSU. pp. 359-369. Tazaki, T. and T. Ushijima. 1963. Studies on the dehydration resistance of higher plants. IV. Dehydration resistance of the leaves in young poplar plants with special reference to the water economy of other plant parts. Bot. Mag. Tokyo 76:237-245. Tobiessen, P. and T. M. Kana. 1974. Drought-stress avoidance in three pioneer tree species. Ecology 55:667-670. Tsel'niker, Y. L. and V. V. Mai. 1979. Growth processes and photosynthetic activity in the leaf of aspen (Populus tremula L.). Sov. Plant Physiol. 26: 859-865. U.S. Department of Agriculture-Forest Service. 1976. Intensive Planations Culture: Five years research. USDA-FS, Gen. Tech. Rep. NC-21. 117 pp. U.S. Department of Agriculture-Forest Service. 19 80. Energy and wood from intensively cultured plantations: research and development program. USDA-FS, Gen. Tech. Rep. 28 pp. U.S. Department of Agriculture-Forest Service. 1983. Intensive plantation culture: 12 years research. USDA-FS, Gen. Tech. Rep. NC-91. 155 pp. Van Norman, R. W. and A. H. Brown. 1952. The relative rates of photosynthetic assimilation of isotopic forms of carbon dioxide. Plant Physiol. 27:691-709. Voznesenskii, V. L.; 0. V. Zalenskii; and R. B. Austin. 1971. Methods of measuring rates of photosynthesis using carbon-14 dioxide. In: Plant Photosynthetic Production: Manual of Methods. Sestak, Z.; J. Catsky; and P. G. Jarvis. eds. Dr. W. Junk, N. V. Publ., The Hague. 818 pp. Chapter 7, pp. 276-293. Watson, D. J. 1947. Comparative physiological studies on the growth of field crops. I. Variation in net assimilation rate and leaf area between species and varieties, and within and between years. Ann. Bot. N. S. 11:41-76. Watson, D. J. 1952. The physiological basis of variation in yield. Adv. Agron. 4:101-145. 155 Watson, D. J. 1956. Leaf growth in relation to crop yield. Ini The Growth of Leaves, Milthorpe, F. L. ed. Butterworths Sci. Public., London. Chapter 14. pp. 178-191. Watson, D. J. 1958. The dependence of net assimilation rate on leaf-area index. Ann. Bot. N. S. 22:37-54. Watts, W. R. 1977. Field studies of stomatal conductance. In: Environmental Effects on Crop Physiology. Landsberg, J. J. and C. V. Cutting. eds. Academic Press, London. Section II. Chapter 4. pp. 173-189, Discussion, pp. 194-196. Weaver, T. 1978. Auxin regulation of branch abscission by aspen (Populus tremuloides, Michx.). Northwest Sci. 52:168-170. Webb, R. A. 1972. Use of the boundary line in the analysis of biological data. J. Hort. Sci. 47:309-319. Wilhelm, W. W. and C. J. Nelson. 1978. Growth analysis of tail fescue genotypes differing in yield and leaf photosynthesis. Crop Sci. 18:951-954. Wilson, B. F. 1970. The growing tree. Press. Amherst. 152 pp. The Univ. Mass. Wray, P H. and L. C. Promnitz. 1976. Controlled-environment selection of Populus clones. In: Intensive Planation Culture: five vears research. USDA-FS, Gen. Tech. Rep. NC-21. 117 pp. pp. 19-24. Yemm, E. W. and R. G. S. Bidwell. 1969. Carbon dioxj^e exchanges in leaves. I. Discrimination between CC^ and CO_ in photosynthesis. Plant Phvsiol. 44: 1328-1334 . Ying, C. C. 1974. Genetic variation of eastern cottonwood (Populus deltoides Bartr.) . Neb. Agric. Exp. Sta. Prog. Rep. #1. 148 pp. Zakhariev, B.; S. Iliev; and T. Mitev. 1975. [Growth and productivity of some hybrid black poplar clones in Bulgaria on sites close to rivers and on dry sites away from rivers]. Gorskostopavska Nauka 12:16-22. Zavitkovski, J. 1971. Dry weight and leaf area of aspen trees in northern Wisconsin. In: Forest Biomass Studies. Proc. XVI IUFRO Congress. Univ. Florida, Gainesville, Fla. March 15-20, 1971. pp. 191-205. 156 Zavitkovski, J. 1976. Biomass studies in intensively managed forest stands. In: Intensive Plantation Culture: five years research. USDA-FS, Gen. Tech. Rep. NC-21. 117 pp. pp. 32-38. Zavitkovski, J. 1981a. Small plots with unplanted plot border can distort data in biomass production studies. Can. J. For. Res. 11:9-12. Zavitkovski, J. 1981b. Structure and seasonal distribution of litterfall in young plantations of Populus 'Tristis #1'. Plant and Soil 60:409-422. Zavitkovski, J. 1982. Characterization of light climate under canopies of intensively-cultured hybrid poplar plantations. Agric. Meteorol. 25:245-255. Zavitkovski, J.; J. G. Isebrands; and T. R. Crow. 1974. Application of growth analysis in forest biomass studies. In: Proc. 3rd N .A. For. Biol. Workshop. Reid, C. P. P. and G. H. Fechner. eds. Colorado State Univ., Ft. Collins, Co. Sept. 9-12, 1974. pp. 196-226. Zavitkovski, J.; J. G. Isebrands; and D. H. Dawson. 1976. Productivity and utilization potential of short-rotation Populus in the Lake States. In: Proc. Symp. on Cottonwood and Related Species. Thielges, B. A. and S. B. Lund, Jr. eds. Greenville, MS. Sept. 28-Oct. 2, 1976. LSU. pp. 392-401. Zsuffa, L.; H. W. Anderson; and P. Jaciw. 1977. Trends and prospects in Ontario's poplar plantation management. For. Chron. 53:195-200.