t-t .-- golllii’ii LIBRARY Michigan State University This is to certify that the thesis entitled Wind-related Control of Tree Form and Density on Lake Superior Ridge Crests presented by Daniel Curtis Nepstad has been accepted towards fulfillment of the requirements for M.S. degree inPlant Ecoiogy @fit out Major professor i Dateii‘7 [it/‘0“? iéigé 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution ‘IVIESij RETURNING MATERIALS: Place in book drop to LIBRARIES remove this checkout from .—_. your record. FINES will be charged if book is returned after the date stamped below. WIND-RELATED CONTROL OF TREE FORM AND DENSITY ON LAKE SUPERIOR RIDGE CRESTS By Daniel Curtis Nepstad A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTEROFSC'IENCE Department of Botany and Plant Pathology 1988 m;- 692/ ABSTRACT WIND-RELATED CONTROL OF TREE FORM AND DENSITY ON LAKE SUPERIOR RIDGE CRESTS By Daniel Curtis Nepstad Certain ridge tops along the southern shore of Lake Superior support shrub- and herb-dominated plant communities with clumps of short, deformed trees. I evaluated environmental and vegetational gradients, tree growth and tree population structure of this vegetation type on Mount Lookout, Keweenaw Peninsula, Michigan, with the objective of describing the community and identifying the factors controlling tree form and density. The community was characterized by gradients of wind speed and snow depth that corresponded to gradients of plant species composition and physiognomy. The short, multiple- stemmed growth form of Quercus rubra individuals was best explained as a consequence of reduced extension growth, tissue damage, and non-vertical shoot orientation. Size class analysis revealed that low density of Quercus rubra was a result of poor seedling recruitment. Both stunted tree form and low tree density were probably the result of frequent, nighttime. high speed wind episodes to which the site was exposed. To my parents. Don and Laurie Nepstad. iii ACKNOWIEJGEMENI'S Many peOple helped bring this project to a conclusion. Dan Duncan, Peg Kohring, Helen Kuhn, Steve Nepstad. John Plough, Mary Whittaker and Jeff Yoest assisted with field work. Dr. Bill Rose suggested the topic. Dr. Jay Harman provided critical orientation in the conceptual organization of the project. made important observations during his visit to the field, and loaned me his wind recorder. Dr. Peter Murphy provided valuable advice in the early stages of the project, supplied me with field equipment and office space, and carefully edited the early drafts of the thesis. Dr. John Beaman made many helpful suggestions. John Kassenene, Gerry Donnelly, Vicki Dunevitz, Jim Kelley, Bill Larsen and Mike Scott provided comradery and levity. My wife, Andrea Moore, was extremely supportive and patient during the prolonged write-up. To all of these individuals. I am grateful. iv TABLE OF CONTENTS LIST OFTABLES .............................................................................................................. vii LIST OF FIGURES ............................................................................................................ viii CHAPTER 1. INTRODUCTION ......................................................................................... 1 A The Problem ................................................................................................. 1 B. The Study ....................................................................................................... 1 C. Distribution of Balds .................................................................................... 2 D Causes of tree deformation and low tree density ................................... 4 Tree Deformation in Cliff—Summit Balds .......................................... 4 Tree Deformation in Other Plant Communities ............................... 5 Low Tree Density in Other Plant Communities ................................ 12 CHAPTER 2. METHODS ................................................................................................... 14 A. The Study Site ............................................................................................... 14 B. Environmental Conditions On Mount Lookout ....................................... 18 C. Species Composition .................................................................................... 19 D. Physiognomy ............................................................................................... 20 E Tree Growth .................................................................................................. 21 F Winter Water Relations .............................................................................. 22 G. Tissue Damage .............................................................................................. 22 H Population Structure of Quercus rubra .................................................. 22 CHAPTER 3. RESULTS .................................................................................................... 24 A. Environmental Conditions on Mount Lookout ....................................... 24 Soil ........................................................................................................... 24 Snow Depth ............................................................................................ 24 The Wind Regime .................................................................................. 26 Air Temperature ................................................................................... 26 B. Species Composition .................................................................................... 28 C. Physiognomy ................................................................................................ 35 D. Tree Growth....- .............................................................................................. 35 E. Winter Water Relations .............................................................................. 38 G. Tree Reproduction ....................................................................................... 42 H. Population Structure of Quercus rubra .................................................. 42 V CHAPTER 4. DISCUSSION .............................................................................................. 4 8 A. Environmental Conditions ........................................................................ 48 B. Species Composition .................................................................................... 51 C. Physiognomy ................................................................................................ 53 D. Tree Growth .................................................................................................. 53 E. Tissue Damage ............................................................................................... 55 F. Tree Density .................................................................................................. 56 CHAPTER 5. CONCLUSION ............................................................................................. 5 8 LIST OF REFERENCES ...................................................................................................... 60 vi LIST OF TABLES Table 1. Percent cover of vascular plants on Mount Lookout. Table 2. Vascular plant species on Mount Lookout with percent cover exceeding five percent. Table 3. Shoot length and lengthzwidth ratios for Quercus rubra in four microhabitats. Table 4. Shoot water potentials of Abies balsamea and Picea glauca on 22 April 1982. vii . Figure Figure Figure 1. 2. 3. LIST OF FIGURES Location of Mount Lookout in the western Great Lakes Region. Profile of Mount Lookout, a bald Lake Superior ridge. Environmental profiles along the slope of Mount Lookout. Figure 4. A comparison of wind speed on Mount Lookout and at Houghton International Airport. Figure Figure Figure Figure Figure Figure Figure Figure Figure 5. 6. 7. 8. 9. 10. 11. 12. 13. Plant cover on Mount Lookout. Vegetational profiles along the slope of Mount Lookout. Profiles of a vegetation clump at the summit of Mount Lookout. Stem diameter versus age of Quercus rubra. Height versus age of Quercus rubra. Stem height class distribution of Quercus rubra. Stem diameter class distribution of Quercus rubra. Seedling density of Quercus rubra in different microhabitats. Microhabitat composition along the slope of Mount Lookout. viii CHAPTER 1. INTRODUCTION A. The Problem Certain ridge tops along the southern shore of Lake Superior support shrub- and herb-dominated plant communities with clump of short (2 to 3 m), multiple-stemmed Quercus rubra L. var. borealis (Michx. f.) Farw. and prostrate or flagged Abies balsamea Mill" Picea glauca (Moench) Voss, and Thuja occidentalis L. These ridge crests are lower in elevation than neighboring forested hilltops, 1500 m lower than theoretical treeline for this area (Daubenmire 1954). and bear no evidence of fire. The unforested ridge tops, referred to here as "balds," present an opportunity to study the mechanisms by which local environmental factors suppress forest deveIOpment. Such analyses have been conducted for some high elevation alpine communities (review by Bliss 1985), neotropical elfin forests (Lawton 1981) and coastal salt-spray vegetation (Barbour 1978, Boyce 1954). No studies have been conducted of the factors limiting forest development on Lake Superior ridge tops. B. The Study I conducted a study of one unforested Lake Superior ridge top with the objective of: 1) describing the environmental conditions, 2) describing the vegetation, and 3) identifying the most probable controls of tree form and density on the ridge crest. I broke the third objective down into three questions: 1) Is tree form restricted on the ridge top by low rates of shoot extension? 2) Is tree density limited by seedling availability? 3) What environmental feature of the ridgetop limits tree form and density? The first of these questions was addressed through analysis of tree shoot growth rates and age-height relationships. I addressed the second question through analysis of tree population structure. I employed a correlative approach to address the third question, comparing environmental and vegetational gradients on the ridge top. 2 Since the interpretive portion of this thesis (objective 3) draws on the descriptive portions (objectives 1 and 2). I have combined all studies into Methods, Results and Discussion chapters. I preface these sections with a discussion of the distribution of unforested Lake Superior ridge tops and a review of the literature pertaining to controls of tree form and density. C. Distribution of Balds Ridge top communities are distinctive because they lack erect trees and yet are surrounded by forest. I therefore refer to these communities as "balds" for this term has historically been used to describe non-forest plant communities that occur where forest is expected (Whittaker 1956). An attempt was made to identify plant communities physiognomically similar to Lake Superior balds occurring in Michigan and eastern North America in general. Several approaches were taken to this problem including: 1) correspondence with ecologists and botanists of different regions. 2) survey of infra-red aerial photographs from selected areas of the western Upper Peninsula of Michigan, 3) ground reconnaissance in the Huron Mountains and other areas of dramatic topographic relief in northern Michigan, and 4) a literature search. Communities with small- statured, prostrate trees showing asymmetrical and otherwise deformed growth habits are restricted, in Michigan, to the unnamed escarpment in the Porcupine Mountains and several ridges in the Keweenaw Peninsula. including Mount Lookout, two unnamed bluffs and Mount Brockway. Several rock outcrops on the upper slopes of the Huron Mountains had small-sutured trees but ,are not considered balds because of the lack of tree deformation. The vegetation along the summit of Mount Brockway has been drastically altered through construction of a scenic drive and tourist shop. Bald communities on the unnamed Keweenaw ridges and much of the escarpment in the Porcupine Mountains have sawn or charred tree stumps. Only Mount Lookout and two sections of the escarpment bore no evidence of alteration by humans or fire. Of these, Mount Lookout has the largest area (ca. two ha) and the greatest width (extending 70 to 80 m from the cliff summit). It is of particular interest to note that all of these communities occupy the summits of cuesta-form ridges with precipitous southwest- to southeast- facing sIOpes and gradual grades in the opposite directions. In fact, all of the I 3 sites have the same geological history. originating as abruptly truncated edges of Precambrian strata that underlay the Lake Superior Syncline (Dorr and Eschman 1970). For this reason, I refer to these communities as ”cliff-summit balds.” The topographical differences between the sites are subtle but may have important ecological consequences. While Mount Lookout rises steeply to the ridge summit then falls abruptly to the north. the other sites are somewhat rounded on the south-facing sides of the crest. As a result, the bald on Mount Lookout has a northerly aspect while the others are south-facing. The summit of Mount Brockway has a configuration similar to that on Mount Lookout, but it has been severely altered by human activity. Adams (1905) was the first to note that the balds of the Porcupine Mountains, at 450-460 m elevation, are lower than the forested summits of nearby mountains. The same holds for the Keweenaw Peninsula balds which range from 388 to 407 m above sea level. No reports were found for cliff-summit balds outside of Michigan. Given and SOper (1981) do not mention such communities in their description of vegetation with arctic-alpine elements along the Lake Superior shoreline except those on the Keweenaw Peninsula. Similarly, field ecologists from New England (T. Rawland, New England Natural Heritage Program; T. Siccama, Director of Field Studies, Yale School of Forestry and Environmental Studies) and the southern Appalachians (S. Bratton and P. S. White, Ecologists, Great Smokies National Park Research Station. United States Forest Service) knew of no inland communities of this sort below altitudinal timberline in their respective regions. The grass and heath balds of the southern Appalachians (Whittaker 1956) differ from cliff-summit balds in that they do not support prostrate. deformed trees. At least two plant community types of the east coast resemble cliff- summit balds physiognomically though they are not topographically distinct. The New Jersey pine barrens are dominated by small-statured, multiple- stemmed Pinus spp and Quercus spp (McCormick and Buell 1968). Shoreline communities, such as those which occur on Cape Cod, are frequently characterized by short. multiple-stemmed. strongly asymmetrical trees (Boyce 1954, Art {441. 1974). These systems remain physiognomically suppressed by steep vertical stress gradients; tree growth is restricted to wind-sheltered microsites (Boyce 1954). 4 D. Causes of tree deformation and low tree density I DE . . CHI-S . Ell Several explanations have been offered for the unusual tree forms of cliff-summit balds. Wells and Thompson (1974) suggested that the short stature of plants on Mount Lookout is a consequence of exposure to "the full force of strong, sweeping winds which are often colder than those of surrounding locations and normally contain moisture collected from the passage over lake waters”. Similarly. Glime et al. (1980) invoke ”continuous winds which carry moist air from Lake Superior and provide a noticeably colder habitat” as the likely control of plant stature on the site. Neither author provides data to support their hypotheses, nor propose a mechanism by which cold, moist winds might limit plant growth. Richards (1954) explains the occurrence of cliff-summit balds as a consequence of soil erosion by rain, wind and frost, though he does not suggest a cause of tree defamation. ‘ In his classical Cowlesian analysis of balds in the Porcupine Mountains, Adams (1905) cites soil erosion through wind and rain as the factor which limits the "succession of plant societies” from reaching forest at the cliff summit. He states that the ”shrubby mat" growth forms of Quercus rubra are caused by ”breaking off of their tops by wind”. The tops of many pines are dead, he suggests. because of wind exposure. Darlington (1930) claims that the forest does not cover the Porcupine Mountain escarpment. because of lack of soil due to erosion from rain run-off and because tree encroachment is limited from the south by the escarpment face. He explains tree asymmetry on the ridge as a consequence of prevailing southwesterly winds. Soil depth, wind and prevention of forest encroachment by the cliff are the proposed limiting factors to tree establishment on cliff-summit balds. Wind and heat deficiency are the suggested agents of tree defamation. A more in-depth consideration of the mechanisms by which tree growth form and density can be limited is needed to determine the plausibility of these and other controls of community structure on Mount Lookout. 5 I DIE °°Ql El E2 .. Much work has been done to elucidate the factors limiting arborescent growth forms at high elevations. Some of these factors are operative only at very high altitudes and would not apply to Mount Lookout. For instance, carbon dioxide deficiency (Decker 1947) and excessive light (Collaer 1941), in addition to being of dubious importance at timberline, are obviously not possible causes of tree stunting on Mount Lookout given the low elevation of this site and considering the fact that many ridges and peaks close to it are higher but have forested summits. In his review paper, Daubenmire (1954) concludes that heat deficiency is the ultimate control of timberline, though wind and snowdrift may lower its position locally. This concept is based on the altitudinal temperature lapse rate [which is roughly 0.5° C per 100 m (Grace 1977)] and rests on the notion that arborescent growth forms are limited at elevations at which the growing season is just long enough for trees to assimilate enough carbon each year to replace that lost in respiration. Since the heat balance is more favorable close to the ground. low growth forms are able to grow at higher elevations than trees. Wardle (1974) has presented evidence in support of an earlier hypothesis (Michaelis 1932) which proposed that heat deficiency determines the upper limit of tree growth by preventing new tissues from fully "hardening." Given a short growing season, leaves do not fully cutinize and die from desiccation during the subsequent winter. Both of these hypotheses are unlikely for the vegetation on Mount Lookout on several accounts. First, there is no reason to believe that average temperature of this site is any more than one degree C cooler than that of low- lying areas based on the air temperature lapse rate. Second, the existence of forested summits on higher peaks close to Mount Lookout also refute these explanations. However, since Mount Lookout is purportedly exposed to unusually cold winds (Wells and Thompson 1974), heat deficiency as a control of tree growth merits attention in this study. 6 The mechanisms of tree deformation proposed for alpine timberline communities that do not depend on factors tied to absolute elevation must be carefully considered as possible controls of vegetation structure on Mount Lookout. Much evidence has been presented that wind is an important agent of structural control near alpine timberline including the following: 1) timberline vegetation may be reproduced locally on exposed knobs, shoulders, or passes far below the average elevation of upper timberline and tends to "parallel the contour of a ridge situated at right angles to the wind" (Daubenmire l954); 2) high elevation trees are often asymmetrically deformed in a consistent direction (e.g., Holroyd 1970); and 3) trees which grow above timberline are often restricted to depressions or other microsites which are somewhat protected from the wind (e.g. Griggs 1938. Marr 1977). Winter Desiccation Although the importance of wind in controlling vegetation structure at timberline is widely acknowledged (Griggs 1938. Johnson and Billings 1962, Klikoff 1965. Lindsay 1971, Sakai 1970, Schimper 1903, Smith 1972, Wardle 1965). the mechanisms of its action on plant growth form are poorly understood. Of these. desiccation has received the most attention. The widespread occurrence of tree foliage and shoot death in early spring at high altitude areas has focused attention on winter as the season during which damage takes place. One school (e.g. Lindsay 1971, Schimper 1903) invokes leaf desiccation damage. particularly in the winter, through increased tran- spiration as the primary means by which high speed winds shape trees. Marchand (1972) has challenged this hypothesis based on biophysical considerations. He states that wind decreases transpiration of winter-exposed plant tissues by reducing the thickness of the boundary layer of air on the leaf surface. The thinner boundary layer allows greater convective heat loss which reduces the leaf-air temperature differential. Since stomates are closed or plugged in the winter, moisture loss is limited to cuticular transpiration. which is largely a function of the leaf-air temperature differential. (When stomates are functioning, transpiration is a function of the humidity differential between leaf and air.) 7 Marchand (1972) fails to point out that wind will have this effect only on irradiated tissues: at nighttime. exposed plant parts may cool to below ambient air temperature because of long-wave emission (Jones 1983) in which case wind acts to increase transpiration. But, the leaf-air temperature differential is greater during clear days than at night, so consistently wind- blown plants should lose less water in winter than wind-sheltered plants or plants exposed only to nighttime winds. It is important to note. however. that the effect of wind on transpiration is realized at low wind speeds (Marchand and Chabot 1978); water loss of irradiated. winter-exposed plant tissues does not decrease linearly with wind speed but approaches its minimum value at velocities usually less than 2 mlsec (7.2 kmlh). depending on leaf size. . These considerations also point to a large role which solar radiation may play in causing desiccation damage. For isolated trees, such as are common near timberline. tissues are subjected to large radiation loads in the winter, and therefore large leaf-air temperature differentials, since there is little shading from neighboring plants. In particular, tissues just above the snow surface are subject to desiccation since they are heated by direct and snow- reflected radiation (Sakai 1970). Wind may cause tissue desiccation in other less direct ways. Snow of only a few dm effectively contains ground heat and can prevent freezing of soil moisture despite extended periods of below-freezing air temperatures (Geiger 1966). By blowing ground surfaces free of snow wind may allow soils to freeze deeply in the winter, blocking or reducing moisture uptake by trees (Sakai 1970. Tranquillini 1976). Trees in some areas of frozen soil are apparently able to prevent development of severe foliar water deficits through redistribution of water contained in the stem (Hygen 1965). Kincaid and Lyons (1981) found that the interior temperature of red spruce (Picea rubra) stems, which were above snow level, closely tracked ambient air temperature. suggesting that bulk flow of moisture in above-snow tissue could replenish water lost through transpiration during temporary winter thaws, even if the soil remained frozen. Exposed plant tissues may also succumb to winter desiccation through abrasion by wind-home ice particles and consequent increase in transpiration. Johnson and Billings (1962) and Klikoff (1965) placed stakes in the ground above timberline in early fall and found that they were severely 8 abraded by spring at a height which they assumed to correspond with the surface of the snowpack. In the timberline communities of the northern Rocky Mountains. Griggs (1938) found widespread occurrence of trees with a basal mat of dense foliage. bare trunks and. at most. a ”tassle" of foliage at the trunk apex. He interpreted this widespread growth-form as the consequence of ice crystal blast at the snowpack surface which ”injures” plant tissues. Such trees were absent where the snowpack remained deep through the winter and in extremely windy areas where erect stems were prevented from developing. But the link between this evidence of abrasion and foliar desiccation is not firmly established. Kincaid and Lyons (1981) found. through electron microsc0py. that needles of New rubra above timberline which were brown by the ‘ end of the winter had undamaged cuticles. though stomatal plugs appeared granular compared to those of wind-protected leaves. Winter desiccation of coniferous foliage may. therefore. result from abrasion of stomatal plugs by wind-bome particles. Hence. there are several reasons for expecting desiccatibn of winter- exposed plant tissues. In particular. on windless. clear winter days when there is a light snowcover on the ground. many factors may combine to produce a tissue-killing episode. Large tissue-air temperature differentials develop because of direct and snow-reflected radiation load and lack of wind. The heated leaves and buds lose water rapidly through abraded cuticles. Since the soil is frozen. this water is replaced only by stem reserves. When moisture reserves are exhausted. lethal water deficits develop. In view of this multiplicity of factors which may cause winter desiccation. it is surprising that few studies have demonstrated development of large water deficits in winter-exposed tissue. In the Alps and mountains of New Zealand. lethal water deficits have been found to develop cumulatively through the winter in foliage which turns brown and dies by early spring (Tranquillini 1976. Wardle 1965. 1974). This effect is most pronounced in leaves which are not fully mature because of a short growing season (Michaelis 1932. Wardle 1974. Tranquillini 1976). In the Canadian Rockies. Richards and Bliss (1986) measured very low (<-3.0 MPa) xylem potentials in the winter for above-snow twigs of Larix lyallii. However. buds of this deciduous tree avoid winter desiccation by becoming physiologically isolated from twig xylem during the winter. 9 In the Snowy Mountains of Australia (Slatyer 1976). and the White Mountains of New Hampshire (Kincaid and Lyons 1981. Marchand and Chabot 1978) the winter deficits of timberline tree foliage fluctuated but did not increase as winter progressed and did not reach very low levels. even though much foliage was dead at winter's end. Kincaid and Lyons (1981) found greater variation in water status within individual trees than between trees of different ages or from different elevations. No studies have documented foliar water status with sufficient frequency to detect the rapid development of lethal water deficits such as might occur on a single sunny winter day. however. nor have any investigations concentrated on the water relations of foliage which is most susceptible to abrasion. It is clear that the role of wind- related winter desiccation in limiting arborescent growth likely varies between regions (Bliss 1985) and will require intensive monitoring of the winter water status to be unravelled. Nonetheless. since all of the proposed mechanisms of wind-induced winter desiccation could be operative at low elevation. windy sites. it was given careful consideration in this study of Mount Lookout. Summer Desiccation There are several mechanisms by which wind can affect. desiccate or otherwise damage plant tissues. For instance. Wilson (1984) found that collisions between wind-blown leaves of Acer pseudoplatanus caused collapse of epidermal and mes0phyll cells and abrasion of the cuticle resulting in lesions and desiccation. In his investigation of the grass species Festuca arundinacea. Thompson (1974) demonstrated a direct effect of wind on epicuticular wax structure. More severe damage. including rupture of epi- dermal cells. was found to result from collisions between leaves of the wind- exposed grass. Grace (1974) showed that alterations of the wax structure reduced cuticular resistance to water loss (increasing cuticular transpiration). Damage of epidermal cells may permanently increase evaporative loss of a leaf since ruptured cells may continue to be supplied with water by neighboring cells. Wind decreases leaf transpiration when leaf conductance is low (i.e.. when stomates are shut). but may increase transpiration if conductance is high (i.e.. when stomates are Open). Since stomatal closure is often triggered 10 by large vapor pressure deficits (Caldwell 1970). such as develop when leaves are exposed to wind. transpirational water loss alone is not likely to cause lethal water deficits during the growing season. Reduced Carbon Assimilation By inducing stomatal closure. wind may indirectly reduce. carbon assimilation of leaves. Caldwell (1970) exposed Rhododendron ferrugineum to high speed winds (15 m/s) and observed reduced photosynthesis and transpiration due to stomatal closure. Tranquillini (1969) subjected six tree species to a range of wind speeds and found that two of them (Larix sp. and Pinus cembra) had slightly increased photosynthesis at low wind speeds. Photosynthesis was reduced for all species at high wind speeds. To the extent that photosynthesis is limited by boundary layer resistance to carbon dioxide transfer. wind should increase carbon assimilation. It reduces photosynthesis. however. particularly at high wind speeds. by triggering stomatal closure (Grace 1977). Reduced growth because of reduced carbon assimilation may partly explain tree defamation on Mount Lookout. Salt-spray The widespread phenomenon of tree defomation along ocean shorelines is evidently a result of tissue death through wind-bome salt impaction. Working on Cape Cod. Boyce (1954) found far greater accumulation of salt particles on greased slides positioned just above the smooth canopy of stunted Quercus than he found on slides located at the canopy contour. Within the canopy. there was almost no interception of salt. Shoots which extend above the canopy contour of these ”sculpted” shoreline trees evidently succumb to salt damage. Saltspray is obviously not a control of tree form on Mount Lookout though the physiognomic similarity between shoreline and bald vegetation suggests that the control on Mount Lookout is wind-related. Physical Force Wind may also influence community structure through the mechanical force it exerts on plant tissues. Though trees are generally able to prevent ll breakage of trunks and limbs through growth of reaction wood. foliage may be stripped away by high-speed winds (Smith 1972). Broad leaves should be most vulnerable to this since they have large drag coefficients (Grace 1977). Since wind speeds increase dramatically only short distances above the ground, leaf removal by wind should strongly favor prostrate growth foms. Mechanical damage to trees can be brought about indirectly by wind through the fomation of rime ice. When supercooled water droplets impact on leaves and twigs they freeze immediately. Ice builds on tissue in this way at a rate proportional to wind speed (Geiger 1966) so that shoots which are highest off the ground may accumulate large ice loads and break off. Sprugel (1976) made observations in the fir-wave forests of Whiteface Mountain which strongly implicated rime-ice damage as one cause of tree death in wind- exposed fronts of the waves. Similarly. Lawrence (1939) found that asymmetrical growth foms of conifers in a windswept river gorge were due in part to the damage caused by "glaze ice" accumulation on twigs and branches. Adams (1905) explains the flat crown surfaces of Quercus rubra growing on the escarpment of the Porcupine Mountains as a result of the breaking off of shoot tips by strong winds. He noted several pine trees with snapped tops. presumably due to winds as well. Endogenous Mechanisms Extension growth of plants can be limited endogenously in windy environments through a thigmomorphogenic mechanism (Jaffe 1980). When the stems of bean plants were rubbed every day and when Liquidambar seedlings were grown on a shaker table. stems were significantly shorter and wider than control plants. Lawton (1981) examined the shoots of a tr0pical tree and found that they were shorter and wider towards the summit of a Costa Rican ridge characterized by high wind speeds. He suggested that the thigmomorphogenetic response was partially responsible for the dwarfed stature of the elfin forest. Edaphic Stresses The small stature of trees in some low elevation plant communities has been attributed to local edaphic factors. Perhaps the best example of this is the 12 pygmy forest of California. Here. coniferous trees are stunted as a result of low nutrient levels and high concentrations of exchangeable aluminum in the extremely acidic Spodsolic soil (Westman 1975). Vegetation physiognomy is evidently suppressed by low nutrient availability and aluminum toxicity. If trees on Mount Lookout are edaphically limited. the soils are likely extremely acidic or nutrient poor. Infertile soils are usually acidic because cation exchange sites become occupied by hydrogen ions (Brady 1974). Since aluminum and other potentially toxic elements are converted to their free foms at low pH. soil toxicity is also frequently associated with acidic soils. One popular hypothesis holds that repeated burning maintains the trees as multiple-stemmed. short individuals (McComick and Buell 1968). IID"QIEIC .. Tree density is reduced in some communities by catastrophic disturbances. Vegetation subjected to disturbance which causes extensive damage of above ground tissue often contains a few tall-statured plants which somehow withstood the disturbance and a dense growth of low-statured regeneration. For example. in an area of tropical rainforest vegetation in Australia. periodic cyclones blow down tall trees so that the community is a tangle of shrubs and saplings with an occasional vine-draped emergent (Webb 1958). In the Rocky Mountains. lodgepole pine communities have a flush of new regeneration and scattered. fire-scarred. mature trees following a burn (Daubenmire 1943). Sparse tree cover may result from chronic or frequent damage ,of seedling tissue which acts as a bottleneck to seedling recruitment. In alpine environments. several types of disturbance have been suggested. The extremely harsh conditions which exist just above the snowpack in windy sites may limit seedling recruitment through winter desiccation of seedling apices (Johnson and Billings 1962. Klikoff 1965). Seedling survival may be limited in wind-sheltered areas where deep snowdrifts fom and persist late into the growing season. On the leeward side of subalpine ribbon forests in the central Rocky Mountains (Billings 1969) large snowdrifts develop and persist to mid-July. preventing tree seedling establishment. 13 In other areas. tree seedlings succumb to seasonal drought conditions which mature trees can endure (e.g. Noble and Alexander 1977). This same study found that heat-girdling. frost heaving. and clipping by birds were causes of seedling mortality. Dunwiddie (1977) presents evidence that tree invasion of a subalpine meadow in Wyoming is limited by tree seedling competition with meadow vegetation. The cessation of seedling establishment here coincided with the temination of cattle grazing which evidently had reduced the competitive vigor of the meadow plants. The occurrence of trees on a site may also be limited by seed availability. Trees near timberline are sometimes sterile (Daubenmire 1954. Weidlich and Teeri 1976). or attacked by predators (Klikaff 1965). Black and Bliss (1980) identified seed gemination as a bottleneck to tree seedling establishment at the arctic timberline. citing inadequate heat as the limiting factor. There is indeed a multiplicity of potential causes of tree defamation and sparsity on Mount Lookout. Wind. heat deficiency and edaphic factors are potential causes of tree defamation and scarcity on Mount Lookout. They act by causing tissue death. reducing plant growth. or preventing seed production. In this study I evaluated the significance of each mechanism as a control of vegetation physiognomy in cliff-summit balds by analyzing environmental and vegetational gradients. tree growth rates. tissue damage and population structure of trees on Mount Lookout. CHAPTER 2. METHODS A. The Study Site The study was conducted on Mount Lookout. located an the Keweenaw Peninsula. a 25 to 30 km wide finger of land which extends northeast into Lake Superior from Michigan's Upper Peninsula (Figure 1). At 47° 27‘ N and 88° 06' W. the site is one km south of the lake shoreline. It rises 396 m above sea-level (223 m above lake level). which is 47 m lower than the highest peak on the peninsula. Mount Lookout is an the southern edge of a late Precambrian stratum of conglomerate bedrock (Keweenawan Series) that dips below the Lake Superior Syncline to the north. Like adjacent layers. this stratum has been abruptly truncated by erosion along its southern edge (Darr and Eschman 1970. Figure 2). Mount Lookout is therefore one of a series of parallel. cuesta-fam ridges which run east-west curving ta northeast-southwest at the base of the peninsula and northwest-southeast at the tip of the Peninsula. Climate on the Keweenaw Peninsula is strongly influenced by Lake Superior. The spring season may begin ten to fifteen days later and the fall extends several weeks later than at inland locations because of the ameliorating effect of the lake waters an air temperature (Wells and Thompson 1974). At Eagle Harbor. which is an the coast 2 km northwest of Mount Lookout. the last spring frost has an average date of 20 May and the average first fall frost is 14 October. Mean July temperature is l6.5° C and mean precipitation is 74 chy (Wells and Thompson 1974). Tremendous snawfalls. up to 1000 CM. are received an inland portions of the Peninsula. - The Keweenaw Peninsula lies within the Superior Upland portion of the hemlock-white pine-northern hardwoods region (Braun 1950).. Its pre- settlement vegetation consisted of extensive mixed forests in which Acer saccharum.Betula papyrifera.Pinus strobus.Tilia americana.Tsuga canadensis. and Ulmus americana were prevalent. with Betula alleghaniensis important in law-lying areas. Most of the forest was logged in the late 19th and early 20th century so that today secondary forests of Acer saccharum. Abies 14 15 0 Lake Superior ‘\ Michigan ‘s s‘~‘ ‘\ Wisconsin Figure 1. Location of Mount Lookout in the western Great Lakes Region. 1 (i .835: E 8. x m 25 52:3 .8589»? EB Ensues—223:. me hate» u :a woman 6qu Brogan 8.3 22 a .3303 .552 be 2:95 .N 25E as an ca a; an a: an cw ea c . . . . . . . . . . \x can \ x x - can ‘ \ L - cmm \ n. a m . - can "n x m \ mm ‘ - can “v . O a u 3 a - can a M“ A O u - can - cos hDOxOOJ .22 l7 balsamea.Betula papyrifera.Populus grandldentata and Quercus rubra are the prevalent vegetation type (Wells and Thompson 1974). The Keweenaw Peninsula has a long history of intensive land-use. From the mid-1800's to the early 1900's. copper deposits inspired a flush of mining. clearing and building activities by white settlers which eliminated most of the primary forest on the Peninsula and gutted many of the ridges (Clark I975). Although Indian surface mines have been found within one km of Mount Lookout. there is no evidence that the ridge tap itself was mined for copper (Clark 1975). It is also unlikely that Mount Lookout was cleared or burned by white settlers for there are no charred or cut stumps an the site and I have not found charcoal in the soil. According to the field notes from the original land survey of 1846 (State of Michigan. unpublished). Mount Lookout appeared to be "bare conglomerate on the top and south side”. Only one small Quercus is cited in the survey as a marker tree for a section line which traverses the edge of Mount Lookout. suggesting that there was no tall-statured forest on the site at that time. Since the Keweenaw Peninsula did not begin to receive its first settlers until the early 1840's (Clark 1975), it is likely that the survey records accurately reflect the pre-settlement vegetation of this remote ridgetop. The top of this ridge likely escaped lagging by early settlers for the same reason that modern day loggers ignore it: it's too hard to get to and the trees are small. Whether or not there ever was full-statured forest on Mount Lookout prior to 1846 is impossible to detemine. The important point is that there is no evidence of forest having covered Mount Lookout in the last 141 years. If the vegetation is succeeding to closed canopy forest. it is doing so very slowly. Short. deformed trees are a conspicuous vegetational feature on Mount Lookout. The most common tree species in the community. northern red oak (Quercus rubra). is multiple-stemmed. less than 3 m in height. and often has a flat. dense crown surface. White spruce (Picea glauca) and balsam fir (Abies balsamea) growing near the summit of the ridge have dense basal foliar mats and 2 to 3 m erect stems which tend to be bare just above the basal mat. particularly on the south side. Other conifers. including white pine (Pi nus strobus) and northern white cedar (Thuja occidentalis). occur only as prostrate. shrub-like specimens. Juniperus horizontalis. a common shrub on Mount Lookout. is represented by individuals along the ridge summit which appear to be migrating northward (away from the cliff). The horizontal stems 18 of these plants are rooted up to 1.5 m south of the foliage. Similarly. common juniper (Juniperus communis) has its branches oriented to the north of the root crown. The consistently northward orientation of asymmetry displayed by the woody plants on Mount Lookout indicates that they are subject to climate-induced damage from the south. The low stature and the flat crowns of Quercus rubra suggest that there is also a vertical control of plant growth. The Keweenaw Peninsula has a high concentration of plant species which are widely disjunct from western montane and arctic ranges (Femald 1935. Richards 1954. Marquis and Vass 1981). Several of these plants are typically found near timberline in the Rocky Mountains and the Cascades but inhabit cliff-summit balds on the Peninsula. On Mount Lookout. such species include Collinsia parviflara. Draba arabisans. and Cares rossii. all of which are rare in the state of Michigan and are designated as "Threatened" (State of Michigan 1987). Solidago decumbens and Woodsia X abbeae. listed as ”special concern” by the Michigan Natural Features Inventory (unpublished). are also reported from Mount Lookout. B. Environmental Conditions On Mount Lookout The cliff-summit bald on Mount Lookout is not a homogeneous. discrete community but. rather. grades down-slope into a closed-canopy forest. Gradients of those environmental factors which are significant in controlling tree fom and density on Mount Lookout should correspond to gradients of community structure (Whittaker 1967). I therefore measured environmental and vegetational parameters at intervals along down-slope (south to north) transects. Four environmental variables (soil depth. soil pH. snow depth and wind velocity) were measured incrementally along down-slope (due north) transects. Soil depth and pH were detemined along 2 transects which were also used for vegetation measurements (described below). Sail depth was measured as maximum depth of penetration of a stiff wire probe at each comer of a 0.5 x 0.5 m square placed at two in intervals along the transect. The pH of mineral soil was measured for one randomly selected sample per 8 m interval along 2 transects using a Hellige-Truag test kit. 19 Snow depth was measured using a graduated rod which was pushed through the snow to ground level at 5 m intervals along 6 transects (4 of which were also used for vegetation measurements). Surface wind velocity profiles for southerly winds (which are the strongest on Mount Lookout) were measured on two days using a hot-wire anemometer (W indmeter). Maximum wind speed (up to an instrument maximum of 48 mill) in a 60-second period was recorded at 2.0 m and 0.5 m height in a treeless area at 10 m intervals down-slope. In order to compare temporal wind patterns on Mount Lookout with regional patterns. continuous wind run and direction were measured on the ridge summit for a total of 83 days during the period from 27 May to 15 October 1982. A Mechanical Weather Station (#1041. Meteorology Research. Inc.) that recorded the data in chart fom was used. For each 24-hour period. the 2-hour interval with maximum wind run was selected. The time. average direction. and average velocity during this interval were recorded. The instrument was vandalized at some time during the period from 30 May to 23 June such that directional recordings were skewed 60° counterclockwise. Since the exact time of disturbance could not be detemined. the data were not adjusted; as many as 25 recording days may have this bias. Wind data from Mount Lookout were compared with similar data gathered on the same observation days by the National Oceanic and Atmospheric Administration (NOAA. unpublished) at Houghton International Airport. The airport is situated on a large plain ca. 150 m lower in elevation than Mount Lookout and 40 km to the southwest and was assumed to have ground wind conditions typical of the region. NOAA data consisted of hourly observations of wind velocity and direction between 500 and 2400 h; Mount Lookout night-time data from 2300 to 500 h were therefore omitted from the comparison. The weather station also provided a continuous record of temperature. These data were averaged for May. June and July recording days. Mount Lookout and airport temperature maxima were compared. C. Species Composition The relative importance of each vascular plant species on Mount Lookout was detemined by sampling along 4 transects which ran from the 20 ridge summit dawn-slope into the closed forest margin. One transect ran from the highest point of the summit while the others were randomly assigned to 3 out of 6 possible positions spaced at 20 m intervals along the ridge crest. Each transect was divided into 10 equal intervals; one 5 x 5 m quadrat was randomly positioned along each interval. This stratified design was elected because it insured a more or less even distribution of plots along the down-slope gradient without sacrificing randomness. Coverage of every plant >0.5 m in height (i.e.. trees and tall shrubs) was estimated in each large quadrat by measuring canopy dimensions with a meter stick. Coverage by species of herbs and low shrubs (< 0.5 m in height) was estimated using a 0.5 x 0.5 m grid randomly placed at 4 positions within the larger plot. The number of squares in the grid filled by each species was estimated visually. The total sample areas for each vegetation class (1000 m2 and 40 m2) corresponded to the flat portions of species number vs. sample area curves established in June 1981. With these data.% cover was detemined for each species in each height class. General characteristics of the compositional gradient were then detemined based on this importance measure. D. Physiognomy A structural analysis of the community focused on tree species. in particular Quercus rubra. the most common species. The canopy height and stem diameter(s) were measured for all trees rooted within the 40 5 x 5 m quadrats described above. To increase the sample size for these measurements. quadrats were laid out along 2 additional transects; physiognomical measures for each slope interval were therefore based on 6 5 x 5 m quadrats. Many trees branched close to the ground. especially near the summit. so diameter was detemined just above the basal region of butt swell. Canopy height was measured directly using a meter stick. Further down-slope. canopy trees were measured which were judged to be typical in height. From these data. maximum tree height and diameter were detemined for each 10 m interval of the gradient. Stem number per Quercus individual was examined along the down- slope gradient. All stems greater than 2 cm diameter were counted for each tree with at least one stem greater than 6 cm diameter which fell within 2 10 x 100 m transects running dawn-slope. Two measures of tree fom were 21 calculated from the resulting data for each 10 m interval: 1) mean stem number per tree. and 2)% of total trees with one stem. To test for the existence of a directional stress in the environment of the ridgetop. the orientation of the longest radius of Abies balsamea and Picea glauca canopies (within 30 m of the summit) and Juniperus communis were measured with a compass. Mean direction of flagging was then calculated. To illustrate the structure of a particularly conspicuous feature of the bald community--the vegetation clump--a typical example was selected and a profile diagram was constructed. A 3 x 15 m belt transect was laid out through the clump and the vertical and horizontal contours of all canopies (>0.5 m height) and stems (>1 cm diameter) were sketched. E. Tree Growth To detemine whether or not growth of Quercus is low on Mount Lookout compared to neighboring forested sites. radial growth rates were detemined using increment borings taken from 31 trees (selected randomly) within 50 m of the ridge summit. The cores were taken near the base of the stem. above the region of butt swell. from a variety of directions. Control growth rates were measured based an increment cores taken from 6 mature Quercus 150 m down- slope. Heights and diameters of all sampled trees were measured. Annual shoot extension was compared for Quercus trees near the ridgetop and 120 m down-slope in the fall of 1982. To sample twigs. a rope was thrown over the crowns of 6 trees up-slape and 3 trees down-slope. The annual longitudinal increment of shoots touching the rope was measured for years 1979 to 1982 as the distance between teminal bud-scale scars. To assess the effect of wind exposure on shoot extension growth. extension increments were measured for unshaded trees growing in wind- sheltered microsites near the ridge summit and 70 m down-slope. At the ridge summit. wind-sheltered trees were defined as those growing down-slope from taller trees. At the transition between bald and forest. trees surrounded by taller trees (but not shaded by them) were selected. A more integrative measure of actual height gain was obtained by calculating the height to age ratio. i.e.. the vertical increment averaged over the life of the tree. Tree age was detemined from increment cores. 22 F. Winter Water Relations A preliminary examination of the role of winter desiccation in conifer shoot damage was undertaken in April 1982. On this date. most of the ground surface on Mount Lookout was covered with snow; beneath the snow. the soil was deemed frozen as it was impenetrable with a soil probe. Water potentials of 8 shoots. clipped under cloudy conditions at 900 h. were measured for each of 3 vertical zones of 4 trees (2 Picea and 2 Abies). The zones sampled included: 1) the ground foliage mat covered by snow most of the winter; 2) foliage along the bare trunk, 1 m above the ground. just above the snow surface for much of the winter: and 3) foliage near the tree apex. 2 to 3 m above the ground. Only shoots with green needles were selected. Once cut. each shoot was sealed in a Ziploc bag and placed on ice in a cooler. All readings of leaf water potential were made within 24 h using a pressure chamber (Ritchie and Hinckley 1977). Pressure was increased at the rate of 0.5 MPa/minute until the cut end of the stem became moist. G. Tissue Damage A qualitative measure of tissue damage was gained by inspecting trees on the bald for cracked or broken twigs and stems. pathological or browse damage. fire scars. dead buds. dead leaves or leafless stem. in late winter and in early fall of 1981 and 1982. H. Population Structure of Quercus rubra All trees measured in the structural analysis were tallied for every 0.5 m height class and every 1 cm diameter class. The resulting class distribution was interpreted to detemine the availability of seedlings on the site. and to identify the size class which might be a bottleneck to seedling recruitment. The density of seedlings along the slope gradient was detemined by mapping all seedlings within 3 5 x 100 m belt transects positioned east-west at 23 10. 30 and 60 m from the ridge summit. The boundaries of mature tree canopies were also mapped and used to assign each seedling to one of 3 microhabitats: 1) beneath tree crown. 2) within 2 m of the vertical projection of a tree crown margin. and 3) >2 m from the nearest tree crown margin (projected vertically to the ground). It was predicted that each microhabitat type represented a different level of protection from harsh environmental factors. CHAPTER 3. RESULTS A. Environmental Conditions on Mount Lookout 8.9.11 The soil on Mount Lookout is shallow at the summit but shows no trend toward increasing depth beyond 8 m down-slope (Figure 3a). Mean soil depth from 0 to 8 m down-slope is 6.2 cm which is significantly less than all other means (p<.02). Further down-slope. significant differences in soil depth are only found between extreme values; the slope of soil depth vs. distance down- slope is not significantly different from zero (p<.01). Moreover. there is no abrupt change in soil depth at the transition between bald and forest (i.e.. at 60 to 80 m down-slope). The soil on Mount Lookout is a sandy loam. derived from weathered conglomerate bedrock. Soil profile pits revealed a 4—8 cm deep Al horizon below which the sail contains gravel and cobbles. The soil could be excavated suggesting that the sail probe method underestimated soil depth. No charcoal was found in any of the three excavated pits. At the very summit of the ridge. much of the ground surface is exposed conglomerate with loose gravel and cobbles. Much soil has apparently eroded in this area due to heavy foot traffic; a comparison of aerial photographs from 1957 and 1978 revealed that the path which now traverses the cliff summit has developed in recent years. On the precipitous southern exposure. soil is found only in small depressions. rock crevices and at the bases of plants. SnnLIchth Snow depth tends to increase along the down-slope gradient (Figure 3b) suggesting that ground-level wind velocities are greatest near the ridge summit. Variability (standard error) of snow depth is greatest in the center of the community; snow is unifomly shallow (0.15 m) at the summit and unifomly deep (0.95 m) near the forest margin. On 27 February 1982. the day 24 25 14 . A. E 121 i i 8 '5 101 I . i i e . i i i G 8- '3 ‘ bald/forest m 6‘! boundary 1 —> ‘ i I ' 1 ' I v 1 1 0 20 40 60 80 100 1.0 . B. i 0.8- i i s . {Ii 0.6- i , {lill} 3 0.4- 8 . i m 0.2- J! 0.0 I 1 T ‘ l ' l I j 0 20 40 60 80 100 20 C l -- 2.0m l A 15 I instrument '0' 0.5m , ‘E ‘ maximum 3 cl: 3 1 B 5- i o ' 1 r T ' I ' I ‘ 0 20 40 60 80 100 Distance Downslope (m) Figure 3. Environmental profiles along the slope of Mount Lookout. A.) Mean soil depth (n=40): B.) mean snow depth on 26 February 1982 (n=6); C.) mean wind speed at 2.0 and 0.5 m above the ground during southerly flow (n=2). Bars in A and B equal one standard error. 26 after these measurements were made. a strong southerly wind blew the snow off several large patches of ground within 35 m of the summit. Mean snow depth after such a wind event increases more slowly with distance from the summit than illustrated in Figure 3b. II EI'IB' As predicted by the snow depth data. maximum wind velocity decreased along the down-slope gradient when there was a southerly wind. in the present case from >13.7 to 3.9 m/s (Figure 3c). Maximum velocities at 0.5 m above the ground were lower than at 2.0 m and varied little along the slope gradient. Wind velocities tended to reach higher daily maxima on Mount Lookout than at Houghton Airport (Figure 4). Velocity exceeded 31 kph on 58% of the days on Mount Lookout compared to 13% at the airport. a difference which is significant at p<.005. Daily wind-speed maxima had a strong central tendency on Mount Lookout with 52% coming from the south. In contrast. maxima were predominantly westerly (31%) or easterly (26%) at the airport. More than half (52%) of the wind-speed maxima on Mount Lookout occurred at night. while significantly fewer (p<.005) nighttime maxima (25%) were recorded at the airport. Furthemore. nighttime wind-speed maxima on Mount Lookout tended to be greater than daytime maxima: 68% of the nighttime maxima exceeded 31 kph compared to 46% during the day. In summary. daily wind speed maxima were greater on Mount Lookout. were more southerly and occurred more frequently at night than those at the airport. AIL—19mm Mean air temperature on Mount Lookout (the average of daily maximum and minimum) for 28 days in May and June was 12.0° C (s.d.=3.82) and for 19 to 23 July was 18.0° C (s.d.=1.17). Average daily air temperature maximum was slightly lower for Mount Lookout (15.3° C) than for the airport (16.1° C). 27 80 q ' 0 Airport E I MtLookout 3 6°“ E i i E :1 Z 0-31 32-47 48-63 84-79 >79 0-8.7 8.8-13.1 13.2-17.6 17.7-21.9 >21.9 Wind Class (km/hr above, mls below) Figure 4. A comparison of wind speed on Mount Lookout and at Houghton International Airport. The distribution of daily wind maxima by speed class for 83 days from 27 May to 15 October 1982. 2 8 B. Species Composition Eighty-two vascular plant species were found in the cliff-summit bald representing 29 families (Table 1). Eighty-six percent of the total vegetative cover (87.1% of the ground surface) was attributable to five families of which the heath family (Ericaceae) covered the greatest amount of area on Mount Lookout (22.3%). Seven plant species comprised 76% of the total plant coverage. with Aster macrophyllus most important (13.1%). Shrubs were the dominant life fom in the community with 45.7% cover, followed by herbs (31.4%) and trees (10%) (Table 2). Voucher specimens are deposited at the Beal-Darlington Herbarium (Michigan State University). The importance of each growth fom varied considerably along the down-slope gradient (Figure 5). Shrubs were most important up-slope while forbs and trees increased dramatically at the bald-forest ecatone. The large increase in total percent cover at the lower boundary was due primarily to the abundance of Aster macrophyllus which carpeted the ground there. Only grasses had greatest percent cover at the ridge summit. The importance (percent cover) of individual species varied greatly along the down-slope gradient. Species modalities. i.e.. the distances down- slope at which species are most important. were clustered at either end of the gradient (Figure 6a). With the exception of Solidago decurnbens. all of the rare vascular plant species reported for Mount Lookout were found. Plants growing in rotting conglomerate along the ridge crest which keyed to this species but graded into more robust individuals of S. hispida down-slope are considered depauperate specimens of the latter species. In addition. two individuals of Clematis verttcillaris. listed as ”special concern” by the Michigan Natural Features Inventory (unpublished) were discovered in the community and Pellaea atropurpurea. listed as Threatened (State of Michigan 1987). was spotted on the south-facing cliff using binoculars. 29 Table 1. Percent cover of vascular plant species on Mount Lookout. Family/Species1 Percent Cover ACERACEAE Acer rubrum L. A. saccharum Marsh. APOCYNACEAE Apocynum androsaemt'folium L. ASTERACEAE Anaphalis margaritacea (L.) Benth. & Hook. Antennaria neglecta Greene Aster macrophyllus L. A. sagittifolius Willd. Chrysanthemum leucanthemum L. Hieracium auriantt’acum L. H. canadensis Michx. H. florentinum All. Solidago hispida Muhl. S. spathulata DC. S. speciosa Nutt. Taraxacum ofl’t’cinale Weber. BETULACEAE Betula papyrifera Marsh. Ostrya virginiana (Mill.) K. Koch BRASSICACEAE Arabis divaricarpa A. Nels. Draba arabisans Michx. CAMPANULACEAE Campanula rotundifoh’a L. CAPRIFOLIACEAE Diervilla lonicera Mill. Linneae borealis L. Lonicera dioica L. var.glaucescens (Rydb.) Butters Symphort'carpus albus (L.) Blake CARYOPHYLLACEAE Dianthus deltoides L. CUPRESSACEAE - Juniperus communis L. J. horizontalis Moench. Thuja occidentalis L. CYPERACEAE Carex rossii Boatt C. rugosa Mack. C. umbellata Willd. C. sp. ELEAGNACEAE Sheperdt'a canadense (L.) Nutt. A é as S”? H pas—s p—s—s as g as as apppaaeee flu 9 9999 999 9 99 U. nth—lad mun u-n usu— 3 0 Table 1 (continued) W PcmentJlom ERICACEAE Arctostaphylos uva-ursi (L.) Spreng. 10.9 Chimaphila umbellata (L.) Bart. 41.1 Epigaea repens L. 0.4 Vaccinium angusttjfalium Ait. 9.3 V. membranaceum Daugl. 1.3 V. myrtilloides Michx. 0.5 FABACEAE Lathyrus ochroleucous Hook. 0.3 L. venasus Muhl. 0.8 Trifolium pratense L. <0.1 T. repens L. <0.1 Vicia amert‘cana Muhl. <0.1 FAGACEAE Quercus rubra L. var. borealis (Michx. f.) Farw.2 8.1 LABIATAE Satureja vulgaris (L.) Fritsch 41.1 LILIACEAE Lilium philadelphicum L. <0.1 Maianthemum canadensis Desf. 2.0 ORCHIDACEAE Goodyera oblongt’foliaRaf. <0.1 PINACEAE Abies balsamea Mill. 0.5 Picea glauca (Moench) Vass <0.1 Pinus strobus L. <0.1 POACEAE Agropyron trachycaulum (Link) Malte 0.4 Agrastt's sp. <0.1 Danthont'a spicata (L.) R.&S. 1.8 Deschampst'a flexuosa (L.) Beauv. 5.0 Festuca saxt‘mantana Rydb. 0.7 Hierochloe odorata (L.) Beauv. <0.1 Oryzopst's asperifolia Michaux 0.9 0. pungens (Sprengel) Hitchc. 0.3 Paa compressa L. 1.2 P. pratensis L. <0.1 Schizachne purpurascen's (Torrey) Swallen <0.1 POLYPODIACEAE Botrychium multift'dum Gmel. <0.1 RANUNCULACEAE Clematis verticillaris DC. <0.1 Hepatica amert'cana (DC.) Ker. 0.5 RmACEAE Amelanchier sp. 0.5 Crataegus sp. <0.1 Fragart'a virginiana Duchesne. 0.5 Potentilla tridentata Soland. 1.1 31 Table 1 (continued) W Percent Cover ROSACEAE (continued) Prunus virginiana L. 0.1 Rosa acicularis Lindl. <0.1 R. blanda Ait. 41.1 Sorbus sp. <0.1 SALICACEAE Populus tremuloides Michaux. <0.1 Salix humilis Marsh. 0.4 SANTALACEAE Comandra umbellata (L.) Nutt. 0.9 SCROPHULARIACEAE Collinst'a parviflora Dougl. <0.1 Melampyrum lineare Desr. 0.3 Veronica ofiicinalt's L. <0.1 SELAGINELLACEAE Selaginella rupestrt's (L.) Spring. <0.1 VIOLACEAE Viola adunca Sm 0.2 lNomenclature based on Vass (1972) (gymnospems and monocots) and Gleason and Cronquist (1963) (all others). 2Nomenclature based on Femald (1950). 32 Table 2. Vascular plant species on Mount Lookout with percent cover exceeding five percent. Species Percent HERBS Aster macrophyllus 13.1 Deschampsia flexuosa 5.0 All herbs 31.4 SHRUBS Arctostaphylos uva-ursi 10.9 Juniperus communis 12.2 J. horizontalis 7.5 Vaccint'um angustifolium 9.3 All shrubs 45.7 TREES Quercus rubra 8.1 All trees 10.0 33 Percent Cover 0 10 20 30 40 50 60 70 Distance Downhill (m) Fi ure 5. Plant cover on Mount Lookout. Percent cover by growth fom distance from the ridge crest. 34 30 a *A 33 25- ; . 3 20- E . ._ 15- ° 4 r. 10-l 0 g d E q a 5 z . O 'r'r'r'r'rfirrr'r'r1 0 10 20 30 40 50 60 70 30 90100 5 ... «B. 5 4- ' ' I i I 15‘ . . 3 3‘ I I i" 2.. I E a 1 E E “ ‘I o 'I'I'U'I‘IjttirI'I' 0 10 20 30 40 50 60 70 80 90100 10 100 .C . 3.. stem number .30 5 (g i ' a E, s- -so 9 i ‘ ’ g 4- -40 5 . . g g 2‘ '20 :1 i 0.. . . . . . .0 0 10 20 30 40 50 60 70 80 90 100 Distance Downslope (m) Figure 6. Vegetational profiles along the slope of Mount Lookout. A.) Number of species modalities (i.e.. greatest percent cover in the plant community); B.) maximum height of W; C) mean stem number per Quercus and percent of adult Wwith single stems. 3 5 C. Physiognomy The maximum height of Quercus rubra encountered in the sample plots increased with distance down-slope from less than 0.5 m at the summit to 4.0 m at the edge of the closed forest (Figure 6b). One hundred twenty to 170 m down-slope. canopy trees judged to be typical in height ranged from 9 to 12 m. A decrease in stem number per Quercus was found along the down-slope gradient (Figure 6c) with a maximum of 9 stems per tree at 20-30 m. Conversely. the percent of trees having one stem increased down-slope. to a maximum value of 80 at 120 to 130 m. An abrupt change in percent single- stemmed trees appeared to be associated with the bald/forest ecotone at ca. 70 m. Crowns of arborescent conifers. Abies and Picea. exhibited a central tendency to the north-northwest (334.9°). The strength of this tendency is indicated by the rather small standard deviation (17.02); 96% of the crowns had orientations between 302 and 10 degrees. The coniferous shrub. Juniperus communis. exhibited a more easterly central tendency (8.3°) which was also quite strong; 96% of the can0pies had orientations between 338° and 38°. The flat-topped crowns and multiple stems of Quercus are conspicuous features of vegetation clumps on Mount Lookout (Figure 7). The occurrence of Juniperus communis around the margins of the clump is also typical. D. Tree Growth Based on regression analysis of diameter and age. Quercus growing in the bald had a mean radial increment of 0.67 mme (r2=0.85: Figure 8). That the relationship is actually curvilinear (i.e.. that the radial growth rate is decreasing with age) is suggested by the positive y-intercept. 1.45; the value 0.67 mm/y is therefore probably an underestimate for young (thin) trees and an overestimate for old (wide) trees. Mean annual increment of 70- to 80- year-old trees in the bald (0.73 mm/y) was significantly (p<.01) lower than that of similar-aged canOpy trees of the closed forest (1.24 mm/Y). Teminal shoots of isolated Quercus near the summit of Mount Lookout grew an average of 4.6 cm in 1982 which is not significantly different than the shoot growth of canopy individuals down-slope (5.0 cm). Wind-sheltered trees on the bald grew 7.0 cm. which is greater than wind-exposed trees up- 36 .39? 02m 8 .25..“ ES 63335.... 3323: u . u 33 . . w I Um .95.: .33»: u . > 2 38.8.— :522 he E55.» 2: .a nE=_W:a_WWw.wn~fiE“ao 2.33:3. u 03 . «3:95 .5 flaw—m 37 i Quercus rubra " u Stem Radius (cm) 0 l .I y-O.7794+0.0687x r2235 ' l W r a 1 o 20 40 so so 100 moan) Figure 8. Stem diameter wage of W These data are for individuals within 50 m of the ridge summit. 38 slope or down-slope. Quercus of forest gaps grew 12.5 cm which is far greater than all other categories (Table 3). In short. trees which were sheltered from southerly winds grew faster than unsheltered trees and the protected trees down-slope grew faster than sheltered trees on the ridge crest. A comparison of growth rates for a 4-year extension increment showed identical trends. The lengthzdiameter relationship for I982 twigs also varied greatly depending upon slope position and degree of wind exposure (Table 3). Down- slope. canopy (wind-exposed) trees had the thickest twigs with a ratio of 13.8 and wind-protected trees had the most slender shoots (39.7). The ratio was intemediate for both exposed and sheltered trees at the ridge summit (18.0 and 20.6. respectively) which were not significantly different. All other mean ratios were different at p<.05. Surprisingly. no correlation was found between height and age for Quercus rubra growing on the bald (Figure 9). Maximum crown height (ca. 3 m) had been reached by individuals as young as 20 years. even though trees more than 70 years of age were present on the bald. No Quercus stems within 60 m of the summit exceeded 3.5 m in height. Quercus in the forest canopy 170 m down-slope were three times higher than trees of similar age growing in the bald (Figure 9); the forest trees had grown an average of 13.1 cmly vs. 3.8 cmly for trees in the bald. E. Winter Water Relations Mean values of leaf water potential ranged from -0.72 to ~2.03 MPa and were always lower for mid-stem foliage than for apical or basal foliage (Table 4). The difference in means was not significant for the top and middle foliage of one Picea and the large variance in values for mid-stem foliage of the Abies. precluded statistical comparison. Mean values of leaf water potential were lower for mid—stem foliage than for basal foliage in each tree. although the difference was not significant in one case. In short. middle foliage tended to be drier than tap foliage which tended to be drier than basal foliage. It is interesting to note that many Abies and Picea trees had brown needles 0.8-2.0 m above the ground in early spring. Evidence of several types of tissue damage was found in trees of the bald. Many teminal Quercus shoots were found dead with bark worn away by friction from neighboring twigs. Other dead or dying shoots had a series of 39 Table 3. Shoot length and lengthzwidth ratios for Quercus rubra in four microhabitats. Data are for 1982. Mean values with the same superscript are not significantly different (p<.05). Mean Mean Length Length] Microhabitat (cm) s.d. Diameter (cm) s.d. n Upslope/Expased 4.61' 3.55 . 18.00 11.97 36 DownslopelExposed 5.0a 2.51 13.89 7.56 29 UpslopelProtected 7.0b 4.78 20.6° 15.36 24 Downslope/Protected 12.5° 6.73 39.7f 20.11 18 40 12 - Quorwsrubra 10- n 9 . '3, a: c 8- g I a 6- I bald .5 l 2 I forest ‘3 4‘ y.12525+o.o21x r =.31 1 - . . l I. 2- " a ' . I O'l—IT r j l ' ' ' ' f o 20 40 so so 100 Arrows) Figure 9. Height versus age of mm The slope of this relationship for trees within 50 m of the summit (filled squares) is not significantly different than zero. Table 4. Shoot water potentials of Abies balsamea and Picea glauca on 22 April 1982. Means and standard deviations. significantly different have the same superscript. 41 Within each tree. means that are not Abies middle foliage was highly variable and could not be compared with other means. Species Basal Foliage __Shont_flater_£mantiaLtM£aL__ Middle Foliage Top Foliage Picea glauca (No. 1) Picea glauca (No. 2) Abies balsamea -0.‘73a (0.12) n=8 -0.77a (0.32) n=8 -1.31‘ (0.22) n=8 -1.36b (0.13) n=8 -1.14b (0.20) n=8 -2.03 (0.43) n=8 -1.25b (0.12) n=8 -0.84a (0.20) n=10 -1.s6b (0.13) n=5 42 unifom lesions in the bark. apparently the result of fungal infection; some shoots bore insect galls. A large number of dead branches was found among the Quercus . A major (5 cm wide) stem of one tree was found freshly broken during a winter visit to the site; other stems were found cracked near the base. some bearing signs of degeneration. Most of the Quercus in the bald had stems growing from large (e.g.. 0.9 x 0.3 m) root crowns. Many Abies and Picea had irregular trunks indicating histories of teminal bud or shoot death. Closer inspection of tree tissues revealed dead buds. broken or brown needles. and diminutive needle size. Some Picea seedlings appeared stunted with depauperate foliage and branches. Other tree species represented on the site rarely exceeded 1 m in height. Needle browning was evident in Thuja occidentalis and Pinus strobus. stunted and broken needles were found in Ft nus. G. Tree Reproduction Quercus and Thuja produced large seed crops during each of the three years of observation. All other tree species were sterile throughout the study period. Populus tremuloides (quaking aspen) showed evidence of vegetative Spread through rhizamatous sprouting. Thuja occidentalis accomplished the same with horizontal. above-ground stems, though these had not produced adventitious roots. H. Population Structure of Quercus rubra The Quercus population on Mount Lookout had a bimodal distribution of height (Figure 10). The number of individuals decreased dramatically from the smallest class (0-0.5 m; 579 trees) to the second class (0.6-1.0 m; 21 trees). The number of individuals per size class did not decrease unifomly with greater plant height. but reached a minimum for the 1.1-1.5 m size class. This pattern was more pronounced when only trees close to the ridgetop (<50 m) were considered (Figure 10). Na Quercus seedlings 1.1-1.5 m in height were encountered within 50 m of the ridge summit. The Quercus in the bald therefore famed two fairly distinct canopy layers: 0.1-1.0 and 1.6-4.5 m. The diameter class distribution (Figure 11) also showed a dramatic reduction in number from the smallest category (0.0-0.9 cm, 563 trees) to the 43 255 Quercus rubra * 25° 0-50 m i a 245 E E ‘5 a «- O. m 9 we 9. in 0. c5 '— v— cu N co so as . O I O I I I I O ‘9. ° ". “i " 1". O v- N N G) 6) Figure 10. Stem height class distribution of W These data are for individuals yithin 50 m of the ridge summit. Total sample area was 875 m. . 44 260 Quercus rubra 255 0-50m 250 15 . . No. of Individuals 10 .‘u'. .I I:I:l .I.I - - 9...! 5 I... I I ‘ .I.I. I... I I .I.I' I... I I 'I-I' o I_I I_ ,I I_l_l AI; 0 °. °. °. ° °. °. °. ° '- ‘l‘ ‘1’ 2' "3 “3 '? “P ‘i‘ 9 ". P. P. '- ". r: '1 '- v- N 0 V In 0 N 0 Diameter Class (cm) Figure 11. Stem diameter class distribution of W These data are for tregs within 50 m of the ridge summit. The total sample area was 875 m . 45 second class (1.1-2.0 cm. 16 trees). Furthemore. a bimodal distribution of abundance was again evident with only 4 of 646 individuals falling into the 3 diameter classes between 4.1 and 7.0 cm. In contrast. 28 trees fell into the 3 classes between 7.1 and 10.0 cm. Since diameter and age were quite closely correlated (Figure 8) the age-class distribution would resemble this bimodal pattern as well. Quercus seedling densities increased from 0.19/m2 along the ridgetop to 0.62/m2 55 m down-slope (Figure 12). Density varied considerably among microhabitats. Beneath tree crowns at each slope position there were 0.7 trees/m2. Density increased slightly from 0.32 up-slope to 0.51 down-slope within 2 m of crown margins and increased dramatically. from 0.09 to 0.69. in open microhabitats. The relative pr0portion of each microhabitat type changed down-slope as a result of increasing tree cover (Figure 13); the open type. for instance. decreased from 66% of the ground surface near the summit to 4% at 55 m. The average inter-tree distance was therefore greater for positions in the open ' microhabitat up-slope than it is down-slope. Seedling densities of bath Abies and Picea were very low and reached maximum values (0.013 and 0.018/m2. respectively) 30 to 35 m down-slope. Only 11% of all the seedlings encountered were beneath tree crowns. 46 08 I Total G Beneath Crown as <2n1 E! >2n1 Seedlings / Square Meter 0-5 30-35 55-60 InmanaeDownhHIOn) Figure 12. Seedling density 0W8 in different microhabitats. The crown margin of adult trees was measured as the vertical projection of the crown edge to the ground surface. Data are for one 5 x 100 m belt transect at each slope position. 47 ‘: :1 to 1: 5 E U E 0-5 30-35 55-60 Distance Down-slope (m) Figure 13. Microhabitat composition along the slope of Mount Lookout. Each slope position represents one 5 x 100 m belt transect. CHAPTER 4. DISCUSSION A. Environmental Conditions Soil depth does not change across the boundary between the bald and closed forest. Therefore. even if soil depth is generally limiting to tree growth on Mount Lookout. other factors must be responsible for the physiognomic difference between these communities. Although the soil is shallow compared to sites which are not underlaid by bedrock. it is deep compared to rocky cliff and shoreline communities on the Keweenaw Peninsula in which mature trees are found rooted in rock crevices. Furthemore. the soil probe technique probably underestimated depth. Soil is probably more shallow at the summit of Mount Lookout for a number of reasons. Erosion is greater at the apex of the ridge since it 1) is subjected to higher maximum wind speeds than dawn-slope sites (Figure 3c). 2) is without snow cover more frequently in the winter. and 3) is convex promoting erosion through surface run-off. There is little vegetation to anchor the substrate and produce organic matter at the summit so that soil development proceeds slowly. Based on 1957 aerial photos. there was more plant cover on the crest at that time and there may have been more soil as well. Soil pH is slightly acidic (mean=6.3) on the bald. Therefore. aluminum toxicity or low base saturation levels are probably not limiting to tree growth (Brady 1974). Individual nutrients may be limiting to tree growth on the bald although no leaf discoloration was observed during 2 growing seasons. Snow depth increased down-slope (Figure 3b) reflecting the gradient of decreasing wind speed away from the ridge apex (Figure 3c). Wind momentum is likely dissipated down-slape by friction with the ground surface and vegetation and by loss of pressure which may develop as air is forced over the ridge. Since snow depth was measured on only one day. the pattern of snowcover on Mount Lookout may deviate greatly from that depicted in Figure 48 49 3b. The general trend of greater depth with distance from the summit should be typical. however. especially following periods of southerly winds. The wind environment of the ground surface was far more variable in the center of the bald than at either margin as judged by the standard error of snow depth (Figure 3b). This patchiness is presumably due to the effects of microtopagraphy and vegetation clumps on local wind pattern, and to eddying caused by turbulent flow (Geiger 1966). In the treeless area where wind speed was measured. a strong winter wind removed all snow cover. while drifts remained behind trees and in depressions. Soil depth. snow depth and wind speed change abruptly within 10 m of the ridge crest (Figure 6a-c). These conditions. combined with the foot-traffic the area receives makes the ridge apex a very inhospitable environment for plants. The tendency of wind on Mount Lookout to be strongest from the south can be explained on the basis of the shape and orientation of the ridge. Wind approaching the ridge top from the north. east or west encounters gradual slopes (of less than 18%) and therefore dissipates momentum and slows considerably before reaching the ridge summit. Southerly winds. however. cross the ridge crest without being slowed by friction with ground surface air because the south-facing slope is nearly vertical. One reason that winds are strongest from the south on Mount Lookout is that winds from any other direction are slowed by friction before reaching the ridge top. A similar explanation can be invoked to explain why wind speeds tend to be higher on Mount Lookout than at Houghton Airport. Although the ridge crest is exposed to high speed winds. particularly those emanating from the south. the airport is situated on a large. flat expanse of land that is protected from high speed winds by the boundary layer of air.on the ground surface. Wind speeds may be higher at night than during the day on Mount Lookout because of diurnal changes in the vertical wind speed profile. On a clear day. winds crossing the Keweenaw Peninsula are coupled to slow-moving air an the ground surface by convective mixing. As radiation heats the ground. causing parcels of warm air to rise. high speed air aloft subsides to replace them: the momentum of the high speed air is dissipated to the boundary layer air and the slow-moving, rising air dissipates momentum of the air aloft. Therefore. on clear days. wind speed increases gradually with height above ground until the elevation at which there are no more effects of convective mixing. 50 In the absence of solar radiation. convective mixing ceases and cool air collects on the ground surface. A boundary layer of cool. still air develops that is uncoupled from wamer. fast-moving air aloft. (The fomation of this boundary layer is fastest under clear sky conditions. when long-wave radiational cooling of the ground is rapid.) The gradual vertical wind speed profile of the daytime becomes an abrupt wind speed profile at night. with very slow air movement in the boundary layer and very fast flow just above the boundary layer. Mount Lookout may be windy at night because it juts above the boundary layer of cool. still air and is exposed to high speed winds that flow across the surface of this air layer. The orientation of the ridge may contribute to the nighttime wind phenomenon as well because the scenario of warm air flowing over cool boundary layer air is most likely under conditions of warm air advection and warm air generally flows from the south. The fastest winds measured on Mount Lookout may have been southerly and at night because most of the warm air that moves across the Peninsula emanates from the south and because the summit of Mount Lookout is most exposed to southerly winds. 1 These considerations suggest that landfoms that project abruptly upwards through the blanket of cool. nighttime air will be subjected to wind speeds much higher than those at the ground surface during periods of warm advection. This explanation has been invoked for nighttime winds documented in mountains and on ridges as low as 427 m above sea level (see Baughman 1981). All of the ridges supporting cliff-summit balds in northern Michigan may have .wind conditions similar to those on Mount Lookout because of their cuesta-fom shapes and the southward orientation of their cliff faces. Although it is only 396 m in elevation. Mount Lookout may be windy at night because it projects above cool air which collects at the ground surface at night. As warm winds advect north towards this ridge they must first cross at least 40 km of water surface at the Keweenaw Bay (south of the Peninsula). The winds move across the Keweenaw Peninsula and encounter the cliff face of Mount Lookout. Since the cliff face is nearly vertical. airflow is not gradually diverted up over the ridge apex but. rather. it is compressed upon impact and accelerates over the top of the ridge. This mechanistic hypothesis is supported by other lines of evidence as well. Nighttime wind episodes that 51 were compared with regional weather maps showed a correspondence with southerly. warm air advection patterns. The wind regime on Mount Lookout during the winter months is unknown. Since little radiation is absorbed by snow. and since days are shorter. there is reason to expect that high speed winds. documented primarily at night during the summer, would be more prevalent during the daytime in the winter. In four winter visits to the site. I observed several episodes of daytime. high speed southerly winds. Additional data are needed to characterize the winter wind regime. however. Daubenmire (1954) demonstrated that alpine timberline rarely exceeds the elevation at which the mean temperature for the wamest month of the year is less than 10° C. The mean temperature for recording days was 12° C in May and June and 18° C in July. suggesting that heat deficiency is not a control of tree growth on Mount Lookout. In fact. daily temperature maxima were very similar to those of nearby flat areas. refuting the hypothesis that heat deficiency is limiting to tree growth on Mount Lookout (Wells and Thompson 1974). A more biologically meaningful measure of energy for a plant. however. is leaf temperature. Since Mount Lookout is windier than the the airport site (Figure 4). plant leaves on the bald should generally have thinner boundary layers. and therefore cooler daytime temperatures. than plants at the low-lying site (Gates 1980). Suboptimal leaf temperatures may partially restrict tree growth on Mount Lookout by restricting photosynthesis although additional study would be needed to test this idea. B. Species Composition Vegetational composition varies along environmental gradients described for Mount Lookout (Figures 5 and 6). At the ridge summit 12 species reach their greatest percent cover on the bald including only 2 woody species. Juniperus horizontalis and Shepherdia canadensis. All of these species are components of south-facing cliff communities: vegetation of the ridge crest can therefore be considered as a transition zone between the cliff-face and bald vegetation. The 12 modal species at the ridge apex are also common in unshaded conglomerate bedrock plant communities of the Keweenaw Peninsula shoreline (Wells and Thompson 1974). 52 All angiospem trees (except Populus tremuloides and Betula papyrifera) were most important at the down-slope margin of the bald. All of these species (except Quercus) were restricted to wind-sheltered microsites in the bald. e.g.. beneath or to the leeward sides of Quercus crowns. suggesting that they cannot tolerate the microenvironment of open areas. Shrubs covered more of the ground surface on the bald (46%) than any other growth fom (Table 2) and were the only growth fom which does not become more important at the bald margins. All of the important shrubs were low-growing (generally <0.5 m) and. therefore. not subject to the high speed winds which occur short distances above the ground. Also. they had needle- like. scale-like or coriaceous leaves which may be resistant to water loss. During winter visits I noted that the evergreen shrubs Juniperus communis and Arctostaphylos uva-ursi were always covered by snow. perhaps owing to the boundary layer of still air created by their crowns. 5 3 C. Physiognomy The stature and number of stems of Quercus rubra trees corresponded. roughly. to measured gradients of wind speed and snow depth (Figures 3b.c and 6c). Height of Quercus increased along the down-slope gradient and. thus. was negatively related to observed wind gradients. It was impossible to detemine which of these variables was the dependent one. Increased tree height could cause reduced wind speeds at ground level or reduced wind speeds could allow trees to grow higher. as will be discussed in the following section. Reduced wind speed down-slope can also be explained on the basis of aerodynamic principles. The multiple-stem fom was presumably the result of loss of apical dominance. Seedlings produced several shoots if the teminal bud was damaged and mature trees sprouted if the monopodial stem was damaged or died. The correspondence between number of stems and wind speed suggested that trees higher on the slope of Mount Lookout had been subjected to wind- related damage. Tree height and stem number per tree were probably not independent of one another. Multiple stems may compete for light and root-supplied resources (nutrients and water). particularly if there is a small soil volume or low levels of either resource. D. Tree Growth Radial growth of Quercus (per stem) was twice as slow on the slope as in the closed canopy forest below (Figure 8). There are several possible wind- related causes for this difference including tissue loss and damage due to- winter desiccation (e.g. Johnson and Billings 1962. Sakai 1970). summer desiccation of wind-damaged leaves (Grace 1977). the force of high speed winds. or reduced carbon assimilation due to stomatal closure (Tranquillini 1969). Radial growth may depend on stem number as well if root-supplied resources are limiting. Additional study will be needed to detemine the relative importance of these potential limitations to growth. Shoot extension growth of Quercus decreased along a presumed exposure gradient: down-slope protected>up-slope pratected>down~slope exposed-up- sIOpe exposed. Possible causes of these differences include all of those cited 54 above for variation in radial growth but applied to individual shoots. That is. growth of an individual shoot will be detemined by leaf and bud damage it undergoes. limits on photosynthesis of its leaves. and the availability of stem- supplied resources. Shoot growth of Quercus was apparently under thigmomorphogenic control as well. based on lengthzdiameter ratios. Down-slope. protected shoots were 3 times as slender as wind-exposed shoots of neighboring canopy trees (Table 3). A similar trend was found by Lawton (1981) for a rainforest tree species in Costa Rica which grew along a wind speed gradient. A clear exception to this trend is the lengthzdiameter ratio found for up- slope exposed shoots. While wind profile data show that wind speeds were highest at the ridge crest. these shoots were considerably more slender than down-slope exposed shoots. although their lengths were the same. Assuming the densities of the shoot wood were equal. this may be a reflection of the low vigor of the trees at the upper slope position. as suggested by the radial growth data (Figure 8). Perhaps restrictions on the productivity of up-slope, wind- exposed trees renders them incapable of producing stout shoots. In the absence of microclimate data for the slope and canopy positions used in this comparison. and without an understanding of the limitations to growth on the site. these explanations remain speculative. But. regardless of the physiological bases for observed patterns of shoot growth. up-slope shoots of wind-exposed trees were probably more susceptible to breakage than canopy twigs dawn-slope since they were thinner but of the same length (Grace 1977). Patterns of shoot growth partially explain the difference in tree height along the down-slope gradient (Figure 6b); given the greater extension growth of wind-sheltered trees law on the slope. stems of the same age tend to be taller down-slope than on the ridge crest (Figure 9). But other factors limit the stature of ridge-top Quercus as well. for measured annual extension increment of wind-exposed tissues (4.6 cmly) is greater than mean annual height gain averaged over the life of the stem for trees older than 70 y (3.8 cmly); that of wind-sheltered tissue (7.0 cmly) corresponds to the net vertical increment of 30-40 y old trees. Moreover. trees close to the ridge crest appeared to reach an upper limit to height at about 3 m (Figure 13); old Quercus (70 y) spend decades without increasing in height. This apparent ceiling to height gain may have been due to non-vertical orientation of shoot extension or mortality of teminal shoots. Lawrence (1939) found that conifer 55 branches were bent. or ”trained.” to grow horizontally by prevailing winds. Similarly. Boyce (I954) discovered that bud death on one side of salt-sprayed shoots caused them to gradually grow downwind. Inspection of Quercus crowns on Mount Lookout revealed that teminal shoots were oriented at angles other than vertical. although the direction of orientation was variable. The flat canopies of Quercus on Mount Lookout and the apparent upper limit to height growth were probably due. in part. to wind-induced nan-vertical orientation of teminal shoots. E. Tissue Damage Height gain may also be limited by tissue die-back resulting from high speed winds encountered short distances above the ground. Wind-induced damage is often particularly severe for shoots which project above an otherwise flat canopy surface (e.g. Daubenmire 1954. Boyce 1954). Such damage may be corroborated by the slendemess of these wind-exposed shoots (Table 3). Some of the dead teminal shoots observed in crowns of Quercus on Mount Lookout had apparently died due to abrasion of neighboring shoots or pathogenic infection. Many shoots were dead for no apparent reason and may have succumbed to constraints on carbon gain (Tranquillini 1969) or deterioration of buds and leaves through cuticular or epidemal damage (Grace 1977). Of course. larger scale tissue damage may also limit height gain of wind- blasted trees. Leach (1925) observed that the oldest stems of short-statured Quercus on a windblown slope gradually died. apparently due to pathogen attack. Also. frozen stems are far more brittle than wam stems. During the winter of 1982. a 5 cm thick Quercus stem was found snapped and leaning to the north. as if broken by a strong gust of wind. Several larger Quercus stems were found which were cracked near the base and from many of the Quercus root crowns projected the broken. dried remains of dead stems. Stem breakage and death may limit height gain of Quercus on Mount Lookout. However. death of entire stems would not explain the lack of correlation between age and stem height (Figure 9). The prostrate foms of Thuja occidentalis and Pinus strobus and the bare trunks of Abies and Picea were at least partially the result of tissue damage which occurs at 0.5-1.0 m above the ground during the winter. A large 56 dieback of conifer foliage at this height was observed in early spring of 1982. Published explanations for such a phenomenon suggest that tissue death often occurs at the snow surface because abrasion by wind-blown particles and heating by direct and snow-reflected radiation result in tissue desiccation (Johnson and Billings 1962. Kincaid and Lyons 1981. Klikoff 1965. Sakai 1970). In support of this view. a preliminary investigation of winter water relations of Abies and Picea indicated that xylem water potential above the snow surface tended to be lower than that of foliage at the base or apex of the tree (Table 4). These upright conifers are apparently limited in their vertical extension by repeated death of teminal buds and shoots which occurs more frequently with greater height and wind-exposure. F. Tree Density The occurrence of Abies and Picea on Mount Lookout was probably limited in part by low availability of seeds. No cones were found on either of these species in 1981 or 1982. Seeds are evidently carried into the community by wind from seed sources at the base of the cliff for more seedlings were encountered in the transect located at 30 m down-slope than at 55 m (Figure 12). Also. both of these species had modalities near the center of the slope which would not be expected if they were establishing from seeds blown up— sIOpe from the north. Seedling production does not appear to limit the density of mature Quercus rubra in the bald as a whole. Copious seeds were produced during each year of the study and seedlings were abundant (Figure 10). The drastic reduction in seedling number for the 0.5-1.5 m classes suggests that the community was either recovering from a catastrophic disturbance which eliminated a seedling population or that there is chronic seedling mortality or tissue death associated with this height range. Since no evidence was found of fire or wind storms. the latter scenario is more likely. Height-related mortality or tissue death of Quercus seedlings may result from browsing of shoots which extend above snow cover or from microenvironmental factors. Since no browsing animals. or their sign. were ever observed on Mount Lookout. and since seedlings with clipped teminal shoots were not found. browse damage is an unlikely control of seedling height. However. widespread death of low conifer foliage observed in the spring of 1982 supports the notion 57 that tree mortality or tissue death prevents seedlings from advancing into the 0.5-1.5 m height classes. Since the diameter class distribution of Quercus is strongly bimodal (Figure 11). seedling mortality probably accounts for the height class distribution. If seedlings were prevented from advancing into the 0.5-1.5 m height classes because of repeated death of apical tissues. we would expect the diameter class distribution to be unimadal. The likely mechanism of seedling death is wind-induced winter desiccation. If wind-related damage of Quercus seedlings is taking place. we would expect greater concentrations of seedlings down-slope and in wind-sheltered microsites. which was indeed the case (Figure 12). This observed distribution may also reflect. however. differences in seed availability (i.e.. dispersal) or differences in seed gemination rate between microhabitats. Additional study will be needed to assess the relative importance of these factors. The size-class and seedling distribution data indicate that seedling recruitment was a rare event and was probably most frequent close to or beneath the crowns of adult trees. But even low rates of seedling recruitment can result in high densities of adult plants for long-lived species if recruitment rate exceeds the rate of adult mortality. The absence of Que rcus seedlings 1.1 to 1.5 m in height in the 26 5 x 5 m plots within 50 -m of the ridge summit indicates that adult mortality would have to be extremely low for the density of adult trees to increase. and that such an increase would proceed very slowly. CHAPTER 5. CONCLUSION Mount Lookout is characterized by environmental gradients of decreasing wind speed and increasing snowcover with decreasing elevation. Both of these gradients are steepest near the ridge summit. The vegetation structure corresponds to these environmental gradients. There is greater vegetative cover down-slope and the trees are taller. A multiplicity of environmental factors accounts for the defamation and low density of trees on Mount Lookout. Although the relative importance of each factor remains to be quantified. several lines of evidence implicate wind as the primary control of vegetation structure. These considerations are here combined as an integrative. albeit speculative. interpretation of the factors which control this and other ridge-crest communities. Mount Lookout has a steep. south-facing cliff and a gradual slope to the north. With warm air advection from the south. it is swept by high-speed winds. particularly at night. when there is no convective mixing of the air. The wind encounters the cliff face. is compressed and accelerated over the cliff summit. At the apex of the ridge. wind velocity is often very high. even close to the ground surface. Wind speeds decrease along the northern slope. especially close to the ground. because the ground surface and vegetation dissipate momentum of the air, as does the decompression which is allowed by the slope. Near the lower edge of the community. the vertical wind speed profile is gradual: high speed winds are encountered only several meters off the ground. Snow depth therefore increases down-slope but southerly winds . periodically remove snow from patches of ground high on the slope. Only a small guild of hardy. predominantly herbaceous plants can survive on the windswept crest of the ridge. Ten to 50 m down-slope. some trees have successfully established in depressions in the ground surface where wind speeds are lower and snow accumulation is greater. New trees have colonized the sheltered microsites downwind (north) of established trees. Large areas around the resulting tree clumps are kept free of seedlings by wind-induced seedling mortality and. perhaps. the unavailability of seed. 58 59 Quercus rubra individuals are maintained as flat-crowned. low-statured plants by high speed winds encountered short distances above the ground. The winds act on these trees by reducing extension growth. in part by triggering a thigmomorphogenic response. and by damaging leaves. shoots. and. occasionally. large stems. Tree height increases down-slope since the vertical wind speed gradient is more gradual near the lower margin of the community. Conifers are also restricted in their vertical extension. in part because of teminal bud and shoot mortality. The bare trunks and strongly flagged crowns of Abies and Picea reflect the harsh conditions which exist above the snow surface. The vegetation on Mount Lookout and perhaps other cliff-summit balds appears to be maintained in a structural steady state by recurring wind- related disturbance. Sharp vertical gradients of stress confer great advantage to plants with low stature. thereby favoring shrubs and morphologically plastic tree species. The community is arrested in its vertical development because individual plants undergo cycles of tissue production and destruction. While the vegetation is static at the level of the population or individual. it is dynamic at the level of the shoot because of frequent dieback and regrowth. The cliff-summit bald on Mount Lookout resembles shoreline and high elevation alpine communities in which vertical gradients of stress disturbance favor short plant foms (e.g.. Art et a1. 1974. Boyce 1954. Marr 1977). Although the nature of the stress may vary (e.g.. salt contact vs. abrasion) the net effect remains the same: frequently (e.g. annually) disturbed communities are characterized by spatial and temporal patterns of tissue. as opposed to individual or population. replacement. This study demonstrates that sharp gradients of disturbance intensity and frequency can occur an inland landscapes which are low in elevation. Through a unique confluence of topographic and meteorologic features. a vegetation type has developed which is structurally similar to coastal and subalpine communities. LIST OF REFERENCES LISTOFREFERENCES Adams. C. 1905. An ecological survey of northern Michigan. Michigan Geological Survey Report for 1905. Art, H. W.. F. H. Bomann. G. K. Voigt. and G. M. Woodwell. 1974. Barrier Island forest ecosystem: The role of meteorological inputs. Science 184:60-62. Barbour. M. G. 1978. Salt spray as a microenvironmental factor in the distribution of beach plants at Point Reyes. California. Oecologia 32:213- 24. Baughman. R. G. 1981. Why windspeeds increase on high mountain slopes at night. United States Department of Agriculture. Forest Service. Intemountain Forest and Range Experiment Station. Research Paper INT-276. Billings. W. D. 1969. Vegetational pattern near alpine timberline as affected by fire-snowdrift interactions. Vegetatio 19:192-207 Black. R. A. and L. C. Bliss. 1980. Reproductive ecology of Picea mariana (Mill.) BSP. at treeline near Inuvik. Northwestern Territory. Canada. Ecol. Monog. 50:331-54. Bliss. L. C. 1985. Alpine. Pages 41-65 in B. F. Chabot and H. A. Mooney. editors. Physiological Ecology of North American Plant Communities. Chapman and Hall. New.York. New York. ' Boyce. S. G. 1954. The saltspray community. Ecol. Monog 24:29-67. Brady. N. C. 1974. The Nature and Properties of Soils. Eighth Edition. Macmillan Publishing Co.. New York. Braun. E. L. 1950. Deciduous Forests of Eastern North America. The Blakiston Co.. Philadelphia. Caldwell. M. M. ‘ 1970. The effect of wind on stomatal aperture, photosynthesis and transpiration of Rhododendron ferrugineum L. and Pinus cembr'a L. Cbl. Ges. Forstwesen 87:193-201. Clark. D. H. 1975. The Copper Mines of Keweenaw. D. H. Clark. Calumet. Mich. Collaer. P. 1941. Le role de la lumiere dans l'assimilation chlorophyllienne. III. Ber. Schweiz. Bot. Ges. 51:348-362. Darlington. H. T. 1930. Vegetation of the Porcupine Mountains. northern Michigan. Pap. Mich. Acad. Sci.. Arts and Let. 13:10-65. 60 61 Daubenmire. R. 1943. Vegetation zonatian in the Rocky Mountains. Bot. Rev. 9:325-393. Daubenmire. R. 1954. Alpine timberlines in the Americas and their interpretation. Butler Univ. Bot. Stud. 11:119-136. Decker, J. P. 1947. The effect of air supply an apparent photosynthesis. Plant Physiol. 22:561-571. Dorr. J. A. and D. F. Eschman. 1970. The Geology of Michigan. Univ. of Mich. Press. Ann Arbor. Dunwiddie. P. W. 1977. Recent tree invasion of subalpine meadows in the Wind River Mountains. Wyoming. Arct. Alp. Res. 9:393-399. Femald. M. L. 1935. Critical plants of the upper Great Lakes region of Ontario and Michigan. Rhodora 37:197-222;238-262;272-301:324-341. Femald. M. L. 1950. Gray‘s Manual of Botany. Eighth Edition. American Book Co.. New York. New York. Gates. D. H. 1980. Biophysical ecology. Springer-Verlag. New York. Geiger. R. 1966. The Climate Near the Ground. Harvard University Press. Cambridge. Mass. Given. D. R. and J. H. Soper. 1981. The arctic-alpine element of the vascular flora at Lake Superior. National Museums of Canada. Publications in Botany. No. 10. Gleason. H. A. and A. Cronquist. 1963. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. D. Var Nostrand. New York. Glime. J. M.. J. F. Gereau. and R. E. Gereau. 1980. Bryophytes of Mount Lookout. Keweenaw County. Michigan. Mich. Bot. 19:247-50. Grace. J. 1974. The effect of wind on grasses. 1: Cuticular and stomatal transpiration. J. Exp. Bot. 25:542-51. Grace. J. 1977. Plant Response to Wind. Academic Press. New York. New York. Griggs. R. F. 1938. Timberline in the northern Rocky Mountains. Ecology 19:548-64. Holroyd. E. W. 1970. Prevailing winds on Whiteface Mountain as indicated by flag trees. For. Sci. 16:222-229. Hygen. G. 1965. Water stress in conifers during winter. Pages 89-95 in B . Slavik. editor. Water Stress in Plants. Junk. The Hague. Jaffe. M. J. 1980. Morphogenetic responses of plants to mechanical stimuli or stress. BioScience 30:239-43. 62 Johnson. P. L. and W. D. Billings. 1962. The alpine vegetation of the Beartooth Plateau in relation to cryopedogenic processes and patterns. Ecol. Monog. 32:102-135. Jones. H. G. 1983. Page 248. Plants and Microclimate. Univ. of Cambridge. Cambridge. UK. Kincaid. D. T. and E. E. Lyons. 1981. Winter water relations of red spruce on Mount Monadnock. New Hampshire. Ecology 62:1155-61. Klikoff. L. G. 1965. Microenvironmental influence on vegetational pattern near timberline in the central Sierra Nevada. Ecol. Monog. 35:187-211. Lawrence. D. B. 1939. Some features of the vegetation of the Columbia River Gorge with special reference to asymmetry in forest trees. Ecol. Monog. 9:218-257. Lawton. 11.0. 1981. Wind stress and elfin stature in a montane rain forest tree: an adaptive explanation. Amer. J. Bot. 69:1224-30. Leach. W. 1925. Two relict upland oak woods in Cumberland. J. Ecol. 13:219-99. Lindsay. J. H. 1971. Annual cycle of leaf water potential in Picea engelmannii and Abies lasiocarpa at timberline in Wyoming. Arct. Alp. Res. 3:131- 138. Marchand. P. J. 1972. Wind and the winter-exposed plant. Rhodora 74:528-31. Marchand. P. J. and B. F. Chabot. 1978. Winter water relations of treeline plant species on Mount Washington. New Hampshire. Arc. and Alp. Res. 10:105-16. Marquis. R. J. and E. G. Vass. 1981. Distribution of some western North American plants disjunct in the Great Lakes Region. Mich. Bot. 20:62-82. Marr. J. W. 1977. The development and movement of tree islands near the upper limit of tree growth in the southern Rocky Mountains. Ecology 58:1159-64. McComick. J. and M. F. Buell. 1968. The plains: pygmy forests of the New Jersey pine barrens. a review and annotated bibliography. N. J. Acad. Sci. Bull. 13:20-34. Michaelis. P. 1932. Oekologische studien an der alpinen baumgrenze. Ber. Deutsch. Bot. Ges. 50:31-42. Noble. D. L. and R. R. Alexander. 1977. Environmental factors affecting natural regeneration of Engelmann spruce in the central Rocky Mountains. For. Sci. 23:420-29. Richards. C. D. 1952. Phytogeographical studies in the northern peninsula of Michigan. Dissertation. Univ. of Mich.. Ann Arbor. 63 Richards. J. H. and L. C. Bliss. 1986. Winter water relations of a deciduous timberline conifer. Larix lyallii Parl. Oecologia 69:16-24. Ritchie. G. A. and T. M. Hinckley. 1975. The pressure chamber as an instrument for ecological research. Adv. Ecol. Res. 9:165-254. Sakai. A. 1970. Mechanisms of desiccation damage of conifers wintering in soil-frozen areas. Ecology 51:657-64. Schimper. A. F. W. 1903. Plant-geography upon a physiological basis. Clarendon. Oxford. England. Slatyer. R. O. 1976. Water deficits in timberline trees in the Snowy Mountains of South-Eastem Australia. Oecologia 24:357-66. Smith. A. P. 1972. Notes on wind-related growth patterns of Paramo plants in Venezuela. Biotropica 4:10-16. Sprugel. D. G. 1976. Dynamic structure of wave-regenerated Abies balsamea forests in the north-eastem United States. J. Ecol. 64:889-911. State of Michigan. 1987. Michigan Administrative Code pages 98-105. (Amendment to Michigan Rule 299.1021-299.1028.of 1979.) Thompson. J. R. 1974. The effect of wind on grasses. 2: Mechanical damage in Festuca arundinacea Schreb. J. Exp. Bot. 25:965-972. Tranquillini. W. 1969. verschiedene starkere wind. Photosynthese and transpiration einiger holzarten bei Cbl. ges. Forstwesen 86:35-48. Tranquillini. W. 1976. Water relations and alpine timberline. Pages 473-491 in O.L. Lange. L. Kappen. E. D. Schulze. editors. Water and Plant Life. Ecological Studies 19. Springer-Verlag. Berlin-Heidelberg-New York. Vass. E. G. 1972. Michigan Flora. Part I. Gymnospems and Monacots. Cranbrook Inst. Sci.. Bloomfield Hills. Michigan. Wardle. P. 1965. A comparison of alpine timber lines in New Zealand and North America. New Zealand J. Bot. 3:113-35. Wardle. P. 1974. Alpine environments. Pages 371-402 in J. D. Ives and R. G. Barry. editors. Arctic and Alpine Environments. Weidlich. W. H. and J. A. Teeri. in subalpine black Webb. L. J. 1958. Cyclones as an ecological factor in tropical lowland rain forest in north Queensland. Aust. J. Bot. 6:220-228. Methuen. London. 1976. The occurrence of bisporangiate strobili spruce. Rhodora 78:6-15. Wells. J. R. and P. W. Thompson. 1974. Vegetation and flora of Keweenaw County. Michigan. Mich. Bot. 13:107-151. Westman. W. E. 1975. Edaphic climax pattern of the pygmy forest region of California. Ecol. Monog. 45:109-135. all. In I! 1..» .4-,-_I#IIM 64 Whittaker. R. H. 1956. Vegetation of the Great Smoky Mountains. Ecol. Monog. 26:1-80. Whittaker. R. H. 1967. Gradient analysis of vegetation. Biol. Rev. 42:207-264. Wilson. J. 1984.; Microscopic features of wind damage to leaves of Acer pseudoplatanus L. Ann. Bot. 53:73-82. ill/l l l 43 Jill) 00993 28 l 3 1293 l llllllllllllllllll l llllll