1:5
. r

 

.. I” o...
r: .M:

.: u #1.... .
2:3.#
I!» . f

.

«C. .
. f , a.-.
...mw;h.m.».§fl§
.3 «zihwti c e.
Wynne ;

.r. 41. 1.3....
5.1 L

b rixcdtha.
a v r a.

I...»
it}: 1

« .112-
1 £15.51:

«an

..
.1
“.5. our
4.0.!
. 1.;
e}..- a...
......E.

4. 13...?! 5.
:we £1. fine.
:1 .- .I .1. i;

Rafi. Irv-V...“

5 ‘Kwflff

(I

c .7r. .219“

 

IsfaXYL. v .12 .

V» « .01.}?
. r .A.‘)

.h..u..!i
.lveeJ.

y ~. .
e} I 2 J:
7 .3

I1

 

 

 

Wk.) H

(Tn

"‘
"IQ
1\|

Qg

This is to certify that the
thesis entitled

HYDRAULIC, BIOMECHANICAL, AND ANATOMICAL STUDY OF
XYLEM FROM FIVE TREE SPECIES OF ACER.

presented by

CARRIE LEIGH WOODRUM

has been accepted towards fulfillment
of the requirements for

M. S: degree in" Botany and Plant Pathology

id

Major professor

 

 

Date é/H/OL

0-7 639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

 

LIBRARY
Michigan State
University

 

 

 

PLACE IN RETURN BOX to remove this checkout from your record.
To AVOID FINES return on or before date due.
MAY BE RECALLED with earlier due date if requested.

 

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6/01 c:/CIRC/DaIeDue.p65-p.15

HYDRAULIC, BIOMECHANICAL, AND ANATOMICAL STUDY OF XYLEM
FROM FIVE TREE SPECIES OF ACER

By

Carrie Leigh Woodrum

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Botany and Plant Pathology

2002

ABSTRACT

HYDRAULIC, BIOMECHANICAL, AND ANATOMICAL STUDY OF XYLEM
FROM FIVE TREE SPECIES OF ACER

By
Carrie Leigh Woodrum

Possible tradeofl‘s between hydraulic conductivity and mechanical properties of
Acer negundo, A. saccharinum, A. rubrum, A. nigrum, and A. saccharum were assessed.
It has been shown that tradeoffs between xylem specific conductivity (K. max) and
modulus of elasticity (MOE) and modulus of rupture (MOR) occur in cogeneric Chaparral
shrubs and vines versus trees and shrubs. The current study attempted to determine if this
tradeoff is present in five cogeneric tree species in a similar habitat. Acer negundo, A.
sacchan‘num, and A. rubrum are considered soft maples and are known to be fast growing
and shade-intolerant. Conversely, A. nigrum and A. sacchamm are classified as hard
maples and are slow-growing and shade-tolerant. It was hypothesized that the soft
maples would have lower MOE and MOR, but higher K. m than the hard maples.
Minute anatomical and general morphological characteristics were measured in an
attempt to correlate them to any water transport and/or mechanical strength differences
between species. No difference was found between species in vessel diameter, water
conductivity, or percent embolism. Similarly, no tradeofl‘ was found between K. m and
MOE or MOR across the genus. Fiber lumen diameter was inversely correlated to both
MOE and MOR. Surprisingly, percent ray parenchyma was positively linearly related to
MOE. This suggests that transport/mechanical tradeofi‘s are not universal across every

genus within a similar environment.

DEDICATION

To my parents; for their eternal love and support.

iii

ACKNOWLEDGEMENTS

I would like to thank Dr. Frank Ewers for serving as my advisor and mentor. His
infinite knowledge and experience helped to make sense of a gaggle of data. Also, his
encouragement furnished reassurance when I needed it most. Thank you to my other
committee members, Dr. Frank Telewski and Dr. Pete Murphy for their advice and
discussions on interpretation of results.

I would especially like to thank Dr. Bradley Marks, Agriculture Engineering, for
teaching me the ins and outs of the Instron and its archaic software. Thank you to Dr.
Alan Jones, Plant Research Lab, for use of his Leaf Area Meter and lab space.

Many thanks to Kris Kern, Nick Mirsky, and Dawn Langdon for the pre-dawn
mornings and long days in the lab fighting with the Sperry Apparatus. I could not have

finished this project without you!

iv

TABLE OF CONTENTS

GENERAL BACKGROUND ....................................................................... l
Xylem Components ......................................................................... 1
Hydraulic Conductivity ...................................................................... 2
Biomechanics ................................................................................. 4
Tradeofi‘s ...................................................................................... 7

MATERIALS AND METHODS .................................................................. 11
Plant Material ............................................................................... 11
Site Description ............................................................................. 12
Vessel Lengths .............................................................................. 12
Hydraulic Conductivity: Variation among Species ................................... 13
Biomechanics: Variation among Species ................................................ 15
Intra-tree Hydraulic Conductivity and Biomechanical Variation .................... 17
Anatomical Study ........................................................................... 18

Maceration ......................................................................... 18
Cross Sections ..................................................................... 18
General Morphology .............................................................. 19
Data Analysis ............................................................................... 19

RESULTS ............................................................................................ 20
Vessel Lengths .............................................................................. 20
Intra-tree Hydraulic Conductivity and Biomechanical Variation .................... 20
Hydraulic Conductivity: Variation among Species ................................... 20
Biomechanics: Variation among Species ............................................... 20
Anatomical Study ........................................................................... 21

Macerations ........................................................................ 21

Cross Section ...................................................................... 21

General Morphology .............................................................. 22

Tradeoffs .................................................................................... 23
DISCUSSION ....................................................................................... 39
LITERATURE CITED ............................................................................. 46

vi

LIST OF TABLES

Table 1: Mean Maximum Vessel Lengths. Each mean is followed by one standard error.
11 = 10 ........................................................................................ 24

Table 2: Intra-tree Hydraulic Conductivity Variation. Each mean is followed by one
standard error. Values for K. in“... (kg m MPa'1 3") are 105. Values for KI. m
(kg m MPa‘1 5"), HV (m2 m'z), and LSC (kg MPa'l s'1 m") are 10". Units for
Km“ are (kg MPa'l s' m'l). n = 6 ...................................................... 24

Table 3: Intra-tree Biomechanical Variation. Each mean is followed by one standard
error. Values for El (N m2) and MOE SN mm'z) are 10’. Values for MORo
(Nmm'z) are 103. Units forIareCNmm ). n=6 .................................... 25

Table 4: Hydraulic Conductivity. Each mean is followed by one standard error. Values
for Kb initial (kg m MPa'1 5") are 10". Values for HV m2 m"? are 10". n = 20 for
K1. initial, and % Embolism. n = 18 for Km (kg MPa' 8'1 m' ), and HV ........... 26

Table 5: Dry Modulus of Rupture (MORD) (N mm'z). Each mean is followed by one
standard error. 11 = 6 ....................................................................... 29

Table 6: Macerations: Each mean is followed by i one standard error. n=8 for all
variables except percent vessel lumen area and fiber lumen diameter (n=18).
%V = percent vessels, %FT = percent fibers, %AP = percent axial parenchyma,
Vng = average vessel lumen diameter, VLm = maximum vessel lumen
diameter, V1.4“,fl = minimum vessel lumen diameter, VW = vessel wall thickness,
FL = fiber lumen diameter, FW = fiber wall thickness, PL = parenchyma lumen
diameter, PW = parenchyma wall thickness ............................................ 30

Table 7: Cross Sections. Each mean is followed by i one standard error. it = 18.
% Vessels = percent vessel lumen area, % Fibers = percent fiber area, % Rays =
percent ray parenchyma area, Vng = average vessel lumen diameter, VLm =
maximum vessel lumen diameter, VLm = minimum vessel lumen diameter,
HI) = hydraulic diameter, Vessel Frequency = 10“ vessels per urn2 ............... 31

Table 8: General Morphology. Means are followed by i one standard error. it = 20 for

Stem Diameter and Xylem Diameter. 11 = 18 for % Conductive Growth Rings and
Conductive Xylem Area .................................................................... 35

vii

LIST OF FIGURES

Figure 1: Mean Maximum Hydraulic Conductivity (KI. um) i one standard error.
n = 20 ........................................................................................ 25

Figure 2: Mean Leaf Specific Conductivity (LSC) : one standard error. n = 18. . . . . . ...26

Figure 3: Mean F lexural Stiffness (El) : one standard error. 11 = 20 ....................... 27
Figure 4: Mean Second Moment of Cross Sectional Area (I) i one standard error.

n = 20 ........................................................................................ 27
Figure 5: Mean Modulus of Elasticity (MOE) : one standard error. 11 = 20 ............... 28
Figure 6: Green Modulus of Rupture (MORG) : one standard error. n = 14 .............. 28
Figure 7: Mean Fiber Lumen Diameter : one standard error. 11 = 18 ....................... 29
Figure 8: Mean Pith Area 1 one standard error. n = 20 ....................................... 32
Figure 9: Mean Number of Growth Rings 3; one standard error. 11 = 20 ................... 32
Figure 10: Mean Bark Thickness : one standard error. n = 20 .............................. 33
Figure 11: Mean Green Wood Density (Densityo) : one standard error. it = 18 ......... 33

Figure 12: Dry Density (Densityp) versus Green Density (Densityg). Vertical lines are
one standard error of Densityo (n = 6). Horizontal lines are one standard error of
Densityo (n = 14). A = A. negundo, O = A. saccharinum, I = A. rubrum,

x =A. nigrum, )It =A. saccharum .......................................................... 34

Figure 13: Mean Distal LeafArea : one standard error. n = 20 ............................. 34

Figure 14: Modulus of Elasticity (MOE) versus Maximum Specific Conductivity
(K. m). Vertical lines are one standard error of MOE (n = 20). Horizontal lines
are one standard error of Km”: (11 = 18). A = A. negrmdo, 6 = A. saccharinum,
I = A. rubrum, x =A. nigrum, )K =A. saccharum ...................................... 35

Figure 15: Green Modulus of Rupture (MORG) versus Maximum Specific Conductivity
(K. m). Vertical lines are one standard error of MORG (n = 14). Horizontal lines
are one standard error of K. .m (n = 18). A = A. negundo, o = A. sacchan‘mm,

I = A. rubrum, x =A. nigrum, )It =A. saccharum. ..................................... 36

Figure 16: Modulus of Elasticity (MOE) versus Fiber Lumen Diameter. Vertical lines
are one standard error of MOE (n = 20). Horizontal lines are one standard error
of fiber lumen diameter (11 = 18). A = A. negundo, O = A. saccharinum,

I = A. rubrum, x =A. nigrum, X = A. saccharum ....................................... 36

viii

Figure 17: Green Modulus of Rupture (MORG) versus Fiber cell wall thickness : Fiber
lumen diameter. Vertical lines are one standard error of MORG (n = 14).
Horizontal lines are one standard error of FW:FL (n = 8). .......................... 37

Figure 18: Green Modulus of Rupture (MORg) versus Fiber Lumen Diameter. Vertical
lines are one stande error of MORG (n = 14). Horizontal lines are one standard
error of fiber lumen diameter (n = 18). A = A. negundo, O = A. saccharinum,

I = A. rubrum, X =A. nigrum, X = A. saccharum ...................................... 37

Figure 19: Modulus of Elasticity (MOE) versus Percent Ray Parenchyma. Vertical lines
are one standard error of MOE (n = 20). Horizontal lines are one standard error
of percent ray parenchyma (n = 18). A = A. negundo, O = A. saccharinum,

I = A. rubrum, x =A. nigrum, )rc = A. sacchamm ...................................... 38

ix

KEY TO ABBREVIATIONS

moment 0 area
C

vrty

c vrty

us 0 e
Dry us 0 rupture

o rupture
e 0 in a maceratron
identified as axial parenchyma
e o m a maceratron
identified as fibers.
0 cross sectr area
fiber
0 cross sectr area
ray parenchyma
e o m a maceratron
identified as vessels
0 cross sectr area
vessel lumen area
umen
ce
Average vesse umen ameter
vesse
um umen

 

GENERAL BACKGROUND
Xylem Components

The secondary xylem tissue (wood) of angiosperm trees is not a homogenous
material. It is a complex tissue composed of parenchyma, vessel elements, and fibers.
Each of these cell types serves an important role and will be addressed in turn.

Parenchyma is living tissue and serves several functions in wood. The pith of
a stem or branch is comprised of parenchyma. The cells are filler-tissue and the
surrounding wood takes on structural properties of a hollow cylinder (Niklas, 1992).
Additionally, rays in secondary xylem are made of parenchyma. Here, the cells assist
in storage of carbohydrates and minerals and in lateral water and mineral transport
(Mauseth, 1988). There can also be axial parenchyma within the wood, however, this
cell type is sparse in Acer and could be ignored for purposes of this study (Panshin
and de Zeeuw, 1980).

Vessel elements fulfill a plant’s need for water by acting as a transport
conduit. They have large diameter lumens and thin cell walls relative to other xylem
components. Since they lack end walls, vessel elements can stack end to end and
form long, uninterrupted tubes conducive to water flow. A negative pressure in the
xylem column occurs during transpiration when water is evaporated through stomata
of leaves (Dixon, 1914). Combined with the cohesion properties of water, these
factors allow xylem to act like a capillary tube, pulling water up through the trunk and
branches (Zimmerman, 1983; Pockman et al., 1995; Sperry et al., 1996).

Fibers are the principal load bearing cells in dicot angiosperm wood. They

serve as mechanical support and protect the other cell types fi'om mechanical damage.

These cells are elongated in the direction of the stem or branch axis. Their lumens
are narrow and they possess relatively thick cell walls (Niklas, 1992).

Theoretically, there is an infinite number of ways to construct wood. As long
as basic requirements are fulfilled, only the available resources and
evolutionary/gentic constrictions restrict the tree. On the contrary, ensuring the
ability to effectively execute basic functions, such as water conduction and
mechanical support, can be more complicated than one might think.

Hydraulic Conductivity

In order to photosynthesize and grow, a plant must be supplied with adequate
water. This immediately brings to mind rainfall or irrigation. Although having a
sufficient source of water is of course crucial, the physics of how it is transported is
just as important. Ideally, a plant needs to optimize water flow rates through the stem
and branches in order to maximize growth rate.

According to Poiseuille’s Law, theoretical hydraulic conductivity of a
capillary tube is related to the radius to the fourth power (Zimmermann, 1983).

Lp = rtr4/8u
Where, r is the radius of the lumen and u is the viscosity of the liquid. Since r is
raised to the fourth power, if radius is increased even slightly, hydraulic conductivity
dramatically increases. Thus, the hydraulic conductivity of a branch or stem segment
would be equal to the sum of the hydraulic conductivity for all vessels in a cross
sectional area. Theoretically, a tree with a few wide vessels can have the same
hydraulic conductivity as a tree with many, narrow vessels. However research

suggests that species tend to increase vessel diameter rather than vessel fi'equency to

increase water transport (Arnold, 1999; Gartner, 19913; Villar-Salvador et al., 1997).
This is probably because the larger diameter vessels contribute disproportionately
more to total conductivity than more narrow vessels. Furthermore, the production of
cell wall material is an energetically expensive process.

It was once widely accepted that larger vessel diameters increased the
susceptibility to embolism (Baas, 1983; Carlquist, 2001; Hargrave et al., 1994).
Embolism reduces the ability of a plant to conduct water by blocking the water
column with a bubble of air. If the extent of embolism is great enough metabolic
processes are disrupted and growth is limited (Schultz & Matthews, 1988).
Zimmennann (1983) proposed the idea that drought-induced embolism occurs via air-
seeding. This occurs when air is pulled into a vessel from an adjacent embolized
vessel. Sperry and Tyree (1988, 1990) and Sperry et a1. (1991) supported his
hypothesis. Sperry and Tyree (1988) developed an equation that could predict the
pressure gradient at which air seeding would occur. Their equation was dependent,
not on the conduit diameter, but the diameter of the pit pores on the pit membranes
adjoining two cells. Further research has supported their hypothesis (Sperry and
Tyree, 1988; Jarbeau et al., 1995; Alder et al., 1996).

The relationship of hydraulic conductivity to the architecture of the tree can be
determined by calculating specific conductivity (Ks). Specific conductivity is a
measure of the porosity of a branch or stem segment. It is equal to maximum
hydraulic conductivity divided by the cross-sectional area of the sapwood (Tyree and

Ewers, 1991).

A common value used to quantify an individual tree’s efficiency of xylem per
leaf area is the Huber value (HV). This value is defined as the cross sectional area of
the conductive tissue divided by the leaf area distal to that area (Zimmerman, 1983;
Tyree and Ewers, 1991). A tree that is more adapted to support rather than
conductivity will have a greater HV (Patino et al., 1995). Since water is less
available, cell lumens must be narrow to prevent extensive embolism, the occlusion
of a cell’s lumen by an air pocket (Sperry and Sullivan, 1992). Therefore, the tree
should be less efficient at conduction (i.e. more xylem area per leaf area, a greater
HV) but have greater mechanical strength due to a large cell wall to lumen area ratio
(i.e. greater wood density).

Trees with identical HV can have different amounts of water traversing that
area of xylem. For a given area, the number and lumen diameter of conduits affects
the quantity of available water. Therefore, another value, leaf specific conductivity
(LSC) is also a useful tool for describing the water conducting sufficiency of a tree.
This is equal to Kh divided by the leaf area distal to the segment. This value is
indicative of the relative amount of water available to the leaves. Leaf specific
conductivity is also equal to HV multiplied by K5 (Zimmerman, 1983; Tyree and
Ewers, 1991).

Biomechanics

The biomechanics of a tree are very important to consider because a tree’s
strength determines its ability to cope with both static and dynamic loading stress. If
the mechanical strength of the xylem is not sufficient, loss of branches due to

breakage will occur. This may be detrimental to the survival of the individual tree if

infection follows. Breakage of limbs also results in loss of photosynthetic areas and
reproductive structures. Since xylem makes up the majority of the cross sectional
area of a tree, it plays a crucial role in support. Self-loading due to gravity and forces
from environmental factors such as wind, rain, snow, and ice are of utmost concern.

These environmental factors impose a stress, and may cause deformation in
wood, called strain. The 510pe of a stress-strain curve is the ratio of external stress to
the resultant displacement. This value is called the Young’s modulus, or modulus of
elasticity (MOE), and is a function of an elastic strain (N iklas, 1992). An elastic
strain is reversible. The product of the elastic modulus and the second moment of
area (I), the distribution of mass in the cross section, is flexural stiffness (EI).
Flexural stiffness measures the ability of wood to resist bending. When the elastic
strains are exceeded, changes in shape and structure occur which may become
permanent, this is a plastic strain. Modulus of rupture (MOR) is a measure of the
force at which breakage occurs, as can occur with plastic strains (Niklas, 1992).

Trees often form reaction wood in response to displacement with respect to
the gravitational vector, a plastic strain (Telewski, 1995). For instance, reaction wood,
specifically tension wood in angiosperrns, is thought to be a mechanism for restoring
branches and vertical axes to the desired orientation. Tension wood is characterized
by an increase in gelatinous fibers with thicker cell walls than in normal wood
(Telewski, 1995; Telewski et. al. 1995). The larger amount of cell wall material
increases the density of the wood, and thus the mechanical strength.

The ratio of cell wall thickness to conduit lumen diameter and the number of

cells per unit area impacts the density and mechanical strength of wood (Dinwoodie,

1981). The number of cells in a cross sectional area influences strength because
neighboring cells act as buttresses which increases the stiffness of the tissue
(Easterling et al., 1982; Gibson and Ashby, 1982).

Mechanical strength properties of a particular wood will vary depending on
what type of stress the sample is subjected to. There are tension, compression, and
shear stresses. Wood has greater tensile and compressive strength in the longitudinal
direction than in radial or tangential (Wainwright, 1976). In the present study,
breakage of limbs due to self-loading due to gravity and weight from foliage, fruit,
snow, ice, and other environmental factors are of concern. Loss of limbs, and their
leaves and fruits, reduce the photosynthetic area and the contribution of offspring to
the next generation. Therefore, the present study will focus on bending strength.

Theory concerning the biomechanics of dried wood is well established as a
result of the use of wooden beams in construction (Gere and Timoshenko, 1984).
However, the biomechanics of green wood is a relatively new field. In applying
beam theory to trees we use several assumptions. For simplicity, branches are treated
as solid, homogenous cylinders despite the fact that wood is a anisotropic material
and its MOE depends on the amount and distribution of the various component layers
(N iklas, 1992). Secondly, green wood lacks a precise MOR (Spatz and Bruechert,
2000). A branch can be strained past the elastic phase and still not break, but is
plastically deformed. Although the tissue will be damaged, it is advantageous for the

tree to retain the branch if it is still functional.

Troika/fr

It seems intuitive that as vessel lumen area increases, conductivity would also
increase, but that the strength of the branch would be reduced due to less cell wall
material. However, wood is an anisotropic material and other anatomical variables
can confound the influence of the number and diameter of conduits. These include
variables such as percent tension wood, pith diameter, ray width, percentage of fiber
per unit area, and fiber cell wall thickness, to name a few. Since selective pressures
differ between environments, “optimum” structures vary by habitat.

Due to the complexity of the interactions between anatomy, hydraulic
conductivity and mechanical strength of wood, there have been few studies
addressing all three variables. Early work focused on just two factors at a time.

Gartner (1991a,b) showed that in Toxicodendron diversilobum HV was
smaller but that Ks max was greater, for the vine (supported) growth habit than for the
unsupported shrubs. This was due to the fact that the supported vines had similar
vessel fi'equency but greater lumen area than shrubs. It was found that although vines
had a smaller maximum fiber wall thickness than shrubs, the shrub was significantly
more dense only at p< 0.054. As a result, it was concluded that structural stability
was a fimction of the second moment of area rather than material stiffness.

Ewers and Fisher (1991) found that a vine in the genus Bauhinia had less
xylem per distal leaf area than a tree or shrub of the same genus. Vines allocate fewer
resources to xylem since they are not free standing and instead depend on other plants
or structures for support. To compensate hydraulically for the reduced xylem area,

vines have long and wide vessels. However, that study used specimens from a

botanical garden and not a natural habitat. In addition, the mechanical strength of the
test specimens was not directly measured.

Similarly, Chiu and Ewers (1992) found that free-standing shrubs had a
greater amount of xylem but a smaller percent conductive xylem than a vine of the
same genus (Lonicera). In this case, the plant dedicates the greatest portion of its
wood to mechanical support. Again, this study utilized specimens cultivated in pots
rather than native habitat and mechanical strength was not tested.

Wagner et al. (1998) compared two pairs of Chaparral shrubs in a similar
habitat. Adenostoma sparsifolium had significantly greater mean and maximum
vessel diameters, corresponding to a 34% greater vessel lumen area and a two-fold Ks
max, than A. fasciculatum. As a result, A. sparsifolium had a 14% smaller stem wood
density, 37% smaller MOE, and 30% smaller MOR. Similar tradeoffs occurred
between Ceanothus rnegacarpus and C. spinosus.

Although their study was innovative, there was one confounding factor. The
hydraulic conductivity and mechanical strength tests of a given species were not
performed on the same segments. In fact, the branches for each test came from
different sites. This is a problem because changes in selective pressures between sites
can affect the anatomical development and evolution of each population. In addition,
an average of conductivity and strength of a species is not as accurate as a particular
hydraulic conductivity value to a measured vessel lumen area and a strength value to
percentage of fiber and fiber wall thickness in a given branch. For this reason, the
following research will evaluate the conductivity, mechanical strength, and

anatomical characteristics of each segment.

This research was conducted in an effort to determine if tradeoffs really do
occur in terms of conductivity and mechanical strength, as a result of anatomical
characteristics. In light of past research, this study will examine five species of the
same genus and growth habit to determine if tradeoffs occur across multiple species.
A site where all species are native to the same general habitat has been chosen as the
test site. This will allow for moderate control over habitat-related physiological
differences. All species will be examined to quantify any differences in anatomy.
The results will be used as rationale for any significant variation between species in
hydraulic conductivity and/or mechanical strength properties.

Wood density is inversely correlated with growth and positively related to
longevity (Putz et al., 1983). Furthermore, Lawton (1982) showed that shade
tolerance was positively related to wood density. Thus, trees that are shade tolerant
are, in general, slow growing, long-lived and greater in wood density than shade
intolerant species.

The United States Department of Agriculture conducted research on strength
properties of commercially important woods including A. saccharum, A. nigrum, A.
saccharinum, and A. rubrum (Handbook 72). Measurements were taken in the green
condition (moisture content of 58 — 66%). The specific gravity of A . saccharum and
A. nigrum was 0.56 and 0.52, respectively. The specific gravity of Acer
saccharinum and A. rubrum was 0.44 and 0.49, respectively. They found that the
MOE and MOR for A. saccharum was the largest of all four species. Additionally, A.

saccharinurn had a significantly smaller MOE and MOR than the other species.

The microhabitat in which a tree typically grows may reflect the priorities that
have evolved for that species. For instance, A cer negundo is often found in moist
areas near strearnbeds and is a mechanically weak tree. On the other hand, Acer
saccharum prefers well-drained soils and is a much stronger tree (Barnes 1981).
Therefore, one might expect that A. negundo will have a greater KS max, and thus wider
vessel lumens, than A. saccharum due to a more abundant water source. Perhaps A.
negundo has evolved to conduct more water transport at the expense of mechanical
strength and A. saccharum evolved to maximize mechanical strength over
conduction.

Based on the above, the hypotheses were as follows: Acer nigrum and A.
saccharum are expected to be statistically similar and to have the most narrow vessels
and highest percentage of fiber content. Subsequently, they would have the lowest Ks
max and highest MOE and MOR of the five species. Significantly larger diameter
vessels and a smaller percentage of fiber are anticipated for A. saccharinum and A.
rubrum. Their. Ks max hypothesized to be comparable to one another, yet statistically
greater than that of A. nigrum and A. saccharum. Lastly, A. negundo is expected to
have the widest vessels and lowest percentage of fiber, resulting in the lowest MOE

and MOR with the highest Ks max.

10

MATERIALS AND METHODS
Plant Material

The timber industry divides the relative strength of timber produced by
various species into softwood (conifers) and hardwood (broad-leafed trees) according
to the hardness of the wood. Furthermore, within each division there is a relative
gradient of wood strength. Acer is a hardwood genus that has a diverse range of
strengths by species.

Soft maples include A. saccharinum L. and A. rubrum L and A. negundo.
These three species are included in Section Rubra Pax. Microscopically, these
species are virtually indistinguishable from one another. The woods are diffuse-
porous with rays that are never wider than the broadest vessel. Uniseriate rays are
common in A. rubrum but scarce in A. saccharinum which tends to have up to 5-
seriate rays (Panshin and dc Zeeuw, 1980).

Acer nigrum and A. saccharum are included within hard maples. In fact,
Gelderen (1994) places them both in Section Acer, Series Saccharodendron
(Rafinesque) Murray, A. saccharum Marshall ssp. saccharum being the type species
and the former, a subspecies, A. saccharum ssp. nigrum (Michaux f) Desmarais.
They also have diffuse-porous wood, but have two types of rays (Panshin and de
Zeeuw, 1980). The broader rays are mostly 5-7-seriate contrast with narrow rays,
which are typically uniseriate. A. nigrum and A. saccharum hybridize readily and

thus are often sold indiscriminately as hard maples (Panshin and de Zeeuw, 1980).

ll

Site Description

Several species of Acer are native to the southern Lower Peninsula of
Michigan. Five of these: Acer saccharinum L., Acer nigrum Michx. f., Acer rubrum
L., Acer'saccharum Marsh, and Acer negundo L., occur at Lott Woodlot in Ingham
County. Lott Woodlot is an undeveloped natural area under the management of
Michigan State University. The 17.8 ha area is dominated by Acer saccharum, Fagus
grandifolia, and Ulmus arnericana (Frye, 1976). The diversity of soil type,
topography and seed bank led Darlene Frye (1976) to divide the woodlot into ten
distinct microhabitats. These range from dry upland beech-maple forest in the north,
to the poorly drained floodplain around Felton Drain on the east.

After bud break the canopy of Lott Woodlot becomes closed, creating a
densely shaded understory. However, F elton Drain and several trails cut through the
site creating gaps in the canOpy. With a few exceptions, native maples are relatively
tolerant of filtered light. Acer saccharum and A. nigrum in particular thrive in the
shade (Barnes, 1981). In fact, the understory of Lott is dominated by A. saccharum
saplings (Frye, 1976). Acer negundo, in contrast, occurs mostly in disturbed or open
areas where sunlight is more abundant (Barnes, 1981).

Vessel Lengths

On five days between late May and early July in 2000 entire horizontal, first
order branches were collected from two individuals of each of the five species.
Branches were cut off and placed in black plastic bags with wet paper towels to
prevent dehydration. In the lab, within two hours of collection, the branch tip was cut

off (at a diameter of approximately seven mm) and the distal end of the remaining

12

branch was attached to a pressurized N2 tank. Maximum vessel lengths were
determined via modified air method (Zimmermann and Jeje, 1981). A pressure not to
exceed 172 kPa was applied as the proximal end of the branch was held under water.
The proximal end was cut back with shears until bubbles appeared from the xylem.
Care was taken to ensure bubbles had not originated from the bark or pith. The
length of each segment was then measured with a ruler and added to one half the
length of the last portion excised. The average axis diameter (with bark) of the distal
and proximal ends was measured using a digital caliper.
Hydraulic Conductivity: Variation among Species

On five days of late June and early July of 2000, and five days in July and
early August of 2001, one branch from each of two individuals of all five species was
collected. Branches were first order and from the lower crown. Efforts were made to
pick portions that were relatively straight, about eight millimeters in diameter,
unbranched, and without wounds.

The study of vessel length indicated that none of the species had average
maximum vessel lengths longer than 36 cm. Consequently, the branch was cut from
the tree at a length at least 36 cm proximal to the segment to be used for conductivity
measurements. The branches were then placed in a black plastic bag to reduce
evapotranspiration and subsequent introduction of embolism. In the lab, branches
were recut under water, to prevent introduction of new embolism, to a final length of
approximately 15 cm.

Leaves distal to the segment were collected and stored in a sealed plastic bag

in a refrigerator until leaf areas were measured, usually within a week. Any leaves on

the segment itself were retained on the branch until the termination of the
conductivity measurements and then stored like those aforementioned.

The bark was removed approximately three cm back from the cut ends of the
segments. The segments were then fitted with rubber tubing and connected to a
Sperry apparatus (Sperry et al., 1988). Degassed 10 mMol citric acid was filtered
through a 0.2 pm mesh Gelman filter to discourage microbial growth and allowed to
flow via gravity from a known height through the inlet pipe connected to the distal
and of the branch segment. An outlet pipe collected citric acid in a beaker on a
Sartorious ISO 9001 electronic balance. A timer was used to measure flow rate for
each branch segment in order to calculate hydraulic conductivity in the native state
(K.1 initial). Any emboli, native or mechanically induced during collection, were then
removed via perfirsion with citric acid at 172 kPa for fifteen minutes. Hydraulic
conductivity measurements and perfusion were alternated until conductivity remained
stable (Kh max), within ten percent, between consecutive trials for a given branch,
usually just two perfusions.

Following conductivity measurements, conductive vessels were stained using
0.5% Crystal Violet. Tubing was fitted to the distal end of each segment and filled
with the dye. When Crystal Violet could be seen at the proximal end of the specimen,
double distilled water was added to the tubing and allowed to flow through the stem
to flush out any excess dye. Branch segments were then sealed in a plastic bag with a
moist paper towel and refrigerated until mechanical testing, within ten days.

The average xylem cross-sectional area and heartwood areas were measured

for each segment using digital calipers. Since heartwood does not contribute to

14

conductive tissue this area was subtracted from the total cross sectional area of the
branch before conductivity calculations (Zimmermann, 1978). Hydraulic
conductivity (initial and max) was calculated using the equation:

Kh = F/(dP/dx) (Tyree and Ewers, 1991)
where F is the water flux (kg/s), and dP/dx is the pressure difference causing the flow
(MPa/m). Maximum specific conductivity was calculated and is defined as:

Ks = K, max / As (Tyree and Ewers, 1991)
where A, is the sapwood cross section area (m2). The percent embolism was
calculated via the equation:

% embolism = {(Kh max - Kh initial) / Kh max } "‘ 100 %

Leaf areas were measured using a LI-COR Portable Area Meter model L1-
3000 from Lambda Instruments Corporation. The leaf area of those leaves distal to
the segment was added to one half the leaf area of those leaves on the segment. The
total leaf area was used to calculate leaf specific conductivity via the equation:

LSC = K, / A. (Tyree and Ewers, 1991)
where A. is the total leaf area (m2). The Huber value, defined as:

HV = A5 / A. (Tyree and Ewers, 1991)
was also calculated.
Biomechanics: Variation among Species

In the summer of 2000 branch segments were allowed to remain at room
temperature for one to two hours before biomechanical testing. It has been shown
that at 20°C the MOE of wood drops dramatically. This is approximately room

temperature and thus, small fluctuations in room temperature can potentially have

15

significant effects on the measured strength values of the test specimens.
Consequently, during the summer of 2001 branch segments were transported from
refiigeration to the Instron in Styrofoam coolers. Branches were kept in the cooler at
a temperature of approximately 18°C until just before mechanical testing.

Mechanical strength testing was conducted on an Instron Universal Machine,
model 4202, using a four-point test with a compression load cell of 100 lb. The span
length (L), the distance between the two supported ends, was 13.5 cm. The load was
applied at two points along the span length. The distance between one supported end
and the nearest loading point (a) was 4.5 cm. The load cell was applied at a
crosshead speed of 20 mm/min. Stress versus strain data was collected every 0.1 m
using Cy4200 software on a computer networked with the Instron. Branches were
stressed until the load reached a maximum value (asymptote of the curve).

Due to the size, water content (assumed to be saturated) and juvenility of the
wood, the branches did not rupture. The point at which the bending moment reaches
a maximum is the critical strain and is the limit of the elastic range (Spatz and
Bruechert, 2000). Therefore, modulus of rupture was estimated using the load value
at the asymptote of the curve and the equation:

MOR=Pmax*a*rmajo,/I (modified from Ugural, 1991)
Where Pmax is the load at failure, rmajo, is the major radius of the branch segment
minus the pith, and I is the second moment of the cross sectional area for a hollow
ellipse:

1:“ (rmajor*rminor3)/4

16

as described in Gere and Timoshenko (1984). F lexural stiffness (El) was calculated
using the slope (P/V) of the linear portion (elastic portion) of the curve and the
equation:

EI=P/V (a2/12)(3L-4a) (Gere and Timoshenko, 1984)

F lexural stiffness was divided by second moment of area to get the modulus of
elasticity (MOE) of the wood.

In order to determine any correlation between estimated green MOR (MORG)
and actual dried MOR (MORD) a subset of six branches from each species collected
in 2000 was taken only through its elastic phase and then load was released, not
compromising the property of the wood. These branches were then oven dried at
60°C in a Lipshaw Incubator oven (model #249) until the weight of the segment was
stable (to 100th of a gram) from one day to the next. They were then fractured using a
four-point test and MOR was calculated.

Branches collected in 2000 were placed in brown paper bags until sectioning
(up to 16 months). Those from 2001 were restored in sealed plastic bags with a moist
paper towel in a refrigerator until thin sections were made (within one week).
Intro-tree Hydraulic Conductivity and Biomechanical Variation

Two first-order branches were collected from each of three trees of both A.
nigrum and A. negundo on three separate days during July 2001. Hydraulic
conductivity (initial and maximum), Ks max, % embolism, LSC, and HV were
calculated. Conductive vessels were stained with 0.5% Crystal Violet. Specimens
were tested on the Instron in the green state. Flexural stiffness, I, MORg, MORG

were calculated.

l7

Anatomical Study
Macerations

Macerations were made from eight control segments of each species collected
in 2000 and all samples from 2001. Thick and thin shavings were placed in Jeffiies
solution (1:1 10% Nitric acid: 10% Chromic acid) in a 60 degree oven for four days.
Shavings were then pelleted and washed three times with double distilled water and
stored in Glycerine. Wood macerations were then stained with Safranin and semi-
perrnanently mounted on slides. They were then analyzed using a light microscope
interfaced with a CCD video camera and multi—scan analog monitor (model VB 1000
CCD, Dage—MTI, Inc., Michigan City, Indiana, USA). NIH Image 1.5 analysis was
used to measure cell wall thickness and lumen diameters of 25 vessels, 10 fibers, and
10 axial parenchyma cells from each segment. Fiber cell wall thickness to lumen
diameter ratios were calculated a random cell count of 300 was used to determine the
relative abundance of each cell type.
Cross Sections

Eight branches from each species collected in 2000 and all branches collected
in 2001were used for further analysis of anatomical differences. Thin sections
approximately 40 pm thick were made from the middle of each branch segment using
a sliding rnicrotome. Sections were taken through a dehydration series of ethanol and
xylene (modified from Johansen, 1940) and mounted on slides using Perrnount.

Sections were analyzed using a light microscope interfaced with a CCD video
camera and multi-scan analog monitor. Image analysis was performed on a pie-

shaped wedge, bordered by rays, of a lateral side of each branch cross section. NIH

l8

Image 1.5 software was used to measure vessel lumen areas, fibers per unit area, and
ray parenchyma per unit area in the wood. In addition, two samples from each species
were sectioned and stained with phloroglucinol to qualitatively determine the
presence or absence of lignin in ray parenchyma cell walls.

General Morphology

Green wood density was determined for all branch segments. Mass was
measured on a Mettler AJ100 electronic balance and water displacement was
determined using a graduated cylinder. The dry wood density of the six branches
from 2000 that were oven dried was also calculated in order to access any correlation
between green and dried wood density.

Stem Diameter, xylem diameter, pith area, and cortex thicknesses were
measured with a digital caliper. The number of growth rings and the percentage of
rings that were conductive were determined with a Zeiss light microscope.

Data Analysis

All data were analyzed using SAS version 8.1. ANOVAs were run to identify

any differences between species for all variables tested. Means were graphed in

Excel 97 and trendlines plotted to attain R2 values.

19

RESULTS

Vessel Lengths

Differences in average maximum vessel lengths were not statistically significant
between species (p = 0.2012). Lengths ranged from 26 cm for Acer nigrum to 36 cm for
A cer saccharinum (Table 1).
Intra- Tree Hydraulic Conductivity and Biomechanical Variation

Variance between A. negundo and A. nigrum for Kh initial, Kh max, percent
embolism, Ks max, HV, and LSC were not statistically significant. Likewise, variances
between trees of a species for all parameters were not significantly different (Table 2).

Variances between A. negundo and A. nigrum for MOE, MORg, MORD, and I
were not statistically significant. F lexural stiffness was significantly different between
species (p = 0.0293). Variations between trees of a species for all parameters were not
significantly different (Table 3).
Hydraulic Conductivity: Variation among Species

Of the hydraulic parameters only Kh max (p = 0.0160) and LSC (p = 0.0180) were
significantly different between species (Figure 1, 2). No significant differences were
found for Kh initial, Ks max, percent embolism, HV, or LSC (Table 4).
Biomechanics: Variation among Species

Flexural stiffness exhibited a trend relatively similar to that which was
hypothesized (Figure 3). A. saccharum and A. nigrum had a much higher EI than that of
A. rubrum and A. saccharinum (p <0.0001). In addition, A. negundo had an E1 that was
significantly smaller than that of the hard maples (p < 0.0001). However, it was

statistically similar to the A. rubrum and A. saccharinum. Also, A. saccharum and A.

20

nigrum were significantly different in E1, the former maintaining a greater stiffness (p =
0.0145) (Table 5).

Sampled stems of Acer negundo had a significantly smaller I than the other four
species (p = 0.0360) (Figure 4). The hard maples had a significantly larger MOE (p =
0.0337) and MORG (p = 0.03 62) than A. rubrum and A. saccharinum, as predicted
(Figures 5, 6). Green modulus of rupture was smaller in A. negundo than in the A.
saccharum and A. nigrum (p = 0.0060) (Figure 6). However, no significant difference
between species was found in MORD (Table 5). Nor was there a statistically significant
correlation between MORG and MORD.

Anatomical Study
Macerations

Fiber lumen diameter was significantly different between species (p < 0.0001)
(Figure 7). Hard maples produced fibers much more narrow in diameter than those of
soft maples (p < 0.0001). In addition, A. negundo fiber lumens were greater in diameter
than those of A. saccharum and A. nigrum (p < 0.0001) (Table 6).

No significant differences were found between species for: percent vessels,
percent fibers, percent axial parenchyma, axial parenchyma lumen diameter and cell wall
thickness, average, maximum, and minimum vessel lumen diameters and cell wall
thickness, fiber cell wall thickness, and fiber cell wall thickness : lumen diameter ratio
(Table 6).

C ross Sections

Although there was not a statistically significant difference between species in

percent cross sectional area composed of ray parenchyma, the p-value was only 0.0642.

21

The means of A. saccharum and A. nigrum were greater than those of the other three
species. Acer negundo seemed to have a similar amount of ray parenchyma to that of A.
saccharinum and A. rubrum, but substantially smaller amounts than the hard maples. No
significant differences between species were found for percent vessel lumen area, percent
fiber area, average, maximum and minimum vessel lumen diameters, hydraulic diameter
and vessel densities (Table 7).

Qualitative analysis of lignin in ray parenchyma cell walls revealed that all five
species were similar in having lignified ray cells. Furthermore, the relative amount of
staining was similar for ray parenchyma and vessels of the same branch segment.

General Morphology

Pith area of A. negundo was significantly greater than that of the other four
species (p = 0.0002). Additionally, A. saccharinum possessed a greater pith area than A.
rubrum (p = 0.0261) (Figure 8). Acer saccharum had, on average, more growth rings
than that of A. nigrum (p = 0.0105), meaning the stems were older and slower growing
(Figure 9). Bark thickness was significantly greater in the three soft maples than for the
two hard maples (p = 0.0003) (Figure 10).

As expected, the green wood density of hard and soft maples was significantly
different, A. saccharum and A. nigrum with much more dense wood (p = 0.0214).
Furthermore, A. negundo wood was the least dense of all species (p = 0.001) (Figure 11).
There was a statistically significant correlation between green and dry density (p =
0.8233) (Figure 12).

Significant differences in distal leaf area were found (p=0.0012). Hard maples
had a greater leaf area than that of soft maples (p=0.0017). Also, the leaf area of A.

saccharum was drastically greater than that of A. nigrum (p=0.0268) (Figure 13).

22

No significant difference was found between species for area of conductive tissue,
stem diameter, xylem diameter, and percent conductive rings (Table 8).
T radeoffis

Contrary to the hypothesis, there was no inverse relationship between

MOE and Ks max (R2 = 0.0097) (Figure 14). Similarly, there was no relationship between
MORG and KS max (R2 = 0.0876) (Figure 15). Percent ray parenchyma in the cross
sectional area was highly correlated with MOE (R2 = 0.8006) (Figure 19).

Percent fiber area in a cross section was correlated with differences in MOE (R2 =
0.461) and MORG (R2 = 0.4555). Although there was an inverse relationship between
percent fiber area and MOEG and MORg, fiber lumen diameter is a better predictor of
MOE (R2 = 0.6806) (Figure 16). Fiber cell wall thickness : lumen diameter was highly
correlated with and MORG (R2 = 0.7233) but fiber lumen diameter alone accounted for

more variation in MORG (R2 = 0.8751) (Figure 17, 18).

23

Table 1. Mean Maximum Vessel Lengths. Each mean is followed by one standard error.

 

 

 

 

 

 

 

n = 10.
Vessel Lengths (cm)
A. negundo 27.09 i 3.363
A. saccharinum 36.04 i 3.68]
A. rubrum 31.17 i 4.934
A. nigrum 25.80 i 2.805
A. saccharum 26.05 i 2.084

 

 

 

Table 2. Intra-tree Hydraulic Conductivity Variation. Each mean is followed by one
standard error. Units for Kh initial (kg m MPa'1 5") are 105. Units for K1. max
(kg m MPa’l s"), HV (en2 mi), and LSC (kg MPa" s" m") are 10“. Units for K,

max are (kg MPa'1 3" ml). 11 = 6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Kh initial Kh max % Ks max HV LSC
Embolism
A. negundo 1 6.37 i 1.21 i 43.6 i 3.50 i 1.42 i 4.83 i
1.49 0.265 11.6 0.617 0.130 0.696
A. negundo 2 4.44 _-_l-_ 1.55 i 43.0 i 5.15 i 1.31 i 6.94 i
0.649 0.895 13.4 2.70 0.228 4.11
A. negundo 3 7.23 i 1.33 i 44.5 i 4.63 fig 1.41 i 5.60 i
1.41 0.199 6.60 0.847 0.283 0.639
A. nigrum 1 7.78 i 1.75 i 47.9 i 5.80 i 1.00 i 5.83 i
1.58 0.445 10.5 1.49 0.129 1.70
A. nigrum 2 7.63 i 2.04 i 49.4 i 5.76 i 0.561 1 3.26 i
1.27 0.676 12.0 1.70 0.0188 0.987
A. nigrum 3 4.02 i 0.792 : 46.6 i 2.69 i 0.860 : 2.77 i
0.936 0.186 9.62 0.324 0.298 1.39

 

24

 

Table 3. Intra-tree Biomechanical Variation. Each mean is followed by one standard

error. Units for E1 (N mmz) and MOE (N mm'z) are 105. Units for MORG
(N mm'z) are 103. Units for I are (N mm4). n = 6.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

E1 1 MOE MORG
A. negundo] 7.27 i 0.952 15.2 i 3.13 0.555 i 0.103 0.708 i 0.111
A. negundo 2 7.51 i 1.01 14.0 i 6.08 3.01 i 2.40 1.91 i 1.20
A. negundo 3 5.80 i 0.504 11.2 i 3.97 1.83 i 0.830 1.37 i 0.521
A. nigrum 1 12.7 j: 1.40 13.2 i 2.42 1.08 i 0.157 1.24 i 0.152
A. nigrum 2 13.8 i 1.25 15.0 i 3.30 1.28 i 0.371 1.38 i 0.255
A. nigrum 3 8.55 i 1.56 10.7 i 4.04 1.55 i 0.437 1.51 i 0.252
1 ,. a l
. 7a 5 i
3 '7“ 4 i
, a.
E 3
E
l g 2
i v 1
i 2
‘z 0 :
: E 060 101* {of {05° {011° ,
((613) ‘086 <30 is?” 060° !
b. 090 ' f 50' [
IVS f i

Figure 1. Mean maximum hydraulic conductivity (Kh max) : one standard error. 11 = 20

25

 

 

 

LSC(Itg MPa"s“m°‘)
o-smwbmmxioocoo

 

 

 

 

 

 

 

Figure 2. Mean Leaf Specific Conductivity (LSC) : one standard error. 11 = 18

Table 4: Hydraulic Conductivity. Each mean is followed by one standard error. Units
for K], initial (kg m MPa'l s") are 105. Units for HV (1112 m'z) are 10“. n = 20 for
Kh initial, and % Embolism. n = 18 for Ks max (kg MPa'1 5'1 ml), and HV.

 

 

 

 

 

 

 

 

 

 

 

I(h initial % Ks max HV
Embolism
. negundo 4.66 i 55.2 i 8.56 i 1.24 i
0.793 7.58 1.61 0.158
. saccharinum 4.21 i 55.0 i 4.69 i 1.70 i
0.751 6.97 0.476 0.402
. rubrum 4.36 i 54.7 i 5.85 i 1.21 3:
0.760 7.21 1.03 0.149
. nigrum 4.76 i 54.1 :1; 4.53 i 1.14 1:
0.766 6.46 0.467 0.166
. saccharum 4.91 i 61.1 i 6.24 i 1.13 i
0.833 5.77 0.860 0.191

 

26

 

 

 

 

 

 

 

EI (10‘ N mmz)

 

0
v. 9,5,0 Y‘ vi ng’bo

 

 

 

Figure 3. Mean Flexural Stiffness (EI) : one standard error. 11 = 20.

 

60*

5 5o

:40
E

E 30
Z

: 2o

10

 

 

Figure 4. Mean Second Moment of Cross Sectional Area (I) i one standard error. n = 20

27

 

 

 

 

 

NCO-50103

MOE (10‘ N mm”)

 

 

 

 

Figure 5. Mean Modulus of Elasticity (MOE) : one standard error. 11 = 20.

 

800
700
600
500
400 .
300 ~
200
100

MORG (N mm")

 

6° 9, s“ gee“ (08 vgef l
5‘ k' 1’" 9e00
v.

 

Figure 6. Green Modulus of Rupture (MORg) 3; one standard error. r1 = 14.

28

Table 5: Dry Modulus of Rupture (MORD) (N mm'z). Each mean is followed by one
standard error. 11 = 6.

 

 

 

 

 

 

 

Species MORD

A. negundo 650 + 56.6
A. saccharinum 541 + 36.6
A. rubrum 539 + 76.3
A. nigrum 832 + 140
A. saccharum 807 + 256

 

 

 

 

 

 

E
e 12
0
E 10
§ 8
0
E 6
.2
0 4
5
E 2
3 i
T.‘ 0 . a
3 ° 6‘ 6‘ (o i
._ O ;
LL 090° 0860 60 0Q?) 6‘0‘0

.° 060 V: F" 9°

1" 9'0 e

k“ 1*

 

Figure 7. Mean Fiber Lumen Diameter : one standard error. 11 = 18.

29

 

 

 

 

 

 

 

183 133 183 .83 1:3 1:3 m2 3:. 8.: e: 83 :33

+8; +3.» 323 +8.: +~3 +23 3.2 “So “Nam 33 :2 +33 53382.
[$3 183 1383 123 123 :23 in: 18.". :83 1%.: it: 123

+32 33 +2.3 +2: +22. +33 +3: +3.2 +4.2 +3... +4.: +23 233:.
133 183 123 153 :33 .33 132 no: 183 SN 1:: :23

+3: +23 +3.3 +32 :3 +33 I: :2 +62 .13.: +3” +2; ease.
:33 123 123 123 183 .33 ..S3 18:. :32 no.2 183 :83

+2.: +83 +23 +:._ +3: +23 +2. +3n +3.2 3.: +4.; :3 sausages.
.33 1:3 183 1E3 183 183 to: IFN :3: 123 1:3 123

+8: :3 +33 +8: +3: +33 +3: 1.8 +3.3 :2 +5.2 13 82%»...
A83 A65 A83 A5: A83 A83 9:: :5:

3.. .E is: a: d 25 53> 2.5 9:5 mass. Ex. >3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

$05.25 :95 Egg u 3m .3836

30

:25: 9:20:23 u A: 53:86 :25: ” macs—0.5 :95 :25 u an”? £35.02: :85 RE u 3.”— :8083: :25: Saw

u H..— .mmofiafi :85 3%.? n 25 58:86 :25: $39» 8:83.: .1. car; .3883: :25: .032, 8:83.: u .275 coon—5%

:25: 3%? 0396 u 9:5 6:55:23 Ex: “:83: u .333 .Eonm “:88: .1. EX. £03? “:35: n >..\.. .QWS 36:86
:08... :26 v5 3%? E83: £088 83%.? =m :8 mu: :83 285% 0:0 H .3 330:8 n: :88 comm ”.80quan .w 03:...

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

33 83 £3 a: 83 N2 3.: 83

H 33 H 3.: H 33 H 3:. H E: H :3 H «.3 H 3: 5:385. .w
83 v3 33 N3 33 on: N3 83

H3: H33: H33 H 3v H98 Ho: H33 H _.: SEE .w
23 $3 $3 $3 83 2.: NE 83

H 8: H SN H 33 H 3.2 H 3: H 2.3 H «.3 H «.2 82%: .V
2 3 a: 83 :3 $3 $3 2; 83

H m3 H 3m H v: H 3% H 3.3 H 2.: H 33 H 02 5:25:88 .w
23 $3 83 :3 $3 83 m: 8.:

H :3 H2; Ho: Ho: H3~ H33 H53 H33: :33: .w
3333:: A83 5&3 A83 A83 and 838m £083,
88> a: j 54> M5:, 3 x. .x.

 

 

.u88 3: £0.33 2-2 n 3333:: 39:5 .3388: 02.3.8»: H mm

31

.3388: 38:. 3mm? 8:888 u 54> .3388: 38:— 33? 8:882: n 33> .3388: 383 .033 3333
u 93> .83 388033: +5: 883: n 988 ..\o .33 3:8 883: n 838..— .x. .83 382 .03? 883: .I. m_ommo> .x.
.2 u : :83 83:83 3:: H 3 326:8 mm :88 :oam .m:o:oom 880 s 333

 

 

 

PithAreumm’)
O —l N on h 0| 0)

 

Figure 8. Mean Pith Area i one standard error. 11 = 20.

 

 

Number of Growth Rings
0 -|~ N 0) h» 01 O) \l on (D

 

 

Figure 9. Mean Number of Growth Rings : one standard error. n = 20.

32

 

 

Bark Thickness (mm)

 

6° <0 <0 <0 <0
Q)“ 59° 6° ‘° 0“)
.0“ c)“ x» \N 90°C)

 

 

Figure 10. Mean Bark Thickness : one standard error. n = 20.

 

1.15
1.10
1.05
1.00
0.95
0.90
0.85

 

 

Densityc (g cm")

 

 

Figure 11. Mean Green Density (Densityg) : one standard error. n = 18.

33

 

 

R2 = 0.8233

0.8
0.75

 

.9
N

 

0.65

.0
a:

0.55

Densityo (g cm”)

.0
m

I 0.45
E 0.4
;

 

0.9 0.95 1 1.05 1.1 1.15 f
Densitys (0 cm") !

Figure 12. Dry Density (DensityD) versus Green Density (Densityg). Vertical lines are
one standard error of DensityD (n = 6). Horizontal lines are one standard error of
Densityg (n = 14).

A = A. negundo, O = A. saccharinum, I = A. rubrum, x =A. nigrum,
)K = A. saccharum.

 

 

 

 

 

. "E
l I; ,

2
i <
: ”:7
;' '.
5 I

.2
o l
006° «6° 0“ 0“ 0‘“ I
‘ .0 05‘ V" Y“ 90" i
i V‘ 90 V 1
L r E

Figure 13. Mean Distal Leaf Area : one standard error. n = 20.

34

Table 8. General Morphology. Means are followed by i one standard error. n = 20 for
Stem Diameter and Xylem Diameter. n = 18 for % Conductive Growth Rings and
Conductive Xylem Area.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Species Stern Xylem % Cond. Cond.
Diameter Diameter Growth Xylem Area

(mm) (mm) Rings (mmz)
A. negundo 7.55 i 0.139 5.60 i 0.322 3.83 i 0.326 24.2 i 2.13
A. saccharinum 7.40 i 0.117 5.56 i 0.238 4.89 i 0.332 22.3 i 1.74
A. rubrum 7.49 i 0.168 5.54 i 0.251 5.89 i 0.582 24.0 3: 1.61
A. nigrum 7.38 j; 0.136 5.58 p: 0.275 5.61 i 0.710 25.5 i 1.90
A. saccharum 7.49 _+_ 0.138 5.69 i 0.296 5.11 j: 0.387 27.5 i 1.65

R2 = 0.0097

5.5 T g

1

5 i

E 4.5 —— ...— : X .

z 4 — —— — 1,

1—-.‘——-1 1

E 3.5 I j- L

'5 3 “T'% | v _1

s ._+_. l 1-

2.5 .. L —i

2 p x

3 4 5 6 7 8 9 10 11

K. max (k9 MP3" 8'1 m")

 

 

Figure 14. Modulus of Elasticity (MOE) versus Maximum Specific Conductivity
(Ks max). Vertical lines are one standard error of MOE (n = 20). Horizontal lines
are one standard error of Ks max (n = 18).
A = A. negundo, O = A. saccharinum, I = A. rubrum, x =A. nigrum,
)K = A. saccharum.

35

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

800 1'

. 750 . ._ -- _ - —- —?
; 1 1
. .g- 700 -—» T 5-.

E H—fi l
1 E 650 fi “'4 .55: —— ~

1:? 550 " —< ;
' S 500 T. d ‘ ‘ *
l 400 . ,1
~ 4 5 5 7 8 9 10 11 3

KS m, (kg lillPa'1 s'1 m")

 

 

Figure 15. Green Modulus of Rupture (MORG) versus Maximum Specific Conductivity
(Ks max). Vertical lines are one standard error of MORG (n = 14). Horizontal lines
are one standard error of Ks max (n = 18).
A = A. negundo, O = A. saccharinum, I = A. rubrum, X =A. nigrum,
)K = A. saccharum.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R2 = 0.6806
5.5 - ~ - 5 -
5 ti _____ a . 7".
“7‘ 4.5
E I—x—I
Z 4 ___ \ _ _
E 3 5 i'i_' \ T
8 3 _ _ |—l \T _-
s L .-
2.5 — a:
2 .
7 8 9 10 11 12
Fiber Lumen Diameter (microns)

 

 

Figure 16. Modulus of Elasticity (MOE) versus Fiber Lumen Diameter. Vertical lines
are one standard error of MOE (n = 20). Horizontal lines are one standard error
of fiber lumen diameter (n = 18).
A = A. negundo, O = A. saccharinum, I = A. rubrum, X =A. nigrum,
)K = A. saccharum.

36

 

R2 = 0.7233

 

750
700
650
600
550
500
450

400
0.07 0.09 0.11 0.13 0.15 0.17 0.19 0.21

FW:FL

 

 

MORG (N mm")

 

 

 

 

Figure 17. Green Modulus of Rupture (MORG) versus Fiber cell wall thickness : Fiber
lumen diameter. Vertical lines are one standard error of MORG (n = 14).
Horizontal lines are one standard error of FW:FL (n = 8).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R2 = 0.8751

1 750 1.
1
I 700 ‘
. .. HR T
P‘E 650 . - I
5 E 600 *--—1
‘ 5 -_ N ..
; a, 550 “ ‘ "“‘—‘ ’ l\ (
1 g 500 -—— T 3
-. 450 .. ll

400 . j . *

7 8 9 10 11

Fiber Lumen Diameter (microns) i

 

Figure 18. Green Modulus of Rupture (MORg) versus Fiber Lumen Diameter. Vertical
lines are one standard error of MORG (n = 14). Horizontal lines are one standard

error of fiber lumen diameter (n = 18).
A = A. negundo, O = A. saccharinum, I = A. rubrum, x =A. nigrum,
)K = A. saccharum.

37

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R2 = 0.8006
5.5 . 5
5 ,_ _
'E 4.5 —*—~—~# __m____.:., 3‘ fl
E P /
z 4 " “ _
E 3.5 -- r T/ '
g i l "——i
E 3 — 1 : J.
2.5 —— a
2 . .
4 5 6 7 8 9 10
Percent Ray Parenchyma (%)

 

 

Figure 19. Modulus of Elasticity (MOE) versus Percent Ray Parenchyma. Vertical lines
are one standard error of MOE (n = 20). Horizontal lines are one standard error

of percent ray parenchyma (n = 18).
A = A. negundo, O = A. saccharinum, I = A. rubrum, x =A. nigrum,

)K = A. saccharum.

38

 

DISCUSSION

The present results show a case where differences in mechanical properties
can be related to fiber anatomy. This differs from studies where mechanical
differences were related to vessel diameter (Wagner et al., 1998; Chiu and Ewers,
1992; Ewers and Fisher, 1991; Gartner 1991a,b). Since the five Acer species were
not significantly different in Ks max , the lack of a negative linear relationship between
KS max and MOEG is not surprising. It is important to realize though that all but one of
those studies compared contrasting growth habits within a genus. At best, Wagner et
a1. examined two pairs of Chaparral shrubs but conductivity measurements were taken
on segments collected at one site while mechanical strength properties were assessed
on different segments from an additional two sites.

The explanation for a lack of correlation between Ks max and MOEG or MORG
in the current study is two-fold. First, K1. max was independent of conductive xylem
area (R2 = 0.0013). Second, the method for calculating MOEG and MORG was
biased. Pith and cortex areas were removed.

When the means of all five species were included in regression between
conductive xylem area and K, max, R2 was only 0.0013. Yet, the variance of Kh mm,
was so large for A. negundo that when the species is ignored and the remaining four
are regressed R2 increases to 0.6709.

There are two possible reasons for a large variance in Kh max of A. negundo.
First, this species prefers disturbed habitat. Specifically, amounts of water and light
can fluctuate dramatically temporally and spatially. To the extent that the anatomy is

impacted by microenvironment, conductivity will vary accordingly. Large variance

39

 

in Kh max may also be due to branch order. Abortion of terminal branch buds is
frequent in A. negundo (personal observation) and lateral branches take over as the
dominant leader. These were then considered to be the first order branch.
Zimmermann (1978) found that in diffuse-porous tress there is a hydraulic
constriction at the base of each branch. Furthermore, Ewers and Zimmermann
(1984a, b) showed that HV was greater and LSC smaller in second order branches
than in first order Abies balsamea and Tsuga canadensis. It is possible that second
order branches have fewer and/or smaller diameter vessels than first order,
subsequently reducing Ks max and LSC and increasing the HV. I am unaware of any
literature on this topic to date.

The second reason for lack of a tradeoff between Ks max and MOEG is based in
the calculation of I. Flexural stiffness is determined, in part, by the relative
contributions to the second moment of area by the pith, xylem, and cortex. The
contribution of the cortex is low, except near the tips of stems and branches where it
can make up 10 — 20% of the second moment of cross sectional area (Bruchert et al.,
2000). This results in a decrease in El. Although I was controlled for in the field by
collecting branches approximately eight mm in diameter, only the xylem area was
used in calculating branch second moment of area in this study, based on findings by
Wagner et at. (1998). This could easily explain differences in El between soft and
hard maples found here since the soft maples had significantly thicker cortex. It
might also explain why MOE for A. negundo was greater than expected. Second

moment of area for A. negundo was smaller than the other species. Since MOE was

40

derived by El divided by I, an underestimation of I by eliminating cortex would
overestimate MOE.

There was one additional problem in the calculation of mechanical properties.
There was no consistency in the orientation of the branch segments in the Instron.
Whether the branch rested on its major or minor axis was not controlled in either
year. In 2001 the orientation was recorded but no trend was found. The difficulty
arises with the fact that the compression and tension sides of the branch have different
mechanical strengths. Fredericksen et al. (1994) found that the compression side of a
bent loblolly pine stem had significantly smaller MOE and MOR than control stems.
Ideally, the branches would have been tested on the major axis with tension on the
upper side as they occur in the field. However, since orientation seemed to be
random, strength properties of some branches were inevitably overestimated.

Significant differences in LSC can be attributed to leaf area since Ks max was
not statistically significant between species. Greater leaf areas were found in the
shade tolerant hard maples lowering LSC values dramatically. Although Schultz and
Matthews (1993) found that leaf area did not change significantly between shaded
and sun plants of grapevine, they did find a difference in leaf area ratio (cm2 leaf
area! g plant dry weight). It is possible that the genus Acer has adapted to shade
environments but increasing leaf area ratios instead of, or in combination with,
greater plant dry weight. Additionally, shaded environments require less water use by
leaves and so the relationship between leaf area and xylem area may be altered.

The relative trends in biomechanical data for green branch tips from this

experiment were supported by dried lumber data published by the USDA (Handbook

41

72). Acer saccharum had the highest EI, MOE, and MOR followed by A. nigrum, A.
rubrum, and A. saccharinum, respectively. However, since a statistically significant
linear relationship between MORG and MORD was not found in the current research,
it is not recommended that strength values in the green state be used as predictors for
strength of dried wood.

Specific gravity is equal to the density of wood over the density of water.
Density is the mass per unit volume of the wood. The two measurements are related
in that specific gravity is a unit-less number denoting relative wood density (Panshin
and De Zeeuw, 1980). Increases of percent fiber associated with decreasing percent
axial parenchyma have been shown to be the best predictor of increases in specific
gravity. Increases in fiber wall thickness and decreases in fiber lumen diameter also
significantly related to greater specific gravity but to a lesser extent (McDonald et al.,
1995)

There was no difference in percent area composed of fibers but the number of
cells within that area was not determined. Although cell wall thickness was not
significantly different between species fiber lumen diameter was significantly smaller
in hard maples. It might be expected then, that hard maples have a greater density of
fibers.

Jeronimidis (1980) supports a correlation between decreased fiber lumen
diameter and increased strength as found here for Acer species. Failure under
compression is a function of the modulus of elasticity and cell wall thickness divided
by the lumen diameter. Subsequently, a decrease in lumen diameter could support a

greater load before buckling.

42

The 82 layer makes up nearly 80% of the cell wall area and is where
mechanical strength of the wood is ultimately determined (Jeronimidis, 1980). More
specifically, the angle of the microfibrils in the 82 layer is one factor that affects the
relative stiffness of the wood. In wood formed in response to flexing, the microfibril
angle in the cell wall is increased, resulting in a more bendable tissue (Telewski,
1989). It would be useful then to determine if low microfibril angle is correlated with
high MOEG in these cogeneric species.

Latewood has a high percentage of cell wall material per volume which
translates into a high wood density and specific gravity (Koslowski, 1971). As a
result, as the amount of latewood in the annual rings increases so does the strength of
the wood (Dinwoodie, 1981). The relative amount of latewood versus earlywood was
not quantified. Based on the differences in MOEG and MORG it would be expected
that a greater portion of a given ring would be composed of latewood in hard maples
than in soft maples.

This is the first time that a high correlation between percent ray parenchyma
and compressive MOE has been shown. In contrast, a positive correlation between
volume fraction of rays and transverse or radial tensile strength is well established
(Beery et al., 1983; Schniewind, 1959; Burgert et a1, 2000). However, these
relationships may just be coincidental. Hard maples are slow growing, long-lived
trees. This life history may require a greater volume of ray parenchyma in order to
store adequate amounts of starch for reserve in lieu of prolonged environmental
stresses. The narrower fiber lumens may more that compensate for the potential loss

in structural support from high amounts of parenchyma.

43

Although a qualitative study was conducted on the presence of 1i gnin in the
ray parenchyma, a quantitative study would be more useful. Chafe (1974) and
Murakami et al. (1999) differentiated between two types of ray parenchyma cells,
those that are adjacent to vessels, contact cells, and those that are not in contact with
vessels. Both studies showed that complete lignification of the contact cells was
delayed until heartwood formation. Conversely, ray parenchyma not associated with
vessels differentiated during the same year as their formation. Thus, the trend of
increasing ray parenchyma area and mechanical strength from soft to hard maples
may be explained by a greater amount of non-vessel associated ray parenchyma cells
in hard maples.

One important factor to consider in this study is microhabitat. All the
specimens were from the same 44-acre woodlot where general climate and
photoperiod are similar. However, there exist many microhabitats in the area as
categorized by Frye (1976). Wood density is correlated to a tree’s ability to tolerate
shade and its demographic habitat (Lawton 1984). Shade tolerant species have more
dense wood than shade intolerant species. This correlation was supported by the
distribution of the five species within the woodlot. Acer saccharinum and A. negundo
were found in greater light microenvironments than A. saccharum and A. nigrum.
Thus, environmental factors such as soil moisture, light intensity, and extent of
disturbance may have differed appropriately for each species. In order to reduce
possible effects from microhabitat differences, samples collected in summer 2001

were all sun branches.

44

Further experimentation in a controlled greenhouse would be ideal. There,
access to light and atmosphere conditions would be identical. Several plants of each
of the five species could be grown in individual pots to control population density.
Soil water content and nutrient availability could be manipulated so as to determine
potential phenotypic plasticity in each of the species. The five species may be not
only similar in MOE and MOR, but also in Ks max. In combination with several
treatments of differing light intensity this could determine if differences in MOE and
E1 in the present study were due to genetics, environment, or a combination of the
two.

Additionally, it would be interesting to compare species with similar life
histories but differing wood anatomy. That is, compare diffuse porous species to ring
porous species; both of which were either slow growing and long lived or fast
growing and short lived. However, this comparison would be complicated by genetic
lineage.

This study shows that there is not a direct tradeoff between conductivity and
mechanical strength, at least in the genus Acer. This could happen if water transport
is not limiting, and subsequently there is no selection for wider vessel lumen
diameters and thus, greater Ks max. It is also possible that the division of functional
roles into separate cell types facilitates the decoupling of the water transport and
mechanical roles. Or, maybe inverse relationships shown to date have been a
consequence of environmental differences. Regardless, this suggests that other
cogeneric species of similar growth habit should be investigated as well as the extent

of their phenotypic plasticity as a result of contrasting habitats.

45

Literature Cited

Alder, N. N., J. S. Sperry, and W. T. Pockman. 1996. Root and stem xylem
embolism, stomata] conductance, and leaf turgor in Acer grandidentatum populations
along a soil moisture gradient. 0ecologia. 105: 293 — 301.

Arnold, David H., and James D. Mauseth. 1999. Effects of environmental factors on
development of wood. American Journal of Botany, 86 (3): 367-371.

Baas, P. 1983. Ecological patterns in xylem anatomy. In: T] Givnish, ed, On the
Economy of Plant Form and Function. Cambridge University Press, Cambridge, UK,
pp 321 — 352.

Barnes, Burton V. and Warren H. Wagner, Jr. 1981. Michigan Trees. University of
Michigan Press: Ann Arbor.

Beery W. H., G. Ifiu, and E. McLain. 1983. Quantitative wood anatomy — relating
anatomy to transverse tensile strength. Wood Fiber Science. 15: 83 — 86.

Burgert, 1., D. Eckstein, and A. Bemasconi. 2000. The strength and volume fraction
of rays in trees subjected to radial tensile stresses. In: Plant Biomechanics 2000.
Proceedings of the 3rd Plant Biomechanics Conference. Freiburg-Badenweiler.

Bruchert, Franks, Gero Becker, and Thomas Speck. 2000. The mechanics of Norway
spruce [Picea abies (L.) Karst]: mechanical poperties of standing trees from

different thinning regimes. Forest Ecology and Management. 135: 45 — 62.

Carlquist, Sherwin. 2001. Comparative wood anatomy: systematic, ecological, and
evolutionary aspects of dicotyledon wood. Springer-Verlag: Berlin.

Chafe, S. C. 1974. Cell wall formation and “protective layer” deve10pment in the
xylem parenchyma of trembling aspen. Protoplasma. 80: 335 - 354.

Chiu, Shau-Ting and Frank W. Ewers. 1992. Xylem structure and water transport in
a twiner, a scrambler, and a shrub of Lonicera (Caprifoliaceae). Trees-Structure and
Function. 6: 216-224.

Dinwoodie, J. M. 1981. Timber. Timber Press: Forest Grove, OR.

Dixon, H.H. 1914. Transpiration and the Ascent of Sap in Plants. Macmillan:
London.

Easterling et al. 1982. Proceedings of the Royal Society of London. A383: 31—41.

Ewers, Frank W. and Jack B. Fisher. 1991. Why vines have narrow stems:
Histological trends in Bauhinia (Fabaceae). 0ecologia. 88: 233-237.

46

Ewers, Frank W. and Martin H. Zimmermann. 1984a. The hydraulic architecture of
eastern hemlock (Tsuga canadensis). Canadian Journal of Botany. 62: 940-946.

Ewers, F. W. and M. H. Zimmermann. 1984b. The hydraulic architecture of balsam
fir (Abies balsamea). Physiologia Plantarum. 60: 453-458.

Fredericksen, T.S., R. L. Hedden, and SA. Williams. 1994. Effect of stem bending
on hydraulic conductivity and wood strength of loblolly pine. Canadian Journal of
Forest Research 24: 442-446.

Frye, Darlene M. 1976. A botanical inventory of sandhill woodlot, Ingham county,
Michigan I: The vegetation. The Michigan Botanist. 15: 131-140.

Gartner, Barbara L. 1991a. Stem hydraulic properties of vines vs. shrubs of western
poison oak, Toxicodendron diversilobum. 0ecologia. 87: 180-189.

Gartner, Barbara L. 1991b. Structural stability and architecture of vines vs. shrubs of
poison oak, Toxicodendron diversilobum. Ecology. 72 (6): 2005-2015.

Gelderen, D. M., P.C. de long, and H. J. Oterdoom. 1994. Maples of the World.
Timber Press: Portland, OR.

Gere, James M. and Stephen P. Timoshenko. 1984. Mechanics of materials. 2nd ed.
Pridle, Weber & Schmidt Publishers: Belmont, CA.

Gibson, and Ashby. 1982. Proceedings of the Royal Society of London. A382: 43-
59.

Hargrave, K. R., K. J. Kolb, F. W. Ewers, and S. D. Davis. 1994. Conduit diameter
and drought-induced embolism in Salvia mellifera Greene (Labiatae). New
Phytologist 126: 695-705.

Jarbeau, J. A., F. W. Ewers, and S. D. Davis. 1995. The mechanism of water-stress-

induced embolism in two species of Chaparral shrubs. Plant, Cell and Environment.
18: 189 — 196.

Jeronimidis, G. 1980. Wood, one of nature’s challenging composites. In: The
Mechanical Properties of Biological Materials, XXXIVth Symposium of the Society
for Experimental Biology. J. F. V. Vincent and J. D. Currey, Eds. Cambridge
University Press. 169 — 182.

Johanse, Donald Alexander. 1940. Plant Microtechnique. McGraw-Hill Book Co.:
New York.

47

Koslowski, T. T. 1971. Growth and development of trees. Vol 11. Academic press:
New York.

Lawton, Robert O. 1984. Ecological constraints on wood density in a tropical
montane rain forest. American Journal of Botany. 71 (2): 261-267.

Mauseth, James D. 1988. Plant Anatomy. The Benjamin/Cummings Publishing
Company, Inc.: Menlo Park, CA.

McDonald, Suzanne, G. Bruce Williamson, and Michael C. Wiemann. 1995. Wood
specific gravity and anatomy in Heliocarpus appendiculatus (Tiliaceae). American
Journal of Botany. 82 (7): 855 — 861.

Murakami, Y., R. Funada, Y. Sano, and J. Ohtani. 1999. The differentiation of
contact cells in the xylem ray parenchyma of Populus maximowczii. Annals of
Botany. 84: 429 — 435.

Niklas, Karl. 1992. Plant Biomechanics: An Engineering Approach to Plant Form
and Function. University of Chicago Press: Chicago.

Panshin, A. J. and Carl de Zeeuw. 1980. Textbook of Wood Technology. McGraw-
Hill, Inc.: New York.

Patino, Tyree, and Herre. 1995. Comparison of hydraulic architecture of woody
plants of differing phylogeny and growth form with special reference to freestanding
and hemi-epiphytic Ficus species from panama. New Phytologist. 129 (l): 125 -—
134.

Pockman, W. T., J S. Sperry, and J .W. O’Leary. 1995. Sustained and significant
negative water pressure in xylem. Nature. 378: 715-716.

Putz, Francis E, Phyllis D. Coley, Karen Lu, Arlee Montalvo, and Annette Aiello.
1983. Uprooting and snapping of trees: structural determinants and ecological
consequences. Canadian Journal of Forest Research 13: 1011 — 1020.

Schniewind, A. P., 1959. Transverse anisotropy of wood: a function of gross
anatomic structure. Forest Products Journal. 9: 350 — 359.

Schultz, Hans R. and Mark A. Matthews. 1988. Resistance to water transport in

shoots of Vitis vinifera L.: relation to growth at low water potential. Plant
Physiology. 88: 718 — 724.

Schultz, Hans R. and Mark A. Matthews. 1993. Xylem development and hydrualic

conductance in sun and shade shoots of grapevine (V itis vinifera L.): evidence that
low light uncouples water transport capacity from leaf area. Planta. 190: 393-406.

48

Shumway Durland L., Kim C. Steiner, & Thomas E. Kolb. 1993. Variation in
seedling hydraulic architecture as a function of species and environment. Tree
Physiology. 12: 41-54.

Spatz, Hanns-Christof and Franka Bruechert. 2000. Basic biomechanics of self-
supporting plants: wind loads and gravitational loads on a Norway spruce tree.
Forest Ecology and Management. 135: 33-44.

Sperry, John S. and Melvin T. Tyree. 1988. Mechanism of water stress-induced
xylem embolism. Plant Physiology. 88: 581 - 587.

Sperry, J. S. and M. T. Tyree. 1990. Water-stress-induced xylem embolism in three
species of conifers. Plant, Cell and Environment. 13: 427 — 436.

Sperry, J. S., J. R. Donnelly, and M. T. Tyree. 1988. A method for measuring
hydraulic conductivity and embolism in xylem. Plant, Cell and Environment. 11:35-
40.

Sperry, J. S., A. H. Perry, and J. E. M. Sullivan. 1991. Pit membrane degradation
and air-embolism formation in ageing xylem vessels of Populus tremuloides Michx.
Journal of Experimental Botany. 42 (244): 1399 — 1406.

Sperry, J .S., N. Z. Saliendra, W. T. Pockman, H. Cochard, P. Cruiziat, S. D. Davis, F.
W. Ewers, and M. T. Tyree. 1996. New evidence for large negative xylem pressures

and their measurement by the pressure chamber method. Plant, Cell and
Environment. 19: 427-436.

Sperry, John S. and June E. M. Sullivan. 1992. Xylem embolism in Response to
Freeze-Thaw Cycles and Water Stress in Ring-Porous, Diffuse-Porous, and conifer
Species. Plant Physiology. 100: 605 - 613.

Telewski, F. W. 1989. Structure and function of flexure wood in Abiesfraseri. Tree
Physiology. 5: 113 - 121.

Telewski, F. W. 1995. Wind-induced physiological and developmental responses in
trees. In: Coutts, M. and J. Grace. Wind and Trees. Cambridge University Press.
237 — 262.

Tyree, Melvin T. and Frank W. Ewers. 1991. The hydraulic architecture of trees and
other woody plants. New Phytologist. 119: 345-360.

Telewski, F. W. and M. J. Jaffe. 1986. Anatomical, morphological, and mechanical
analysis of genetically different sibs of Pinus taeda in response to mechanical

perturbation. Physiologia Plantarum. 66 (2): 219 — 226.

U. S. Department of Agriculture. Wood Handbook 72.

49

Ugural, A. C. 1991. Mechanics of materials. McGaw-Hill, New York.

Vander Willigen, Clare and N. W. Pammenter. 1998. Relationship between growth
and xylem hydraulic characteristics of clones of Eucalyptus spp. at contrasting sites.

Tree Physiology. 18: 595 — 600.

Vander Willigen, C., H. W. Sherwin and N. W. Pammenter. 2000. Xylem hydraulic
characteristics of subtropical trees from contrasting habitats grown under identical
environmental conditions. New Phytologist. 145: 51 — 59.

Villar-Salvador, Pedro, Pilar Castro-Diez, Carmen Perez-Rontome, and Gabriel
Montserrat-Marti. 1997. Stem Xylem Features in three Quercus species along a
climatic gradient in NE Spain. Trees — Structure and Function. 12 (2): 90 — 96.

Wagner, Kristofer R., Frank W. Ewers, and Stephen D. Davis. 1998. Tradeoffs
between hydraulic efficiency and mechanical strength in the stems of four co-
occurring speices of Chaparral shrubs. 0ecologia 117: 53-62.

Wainwright, S. A., W. D. Biggs, J. D. Currey, and J. M. Gosline. 1976. Mechanical
Design in Organsims. Princeton University press: Princeton, New Jersey.

Zimmermann, Martin H. 1978. Hydraulic architecture of some diffuse-porous trees.
Canadian Journal of Botany. 56: 2286-2295.

Zimmermann, M.H. 1983. Xylem structure and the ascent of sap. Springer-Verlag:
Berlin.

Zimmermann, M. H. and A. A. Jeje. 1981. Vessel length distribution in stems of
some American woody plants. Canadian Journal of Botany. 59: 1882 — 1892.

50

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

11111liljjjljljjjjljjljtl