boston university graduate school of arts and

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Copyright by

HEIDI J. RENNINGER2010


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Approved by

First Reader ____________________________________________________Nathan G. Phillips, PhD

Associate Professor of Geography and Environment

Boston University

Second Reader ____________________________________________________

Guido Salvucci, PhDProfessor of Earth Sciences, Geography and Environment and

Chairman, Department of Earth SciencesBoston University

Third Reader ____________________________________________________

Mark Friedl, PhDProfessor of Geography and Environment

Boston University

Fourth Reader ____________________________________________________

N. Michele Holbrook, PhDProfessor of Biology and Charles Bullard Professor of Forestry,

Department of Organismic and Evolutionary Biology

Harvard University

Fifth Reader ____________________________________________________Curtis Woodcock, PhD

Professor of Geography and Environmentand Center for Energy and

Environmental StudiesBoston University


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ACKNOWLEDGMENTS

Acknowledgement text here

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HYDRAULIC PROPERTIES OF PALMS VARYING IN ONTOGENY AND

ENVIRONMENTAL GROWING CONDITIONS WITH IMPLICATIONS FOR

HYDRAULIC LIMITATION TO INCREASED HEIGHT GROWTH IN TALL

PALMS

(Order No. )

HEIDI J. RENNINGER

Boston University Graduate School of Arts and Sciences, 2010

Major Professor: Nathan G. Phillips, Associate Professor of Geography andEnvironment

ABSTRACT

Palms are an attractive group for physiological research because their columnar

trunks and simplistic leaf habit allow for estimation of variables important for hydraulic

functioning more easily than in more complex arborescent dicotyledons. Likewise,

palms grow in a wide variety of climates from the very dry (Washingtonia robusta in

Southern California) to tropical rainforests (Iriartea deltoidea andMauritia flexuosa)

which allowed for study of palm species across an environmental moisture gradient. As

well, within species comparisons across an ontogenetic gradient were conducted to

examine hydraulic functioning with changes in palm size in order to characterize the

hydraulic limitations and/or compensations that are made as trees grow taller, and

therefore, move water further distances. While both rainforest species exhibited differing

patterns in height growth rate along boles, height growth rates ultimately decreased in the

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tallest palms. Sapflux per unit leaf area was constant across palm height in all three

species, suggesting that taller palms are not showing evidence of hydraulic limitation.

However, inIriartea deltoidea, transpiration was more restricted by stomatal closure in

the dry season compared with the wet season. Both tropical species exhibited contrasting

patterns in both live frond number and frond leaf area with height that led to a

convergence in the pattern of increasing total leaf area with height in both species. In

contrast, sub-tropical Washingtonia robusta exhibited decreasing total leaf areas with

height. BothMauritia flexuosa and Washingtonia robusta showed an increased reliance

on stored water with height which likely compensated for the increased frictional

resistance to water flow. Regarding petiole conductivities, leaf specific conductivity was

similar both within species and between species forIriartea deltoidea and Washingtonia

robusta. As well,Iriartea deltoidea and Washingtonia robusta had similar P50 values

(point at which 50% of hydraulic conductivity is lost) in petioles averaged across height.

Comparing P50 values with measurements of midday leaf water potentials, as well as a

double-dye staining experiment, suggested that a fairly significant amount of embolism is

occurring on a daily basis. This could mean that these palm species, instead of avoiding

embolisms through tight stomatal control, repair embolisms on a daily basis.

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TABLE OF CONTENTS

APPROVAL PAGEiii

ACKNOWLEDGMENTS..................................................................................................iv

ABSTRACT........................................................................................................................v

TABLE OF CONTENTS..................................................................................................vii

CHAPTER 1 .......................................................................................................................1

INTRODUCTION...............................................................................................................1

CHAPTER 2......................................................................................................................22

COMPARATIVE HYDRAULIC AND ANATOMIC PROPERTIES IN PALM TREES

(WASHINGTONIA ROBUSTA) OF VARYING HEIGHTS: IMPLICATIONS FOR

HYDRAULIC LIMITATION TO INCREASED HEIGHT GROWTH...........................22

CHAPTER 3 ....................................................................................................................59

WET VERSUS DRY SEASON TRANSPIRATION IN AN ...........................................59

AMAZONIAN RAINFOREST PALM, IRIARTEA DELTOIDEA.................................59

CHAPTER 4......................................................................................................................94

INTRINSIC AND EXTRINSIC HYDRAULIC FACTORS IN VARYING SIZES OF

TWO AMAZONIAN PALM SPECIES (IRIARTEA DELTOIDEA AND MAURITIA

FLEXUOSA) DIFFERING IN DEVELOPMENT AND GROWING ENVIRONMENT

...........................................................................................................................................94

CHAPTER 5....................................................................................................................137

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HYDRAULIC PROPERTIES AND VULNERABILITY TO EMBOLISM IN PALM

FRONDS FROM SPECIES OF VARYING HEIGHT GROWING IN RAINFOREST

AND SUBTROPICAL ENVIRONMENTS....................................................................137

CHAPTER 6: OVERALL CONCLUSIONS.................................................................183

LITERATURE CITED....................................................................................................183

CURRICULUM VITAE..................................................................................................211

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LIST OF TABLES

Table 2.1: Basic characteristics of the Washingtonia robusta study palms including

number and locations of Granier sensors for each individual...........................................49

Table 2.2: Equations and r2 values for the regressions in Figure 2.3...............................50

Table 2.3: Maximum, minimum, mean and std. errors for the various physiological

variables measured across palm height where n represents the number of palms sampled

for a given variable............................................................................................................51

Table 3.4: Location (stilt root, bole, petiole) and number of Granier heat dissipation

sensors that were inserted and functioning in Iriartea deltoidea palms. Multiple sensors

were located in separate stilt roots and petioles and in opposite sides of the bole. Only

data from functioning sensors were used in the final analysis..........................................86

Table 3.5: Daily averages (standard errors) of micrometeorological and Iriartea deltoidea

sap flux data for both the wet season and dry season........................................................87

Table 5.6: Means and standard errors for Iriartea deltoidea and Mauritia flexuosa, both

Ecuadorian tropical rainforest species, and Washingtonia robusta, a subtropical, dry

climate species growing in Australia. Means with different letters within the same row

were significantly different at


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LIST OF FIGURES

Figure 2.1: Representative diurnal sap flux (g m-2 s-1) (bottom panel) on Julian day 210

from a single outer sapflow sensor for eight individuals of varying heights. The middle

panel represent vapor pressure deficit (VPD) (kPa) and the top panel represents solar

radiative flux density (W m-2). The solar flux sensor was shaded early in the morning,

therefore solar radiation measurements are artificially low in this time interval. The

dotted line represents a clear-day estimate of solar radiation and the likely light

environment for the crowns of the palms..........................................................................52

Figure 2.2: Tree height (m) vs. means and standard errors of (a) total daily bole sap flux

(kg day-1 m-2 leaf area) and (b) total daily petiolar sap flux (kg day-1 m-2 leaf area). For

bole sap flux, standard error bars represent 4 to 8 replicate days and 2 or 3 sensor

locations. For petiole sap flux, standard error bars represent 3 replicate days and 2

petioles...............................................................................................................................53

Figure 2.3: Daily time courses of (a) stomatal conductance (mol H2O m-2 s-1), (b)

photosynthetic assimilation rate (mol CO2 m-2 s-1) and (c) leaf water potentials

(MPa). Closed circles = 2 m palm, open circles = 8 m palm, closed triangles = 18 m

palm, open triangles = 22 m palm, closed squares = 28 m palm, open squares = 34 m

leaning palm. Equations and r2 values are presented in Table 2.2....................................54

Figure 2.4: Tree height (m) vs. (a) maximum daily stomatal conductance (mol H2O m-2

s-1) calculated from the daily time courses of stomatal conductance (Fig. 2.3a), (b)

maximum daily photosynthetic assimilation rate (mol CO2 m-2 s-1) calculated from the

daily time courses of photosynthetic assimilation rate (Fig 2.3b)(y = 0.23x + 10.2) and (c)

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mean leaf mass per area (LMA; g m-2) where std. error bars represent 5 replicate leaflets.

...........................................................................................................................................56

Figure 2.5: Tree height (m) vs. (a) minimum daily leaf water potential (; MPa) (y =

-1.9 x0.18) and (b) the time at which minimum leaf water potential occurred (y =

0.08x + 16.0), both calculated from the daily time courses of leaf water potential (Fig.

2.3c)...................................................................................................................................57

Figure 2.6: Tree height (m) vs. (a) the number of live leaves per palm (y =

44.6 x-0.24), (b) leaf area (m2)(y = 5.59 x -0.55) and (c) average leaf epidermal cell area

(m2) for a given leaf. Errors bars represent the standard error from the two leaves per

palm measured and the standard error around average leaf epidermal cell size. Leaf

epidermal cell sizes for the 28m and 34m palms are significantly lower than the shorter

palms and well as being significantly different from each other (p-value


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Figure 3.8: Calibration of the Granier sensor performed on a piece of small Iriartea

deltoidea bole with sap flux density (u)(m3 m-2 s-1 * 106) vs sap flux index

(T(0)/T(u) 1)..............................................................................................................89

Figure 3.9: Average petiole sap flux from three small Iriartea deltoidea palms for

representative days during (A) the 2006 wet season and (B) the 2007 dry season,

corresponding vapor pressure deficits (VPD) during (C) the 2006 wet season and (D) the

2007 dry season and average bole sap flux from four medium-sized Iriartea deltoidea

palms during (E) the 2006 wet season and (F) the 2007 dry season. Dotted lines

represent standard errors....................................................................................................90

Figure 3.10: Average sap flux of the stilt roots of three large Iriartea deltoidea palms

during representative days in (A) the 2006 wet season and (B) the 2007 dry season,

corresponding vapor pressure deficits (VPD) during (C) the 2006 wet season and (D) the

2007 dry season and average sap flux of the boles of three large Iriartea deltoidea palms

during (E) the 2006 wet season and (F) the 2007 dry season. The dotted lines represent

standard errors...................................................................................................................91

Figure 3.11: Average sap fluxes corresponding to vapor pressure deficit values ranging

from 0.1 to 3.4 kPa in 0.1 kPa increments for (A) the petioles of small palms, (B) the stilt

roots of large palms, (C) the boles of medium-sized palms and (D) the boles of large

palms. Wet-season sap fluxes are represented by the closed circles, dry-season sap fluxes

by the open circles.............................................................................................................92

Figure 3.12: Average sap fluxes corresponding to photosynthetic photon flux density

(PPFD) values ranging from 0 to 2000 (mol/ m2 sec) in 50 mol/ m2 sec increments for

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(A) the petioles of small palms, (B) the stilt roots of large palms, (C) the boles of

medium-sized palms and (D) the boles of large palms. Wet-season sap fluxes are

represented by the closed circles, dry-season sap fluxes by the open circles....................93

Figure 4.13: Internode lengths (m) along the boles of (A) Iriartea deltoidea and (B)

Mauritia flexuosa. Points with standard error bars represent the mean and variance of

four or five individuals....................................................................................................128

Figure 4.14: Relationship between Palm height (m) and (A) number of fronds lost per

year for Iriartea deltoidea (y = -0.062x + 2.56) and Mauritia flexuosa (y = 1.34x0.44), (B)

number of live fronds in Iriartea deltoidea (y = 3.83 x0.17) and Mauritia flexuosa (y =

0.32x +3.62), (C) individual frond leaf area (m2) in Iriartea deltoidea (y = 0.40x +0.64)

and Mauritia flexuosa (y = 3.25 x0.19) and (D) total palm leaf area (m2) in Iriartea

deltoidea and Mauritia flexuosa where a single regression (y = 2.53x + 5.95) fits the data

for both species................................................................................................................129

Figure 4.15: The relationship between palm height (m) and (A) leaf epidermal cell sizes

(m2) in Iriartea deltoidea (y = 77 +21x-1.3x2) and Mauritia flexuosa (y = -7.2x+350),

(B) stomatal density (mm-2) in Iriartea deltoidea (y = 52x0.47) and Mauritia flexuosa (y

= 37e-0.076x) and (C) guard cell length (m) in Iriartea deltoidea (y = 29.4e-0.028x) and

Mauritia flexuosa (y = 14.7 +0.001x +0.007x2). Guard cell lengths and stomatal

densities were used to calculate stomatal pore area indices (SPI) which are plotted vs.

palm height (inset) for Iriartea deltoidea and Mauritia flexuosa (y = 0.081e-0.066x).. ..130

Figure 4.16: The relationship between palm height (m) and (A) Iriartea deltoidea sapflux

(kg/day) measured in the boles during the wet season and the subsequent dry season.

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Sapflux increased linearly in taller palms (y = 0.27x +0.41); however, there is no

relationship when sapflux is presented on a per leaf area basis (inset). (B) Mauritia

flexuosa sapflux (kg/day) measured in the boles and in the petioles. Sapflux increased

linearly in taller palms (y = 3.95x + 1.82); however, there is no relationship when sapflux

is presented on a per leaf area basis (inset)......................................................................132

Figure 4.17: Bole water storage estimation for Mauritia flexuosa where left panels

represent no lag between bole and petiole sapflux (g m-2 s-1) and right panels

represent a lag that maximizes the r2 of the regression. Top panels, (A) and (B) represent

data from a 6 m tall palm, middle panels (C) and (D) represent data from an 18 m tall

palm and bottom panels (E) and (F) represent data from a 22.5 m tall palm. All data were

collected on the same day; Feb. 28, 2008........................................................................133

Figure 4.18: Percent parenchyma on an area basis vs. height of sample (m) in the outer

bole (lower left picture, x-section, 40X, stained with Toluidine Blue O) and the inner bole

(upper right picture, x-section, 40X, stained with Toluidine Blue O) in Iriartea deltoidea

palms. The inset graph presents bole cross-sectional area measured at breast height (m2)

vs. palm height (y = 0.0018x + 0.0008) in Iriartea deltoidea (Bars =1 mm)...................134

Figure 4.19: Vascular conduit sizes and distributions in the outer bole, inner bole and

stilt roots from Iriartea deltoidea palms of various heights. (A) Metaxylem vessel

diameters (m) vs. sample height above ground (m) for outer and inner bole (y = 71x

0.43) and pooled stilt roots, (B) vascular bundle density (mm-2) vs. palm height (m) for

outer bole (y = 17 x -0.97), the inner bole (y = 27 x -1.6) and pooled stilt roots and (C)

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calculated Hagen-Poiseuille conductivity (kg m-1 s-1 MPa-1) vs. palm height for

outer bole (y = 6.4x 1.2), inner bole (y = 1.3 x 1.6), and pooled stilt roots....................135

Figure 4.20: Vascular bundle density (mm-2) vs. metaxylem vessel diameter (m) for

samples from Iriartea deltoidea palms collected from the outer bole and inner bole at

various heights, and from stilt roots of palms of varying height. The following equation

was fitted to the data: y = 68.5 e -0.023x........................................................................136

Figure 5.21: Vulnerability curves for petioles taken from palm of differing heights of a)

Iriartea deltoidea and b) Washingtonia robusta. closed circles = 1m, open circles = 3m,

closed triangle (pointing down) = 5m, open triangle (pointing down) = 7m, closed square

= 9m, open square = 11m, closed diamond = 13m, open diamond = 15m for Iriartea

deltoidea, 14m for Washingtonia robusta, closed triangle (pointing up) = 16m

Washingtonia robusta. In a) solid line is fitted to all points, b) solid line is fitted to the

average of 1m, 3m, and 5m; dashed line is fitted to the average of 7m, 9m, and 11m; and

a dotted line is fitted to the average of 13m, 14m and 16m.............................................177

Figure 5.22: Palm height (m) vs. a) individual frond leaf area (m2) and b) individual

frond Huber values for Iriartea deltoidea closed circles, Washingtonia robusta growing

in Australia and Los Angeles open and closed squares respectively, and Mauritia

flexuosa open triangles. ..............................................................................................178

Figure 5.23: Palm height (m) vs. a) petiole specific conductivity (KS) (kg m-1 s-1 MPa-

1) and b) leaf specific conductivity (KL) (kg m-1 s-1 MPa-1) from petioles from Iriartea

deltoidea closed circles, Washingtonia robusta - open squares and Mauritia flexuosa

open triangles. .................................................................................................................179

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Figure 5.24: Palm height (m) vs. a) maximum vessel length (cm) in petioles from Iriartea

deltoidea closed circles and Washingtonia robusta open squares, b) average vessel

diameter (m) and c) vascular bundle density (# mm-2) in petioles from Iriartea deltoidea

closed circles, Washingtonia robusta open squares and Mauritia flexuosa- open

triangles............................................................................................................................180

Figure 5.25: Palm height (m) vs. a) the water potential at which 50% petiole conductivity

was lost (P50) (-MPa) obtained from vulnerability curves (Fig. 5.1) and b) midday leaf

water potentials (-MPa) measured on a cloudless day in Iriartea deltoidea closed circles

and Washingtonia robusta open squares.......................................................................181

Figure 5.26: Cross section (20X) of a petiole from a 1m tall Washingtonia robusta palm

where the double dye staining procedure was performed. The red dye (Basic Fuchsin)

was introduced in the afternoon and the blue dye (Toluidine Blue) was introduced the

following morning. Vessels that were stained red were functional in the afternoon, while

vessels that were stained blue were embolized in the afternoon but refilled overnight.

Vessels that are purple presumably received both dyes and did not embolize in the

afternoon..........................................................................................................................182

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1

CHAPTER 1

INTRODUCTION

1.1 OVERVIEWOFTHE HYDRAULIC LIMITATION HYPOTHESIS

Foresters have known for a long time that as trees mature their growth reaches a

peak, at which point further increases in height and diameter begin to decrease (Smith et

al., 1997; Bond-Lamberty et al., 2004; Litvak et al., 2003). It has also been noted that in

very old trees of species such as Douglas-fir and coastal redwoods, height growth is

significantly reduced when compared to shorter trees of the same species (McDowell et

al., 2002a, Koch et al., 2004). There are several possible reasons why forest growth and

tree height in particular would begin to decline as trees get older and taller. One

possibility is that larger trees have larger respiratory demands leaving less carbon

available for growth (Ryan and Yoder, 1997). However, Ryan and Waring (1992) found

that maintenance respiration of woody tissues was only slightly and insignificantly higher

in a 245 yr old lodgepole pine (Pinus contorta) stand compared to a 40 yr old stand.

Another possibility is that as trees get taller, they also become older and that growth is

reduced in older tissues relative to younger ones. However, a study done by Mencuccini

et al. (2005) found that when shoots from the tops of old ash (Fraxinus excelsior),

sycamore (Acer pseudoplatanus), poplar (Populus sp.), and Scots pine (Pinus sylvestris)

trees were grafted onto young rootstock, their relative growth rates and net

photosynthetic rates recovered to values similar to that of younger, smaller trees. It is

also possible that limitations in phloem loading at the tops of tall trees and phloem

transport across long distances may limit height growth in trees (Koch and Fredeen,


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2005; Zimmermann, 1973). Another possibility is that as trees grow taller, the

mechanical stresses on the stem increase, making trees more susceptible to uprooting and

wind damage. Meng et al. (2006) found some evidence for this by tethering tall

lodgepole pines to reduce their bending moment. They found that six years of tethering

resulted in a 40% increase in height growth relative to the previous period when the trees

were not tethered.

With experimental evidence disproving many of these previous hypotheses, the

hydraulic limitation hypothesis remains as a viable explanation for height growth rates

declines in very tall trees (Ryan and Yoder, 1997, Koch et al., 2004). Hydraulic

limitations could begin to impact taller trees because, as trees grow in height, the

gravitational potential increases as well as the path length of water travel. This means

that taller trees are, by virtue, less efficient at transporting water to their leaves relative to

shorter trees. In order to overcome this inefficiency, taller trees require a more negative

leaf water potential to move the same quantity of water as shorter trees. Since trees of a

given species also tend to exhibit representative minimum leaf water potentials in order

to protect their water conducting conduits, the tallest trees of a given species will reach

their minimum leaf water potential sooner than shorter trees, causing stomatal closure,

and reduced photosynthesis and carbon gain (Ryan and Yoder, 1997). This phenomenon

is thought to then act as a negative feedback on further height growth in very tall trees

and would tend to set characteristic maximum heights for given tree species growing in

given environmental conditions.


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Since its proposal, several studies have found evidence in support of the hydraulic

limitation hypothesis. For example, in a study of European beech (Fagus sylvatica),

Schfer et al. (2000) found that at similar environmental conditions, stomatal

conductance, and in turn, the amount of carbon dioxide available in the leaf for

photosynthesis was decreased by 60% for a given 30m increase in tree height. Ryan et al.

(2000) found that water flux and whole tree stomatal conductance was half as much in

36m tall ponderosa pines (Pinus ponderosa) as in 12m tall trees. Additionally, Hubbard

et al. (1999) found that old ponderosa pines had 53% lower whole tree sapflow per unit

leaf area than younger trees. In maritime pines (Pinus pinaster), Phillips et al. (2003a)

found that not only did taller Oregon white oaks (Quercus garryana) have decreased

sapflux compared to shorter trees, but they also had more leaf area for a given sapwood

area, compounding the effects of hydraulic limitation to carbon gain. Another

consequence of the hydraulic limitation hypothesis may also include decreased turgor

pressure at the tops of tall trees which could limit cell expansion unless osmotic

adjustment occurs (Koch et al., 2004; Woodruff et al., 2004; Meinzer et al., 2008).

Decreased turgor represents another way in which height growth could be limited in tall

trees because, although not affecting stomatal conductance, decreases in total leaf area

will reduce carbon gain by decreasing the area of leaf tissue available for photosynthesis.

If osmotic adjustment does occur to maintain turgor, this represents an additional carbon

requirement in tall trees relative to shorter ones.

Although hydraulic limitation seems reasonable given the physical laws of water

transport in tall trees, there are also data which seem to contradict or a least complicate


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this issue. For example, in their study of old-growth Douglas-fir trees, McDowell et al.

(2002a) found that stomatal conductance, photosynthetic assimilation and leaf-specific

hydraulic conductance were not significantly different among trees of different heights

and Barnard and Ryan (2003) also found that taller Eucalyptus trees had similar

photosynthetic assimilation, sapflux per unit leaf area and whole tree stomatal

conductance than their shorter counterparts. Additionally, West et al. (1999) propose that

if conduits taper sufficiently, hydraulic resistance can become independent of path length,

because hydraulic resistance is inversely proportional to the fourth power of conduit

radius, but only linearly related to length (Becker et al., 2000, Zimmermann, 1983).

Tall trees can also make alterations to their anatomy and physiology in order to

alleviate some of the affects of hydraulic limitations on photosynthesis and carbon gain.

One way in which taller trees can move as much water as their shorter counterparts is by

increasing the tension the water is under and exhibiting more negative leaf water

potentials. In fact, both Barnard and Ryan (2003) and McDowell et al. (2002a) saw this

adjustment in leaf water potential made in the trees they studied. According to the

hydraulic limitation hypothesis, taller trees should not exhibit more negative water

potentials (Ryan and Yoder, 1997) because more negative water potentials increase the

risk of disfunction of the water conducting vessels or tracheids in terms of embolism

formation(Zimmermann, 1983). Embolisms occur when air is pulled into a water

conducting conduit that is under a large negative pressure (the air-seeding hypothesis) or

when the water column itself freezes in the vascular tissues of plants (freeze-thaw

embolism) (Zimmermann, 1983; Tyree and Sperry, 1989). These embolism events


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reduce conductivity (Zimmermann, 1983) and cause reductions in transpiration and

photosynthesis (Sperry et al., 1993; Hubbard et al., 2001). However, taller trees could

sustain more negative water potentials if they construct conduits that are more resistant to

embolism formation. This phenomenon was seen in old growth Douglas-fir trees by

Woodruff et al. (2007) and Domec et al. (2008).

There is some contradictory information about whether the conductivity in taller

trees will increase to offset the path length and gravitational effect, or decrease as a result

of increased resistance to embolisms. Increases of sapwood conductivity with height

have been seen in several studies of conifer trees (Pothier et al., 1989; Domec and

Gartner, 2003; Burgess et al., 2006). However, it is generally assumed that there is a

tradeoff between conductivity and embolism resistance. This trade-off is described for

vessels by Hacke et al. (2006) in that vessels with smaller pit pore areas are more

embolism resistant but limited in length and diameter, leading to lower conductances.

One way that tall trees could sustain more negative water potentials than shorter trees, but

avoid any tradeoffs between embolism resistance and conductivity may be efficient

refilling of conduits that embolize. Recent study has shown that trees may be able to

reverse embolisms that have formed in their conduits, in many cases while the leaves are

still transpiring and the water column is still under tension (Zwieniecki and Holbrook,

(1998; Bucci et al., 2003; Stiller et al., 2005).

Tall trees can also make biometric adjustments in leaf area to bole area ratios in

order decrease the water demanding to water supply area and increase their capacity for

bole water storage. Leaf area to sapwood area ratios have generally been found to


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decrease as trees grow taller (Schfer et al., 2000; Phillips et al., 2001; Sterck and

Bongers, 2001; McDowell et al., 2002b; Barnard and Ryan, 2003). However, the

DESPOT model which optimizes carbon gain in trees predicts that leaf area to sapwood

area ratios should increase with tree height (Buckley and Roberts, 2006). Even though

lowering leaf area for a given sapwood area can alleviate restrictions on stomatal

conductance in tall trees, this can still be considered a hydraulic limitation because

taller trees would need to put more of a carbon investment into water conducting tissues

over leaf tissue serving to further limit their carbon gain ability (Phillips et al. 2003a).

Taller trees have also been shown to have a greater reliance on stored water in their boles

than shorter trees (Goldstein et al., 1998; Phillips et al., 2003b). However, Meinzer et al.

(2004) found that although larger tropical trees had greater use of stored water, they also

had greater daily water use than smaller trees. Therefore, the contribution of stored water

to daily water use was equivalent across tree size.

It is obvious that trees are very complex organisms and the study of physiological

phenomena can be difficult. With regard to validating the hydraulic limitation

hypothesis, the complicated branching patterns of many species mean that each leaf on

the tree will have a different path length from the ground. Additionally, the hydraulic

architecture of many trees is designed in such a way that branches receiving more

sunlight are hydraulically favored over lower branches (Protz et al., 2000). Much of this

complexity could be avoided by focusing on tree species that have a much simpler form.

Palms have all the criteria to make them a model organism because, compared to other

tree species, they are structurally, very simple with a relatively fixed crown size and


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vascular system (Zimmermann et al., 1982; Tomlinson, 1987). In particular, palms are a

good tree form to use because unlike most trees, palms lack complex branching patterns

making the path length of water flow easily measurable. In addition, their small, compact

crowns allow for a very accurate estimate of leaf area.

1.2 DISTINCTIVE PHYSIOLOGICAL FEATURESOF PALMS

Palms are very distinctive members of the plant world in many ways. They are

one of the few members of the monocot class that are able to reach significant heights

with the tallest palm species (Ceroxylon quinduiense) reaching 60m in height (Henderson

et al., 1995). In doing so, they are able to transport water very long distances, matching

many dicotyledonous species. However as monocots, palms lack a vascular cambium

and therefore do not have any secondary woody growth for vascular transport. All

vascular transport and mechanical support is accomplished through thousands of vascular

bundles which are comprised of primary xylem vessels, phloem sieve tube cells and

fibers. This makes the boles of palms very heterogenous in nature where they have been

shown to encompass an entire range of published wood density values within a single

stem (Rich, 1987b). Although the crowns of palms are very simple compared with many

other tree species, they are unique in many ways. One important difference between

palms and dicotyledonous trees is that vertical growth in palms is directly tied to leaf

production by the apical meristem (Rich, 1986). Palm leaves are extraordinary within the

plant world holding records for both the longest pinnate self-supporting leaf inRaphia

regalis Becc.at 25m as well as the largest palmately compound self-supporting leaf in

Corypha umbraculifera L. at 8m in leaf diameter (Tomlinson, 2006). Also, because tall


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height have a lower margin of safety against mechanical failure than shorter palms.

Younger palms are also overbuilt for mechanical safety with respect to diameter while

older palms are underbuilt with respect to diameter when compared to both younger

palms and with angiosperm and conifer species (Rich, 1987a). However, instead of

exhibiting large increases in diameter, palms increase the stiffness and strength of their

stem tissues in order to make themselves more mechanically stable (Rich, 1987a). The

upper region of palm stems also becomes increasingly flexible (Rich, 1987a) and the

crown becomes narrower (Rich et al., 1986) in order to make tall palms less susceptible

to wind damage. However, Gale and Barfod (1999) found that manyIriartea deltoidea

palms (47%) died standing, while 45% died from being snapped and 8% were uprooted

(although all of the palms that were snapped or uprooted were pushed over by other

trees). Palms also appear to exhibit decreases in height growth as they get older/taller.

Homeier et al. (2002) found thatIriartea deltoidea reaches its maximum height growth

rates at about 10-12m when the palm reaches reproductive age, at which point vertical

growth rates decrease. Lugo and Rivera Batlle (1987) also found that dominantPrestoea

montana palms grew fast in height when they are small, but height growth slowed once

they reached the canopy.

Palms also provide a unique opportunity to study vulnerability to embolism and

embolism repair because they lack the capacity to make new conducting tissues.

Therefore, their vascular conduits either need to efficiently avoid embolisms or

efficiently reverse embolisms that were to occur in their vascular tissues if they are to

remain functional over a lifetime. Few studies have looked at the rate of embolism


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formation and reversal in palm species. A study done by Drake and Franks (2003) found

that vascular conductivity was significantly decreased in the dry season compared to the

wet season in two species ofCalamus, a rattan in the Palmae. A study done by Sperry

(1986) onRhapis excelsa, found that large tensions were required to induce xylem

embolisms and when embolisms did occur, they were confined to conduits in the petiole

with conduits in the bole remaining intact. Bole conduits were protected by the hydraulic

architecture of the palm, with most of the resistance to water flow located at the stem to

leaf connection and in the leaf itself (Sperry, 1985). If embolisms do occur in palms,

then some mechanism of refilling would be necessary, given that palms cannot replace

embolized vessels. It also seems plausible that the phloem tissue could be involved in

vessel refilling because of its proximity to the xylem vessels in the vascular bundle.

Phloem carries sugars from their origins to locations throughout the plant. When sugars

transported through the phloem exit, the osmotic potential of the phloem drops and the

surplus water that originally transported the sugars also exits and is recycled by the xylem

(Milburn, 1996; Patrick et al., 2001). This surplus phloem water makes up 1 to 3% of

xylem transport and could make up much of the water used to refill embolized vessels

(Milburn, 1996). Measurements of xylem tensions and changes in phloem turgor suggest

that there is a close association of radial water movement from the phloem to the xylem

(Sovonick-Dunford et al., 1981). Several studies have found that inactivating the phloem

by girdling significantly impairs embolism repair (Salleo et al., 1996; Zwieniecki et al.,

2000; Salleo et al., 2004).


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1.3 DISSERTATION STRUCTURE

Chapter 2 describes a test of the hydraulic limitation hypothesis in Washingtonia

robusta palms growing in Southern California. Relationships of sapflux per unit leaf

area, stomatal conductance, maximum photosynthetic rates, leaf water potentials,

stomatal densities, guard cell lengths, leaf dry mass per unit area were evaluated with

palm height to determine whether photosynthesis in taller palms was more hydraulically

limited. As well bole water storage and leaf epidermal cell sizes, and total leaf areas

were compared in palms of differing heights to determine whether any physiological

compensations were occurring to overcome hydraulic limitations.

Chapter 3 describes a study comparing wet season and dry season transpiration in

a tropical rainforest palm,Iriartea deltoidea. Atmospheric data, soil moisture data and

sapfluxes were compared in order to determine if transpiration was more stomatally

limited in the dry season compared to the wet season and if so, was that driven more by

atmospheric vapor pressure deficits or soil moisture availability. Additionally, based on

published tree abundances in this area, measured sap fluxes inIriartea deltoidea were

scaled up to the hectare level.

Chapter 4 tests the hydraulic limitation hypothesis in two species of tropical

rainforest palms,Iriartea deltoidea andMauritia flexuosa. Height growth rates,

sapfluxes per unit leaf area and total leaf areas were compared within species across

palms of differing heights to determine if hydraulic limitations were occurring.

Additionally, physiological comparisons were made betweenIriartea deltoidea and

Mauritia flexuosa because all though they experience similar atmospheric conditions,


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they differ markedly in edaphic conditions (terra firme vs swamp), leaf type (pinnate vs.

palmate) and ontogenetic bole development. This comparison led to speculation as to

whether sustained stem lengthening can occur inIriartea deltoidea.

Chapter 5 focuses specifically on comparing the hydraulic characteristics of

petioles from these three palm species (Washingtonia robusta,Iriartea deltoidea and

Mauritia flexuosa) across an ontogenetic gradient as well as across an atmospheric

moisture gradient. Comparison of leaf area to conducting area ratios, petiole

conductivity, anatomic properties, and vulnerability to embolism shed light on any

hydraulic compensations to increased height that were occurring at the petiole level. As

well, comparison of P50 values (point at which 50% of hydraulic conductivity is lost) with

measurements of midday leaf water potentials, as well as a double-dye staining

experiment, allowed for estimation of the magnitude of daily embolism formation in

these palm petioles. This led to speculation about whether palms avoid embolisms

through tight stomatal control, or refill embolisms that occur on a daily basis.

1.4 LITERATURE CITED

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fast-growingEucalyptus saligna. Plant, Cell and Environment 26: 1235-1245.

Becker, P., Gribben, R.J., and C.M. Lim. 2000. Tapered conduits can buffer hydraulic

conductance from path-length effects. Tree Physiology 20: 965-967.

Bond-Lamberty, B., Wang, C., and S.T. Gower. 2004. Net primary production and net

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Bucci, S.J., Scholz, F.G., Goldstein, G., Meinzer, F.C. and L. da S.L. Sternberg. 2003.

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Buckley, T.N. and D.W. Roberts. 2006. How should leaf area, sapwood area and

stomatal conductance vary with tree height to maximize growth? Tree

Physiology 26: 145-157.

Burgess, S.S.O., Pittermann, J. and T.E. Dawson. 2006. Hydraulic efficiency and safety

of branch xylem increases with height in Sequoia sempervirens (D.Don) crowns.

Plant, Cell and Environment 29: 229-239.

Domec, J.C., and B.L. Gartner. 2003. Relationship between growth rates and xylem

hydraulic characteristics in young, mature and old-growth ponderosa pine trees.


Domec, J.C., Lachenbruch, B., Meinzer, F.C., Woodruff, D.R., Warren, J.M. and K.A.

McCulloh. 2008. Maximum height in a conifer is associated with conflicting

requirements for xylem design. PNAS 105: 12069-12074.

Drake P.L. and P.J. Franks. 2003. Water resource partitioning, stem xylem hydraulic

properties, and plant water use strategies in a seasonally dry riparian tropical

rainforest. Oecologia 137: 321-329.

Dransfield, J. 1978. Growth form of rain forest palms. In Tropical Trees as Living

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Gale, N., and A.S. Barfod. 1999. Canopy tree mode of death in a western Ecuadorian

rain forest. Journal of Tropical Ecology 15: 415-436.

Goldstein, G., Andrade, J.L., Meinzer, F.C., Holbrook, N.M., Cavelier, J., Jackson, P.,

and A. Celis. 1998. Stem water storage and diurnal patterns of water use in

tropical forest canopy trees. Plant, Cell and Environment 21: 397-406.

Hacke, U.G., Sperry, J.S., Wheeler, J.K. and L. Castro. 2006. Scaling of angiosperm

xylem structure with safety and efficiency. Tree Physiology 26: 689-701.

Henderson, A., Galeano, G., and R. Bernal. 1995. Field Guide to the Palms of the

Americas. Princeton University Press. Princeton, New Jersey

Homeier, J., Breckle, S.W., Dalitz, H., Leyers, C., and R. Ortiz. 2002. Demography,

spatial distribution, and growth of three arborescent palm species in a tropical

premontane rain forest in Costa Rica. Ecotropica 8: 239-247.

Hubbard, R.M., Bond, B.J., and M.G. Ryan. 1999. Evidence that hydraulic conductance

limits photosynthesis in oldPinus ponderosa trees. Tree Physiology 19: 165-

172.

Hubbard, R.M., Ryan, M.G., Stiller, V. and J.S. Sperry. 2001. Stomatal conductance and

photosynthesis vary linearly with plant hydraulic conductance in ponderosa pine.


Koch, G.W., Sillett, S.C., Jennings, G.M., and S.D. Davis. 2004. The limits to tree

height. Nature 428(22): 851-854.


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Mencuccini, M., Martinez-Vilalta, J., Vanderklein, D., Hamid, H.A., Korakaki, E., Lee,

S., and B. Michiels. 2005. Size-mediated ageing reduces vigour in trees.

Ecology Letters 8: 1183-1190.

Meng, S.X., Lieffers, V.J., Reid, D.E.B., Rudnicki, M., Sillins, U., and M. Jin. 2006.

Reducing stem bending increases the height growth of tall pines. Journal of

Experimental Botany 57(12): 3175-3182.

Milburn, J.A. 1996. Sap ascent in vascular plants: challengers to the cohesion theory

ignore the significance of immature xylem and the recycling of Mnch water.

Annals of Botany 78: 399-407.

Patrick, J.W., Zhang, W., Tyerman, S.D., Offler, C.E. and N.A. Walker. 2001. Role of

membrane transport in phloem translocation of assimilates and water. Australian

Journal of Plant Physiology 28: 695-707.

Phillips, N., Bond, B.J., McDowell, N.G., Ryan, M.G., and A. Schauer. 2003a. Leaf area

compounds height-related hydraulic costs of water transport in Oregon white oak

trees. Functional Ecology 17: 832-840.

Phillips, N., Bond, B.J., and M.G. Ryan. 2001. Gas exchange and hydraulic properties in

the crowns of two tree species in a Panamanian moist forest. Trees 15: 123-130.

Phillips, N.G., Ryan, M.G., Bond, B.J., McDowell, N.G., Hinckley, T.M., and J. Cermak.

2003b. Reliance on stored water increases with tree size in three species in the

Pacific Northwest. Tree Physiology 23: 237-245.


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Pothier, D., Margolis, H.A. and R.H. Waring. 1989. Patterns of change of saturated

sapwood permeability and sapwood conductance with stand development.

Canadian Journal of Forest Research 19: 432-439.

Protz, C.G., Sillins, U., and V.J. Lieffers. 2000. Reduction in branch sapwood hydraulic

permeability as a factor limiting survival of lower branches of lodgepole pine.

Canadian Journal of Forest Research 30: 1088-1095.

Rich, P.M. 1987a. Mechanical structue of the stem of arborescent palms. Botanical

Gazette 148(1): 42-50.

Rich, P.M. 1987b. Developmental anatomy of the stem ofWelfia georgii,Iriartea

gigantea, and other arborescent palms: implications for mechanical support.

American Journal of Botany 74(6): 792-802.

Rich, P.M., Helenurm, K., Kearns, D., Morse, S.R., Palmer, M.W., and L. Short. 1986.

Height and stem diameter relationships for dicotyledonous trees and arborescent

palms of Costa Rican tropical wet forest. Bulletin of the Torrey Botanical Club

113(3): 241-246.

Ryan, M.G., Binkley, D., Fownes, J.H., Giardina, C.P. and R.S. Senock. 2004. An

experimental test of the causes of forest growth decline with stand age.

Ecological Monographs 74: 393-414.

Ryan, M.G., Bond, B.J., Law, B.E., Hubbard, R.M., Woodruff, D., Cienciala, E., and J.

Kucera. 2000. Transpiration and whole-tree conductance in ponderosa pine trees

of differing heights. Oecologia 124: 553-560.


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Ryan, M.G., and R.H. Waring. 1992. Maintenance respiration and stand development in

a subalpine lodgepole pine forest. Ecology 73(6): 2100-2108.

Ryan, M.G., and B.J. Yoder. 1997. Hydraulic limits to tree height and tree growth.

Bioscience 47(4): 235-242.

Salleo, S., Lo Gullo, M.A., De Paoli, D., and M. Zippo. 1996. Xylem recovery from

cavitation-induced embolism in young plants ofLaurus nobilis: a possible

mechanism. New Phytologist 132: 47-56.

Salleo, S., Lo Gullo, M.A., Trifilo, P., and A. Nardini. 2004. New evidence for a role of

vessel-associated cells and phloem in the rapid xylem refilling of cavitated stems

ofLaurus nobilis L. Plant, Cell and Environment 27: 1065-1076.

Schfer, K.V., Oren, R., and J.D. Tenhunen. 2000. The effect of tree height on crown

level stomatal conductance. Plant, Cell and Environment 23: 365-375.

Smith, D.M, Larson, B.C., Kelty, M.J., and P.M.S. Ashton. 1997. Management of

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Sovonick-Dunford, S., Lee, D.R., and M.H. Zimmermann. 1981. Direct and indirect

measurements of phloem turgor pressure in white ash. Plant Physiology 68: 121-

126.

Sperry, J.S. 1985. Xylem embolism in the palmRhapis excelsa. IAWA Bulletin n.s. 6:

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Sperry, J.S. 1986. Relationship of xylem embolism to xylem pressure potential, stomatal

closure, and shoot morphology in the palmRhapis excelsa. Plant Physiology 80:

110-116.

Sperry, J.S., Alder, N.N. and S.E. Eastlack. 1993. The effect of reduced hydraulic

conductance on stomatal conductance and xylem cavitation. Journal of

Experimental Botany 44(263): 1075-1082.

Sterck, F.J. and F. Bongers. 2001. Crown development in tropical rain forest trees:

patterns with tree height and light availability. Journal of Ecology 89: 1-13.

Stiller,V., Sperry, J.S., and R. Lafitte. 2005. Embolized conduits of rice (Oryza sativa,

Poaceae) refill despite negative xylem pressure. American Journal of Botany

92(12): 1970-1974.

Tomlinson, P.B. 1979. Systematics and ecology of the palmae. Ann. Rev. Ecol. Syst.

10: 85-107.

Tomlinson, P.B. 1987. Architecture of tropical plants. Annual Review of Ecology and

Systematics 18: 1-21.

Tomlinson, P.B. 2006. The uniqueness of palms. Botanical Journal of the Linnean

Society 151: 5-14.

Tyree, M.T., and J.S. Sperry. 1989. Vulnerability of xylem to cavitation and embolism.

Annual Review of Plant Physiology and Molecular Biology 40: 19-38.

Waterhouse, J.T, Quinn, F.L.S., and C.J. Quinn. 1978. Growth patterns in the stem of

the palmArchontophoenix cunninghamiana. Botanical Journal of the Linnean

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West, G.B., Brown, J.H. and B.J. Enquist. 1999. A general model for the structure and

allometry of plant vascular systems. Nature 400: 664-667.

Woodruff, D.R., Bond, B.J., and F.C. Meinzer. 2004. Does turgor limit growth in tall

trees? Plant, Cell and Environment 27: 229-236.

Woodruff, D.R., McCulloh, K.A., Warren, J.M., Meinzer, F.C., and B. Lachenbruch.

2007. Impacts of tree height on leaf hydraulic architecture and stomatal control in

Douglas-fir. Plant, Cell and Environment 30: 559-569.

Zimmermann, M.H. 1973. The monocotyledons: Their evolution and comparative

biology IV. Transport problems in arborescent monocotyledons. The Quarterly

Review of Biology 48: 314-321.

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

York.

Zimmermann, M.H., McCue, K.F. and J.S. Sperry. 1982. Anatomy of the palmRhapis

excelsa, VIII. Vessel network and vessel-length distribution in the stem. Journal

of the Arnold Arboretum 63: 83-95.

Zwieniecki, M.A., and N.M. Holbrook. 1998. Diurnal variation in xylem hydraulic

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and red spruce (Picea rubens Sarg.). Plant, Cell and Environment 21: 1173-

1180.

Zwieniecki, M.A., Hutyra, L., Thompson, M.V., and N.M. Holbrook. 2000. Dynamic

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(Liriodendron tulipifera L.) and northern fox grape (Vitis labrusca L.). Plant,

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CHAPTER 2

COMPARATIVE HYDRAULIC AND ANATOMIC PROPERTIES IN PALMTREES (WASHINGTONIA ROBUSTA) OF VARYING HEIGHTS:

IMPLICATIONS FOR HYDRAULIC LIMITATION TO INCREASED

HEIGHT GROWTH

2.1 INTRODUCTION

It has been frequently observed that as trees mature, height growth reaches a peak,

at which point rates begin to decrease (Barnes et al. 1998; McDowell et al. 2002a; Litvak

et al. 2003; Bond-Lamberty et al. 2004; Koch et al. 2004). There have been several

proposed hypotheses regarding forest growth decline and, in particular, why tree height

growth may begin to decline as trees get older and taller, including increased respiration

(Ryan and Waring, 1992), differences in the vigor of older tissues relative to younger

ones (Mencuccini et al. 2005; Bond et al. 2007; Vanderklein et al. 2007) and increased

mechanical stresses (Meng et al. 2006). However, there are several promising studies that

suggest that hydraulic limitation may not only explain why height growth in tall trees is

limited (Ryan and Yoder 1997) but could be used to predict maximum heights of a given

tree species growing under given environmental conditions (Koch et al. 2004; Burgess

and Dawson 2007). The hydraulic limitation hypothesis is built upon the idea that as trees

get taller, not only does the hydrostatic gradient due to gravity increase, but the path

length of water travel increases, with taller trees overcoming more friction in water

transport than shorter trees. This means that taller trees are, by virtue of their height, less

efficient at transporting water to their leaves relative to shorter trees. This lower

efficiency could lead to lower stomatal conductance and, therefore, reduced


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photosynthesis and carbon gain (Ryan and Yoder 1997). Additionally, the turgor

pressure at the tops of these trees will decrease, unless osmotic adjustment occurs,

making cell expansion more difficult in developing leaves (Koch et al. 2004; Woodruff et

al. 2004). Decreased turgor at the tops of the largest palms could therefore lead to

decreases in leaf cell sizes and increases in leaf mass per unit area (LMA).

Studies in species ranging from ponderosa pine (Pinus ponderosaDougl.ex C.

Lawson) (Hubbard et al. 1999; Ryan et al. 2000), European beech (Fagus sylvatica L.)

(Schfer et al. 2000), eucalyptus (Eucalyptus saligna Sm.) (Barnard and Ryan 2003),

Oregon white oak (Quercus garryana Dougl.) (Phillips et al. 2003a) and the tallest trees

in the world, coastal redwoods (Sequoia sempervirens(D. Don) Endl.) (Koch et al. 2004)

have found evidence that the hydraulic cost of increased frictional resistance reduced

stomatal conductance in tall trees relative to shorter ones (reviewed in Ryan et al. 2006).

However, there are other studies that suggest that the hydraulic costs that taller trees face

can be offset by alterations in their architecture (Becker et al. 2000a). Also, theoretical

models (West et al. 1999; Becker et al. 2000b) as well as empirical measurements show

that hydraulic resistance due to path length can be significantly reduced (Weitz et al.

2006, Coomes et al. 2007), but in very tall trees not completely overcome (Anfodillo et

al. 2006; Petit et al. 2008), by the tapering of vascular conduits along the length of trees.

However, these compensatory features of taller trees are, in fact, consistent with the

presence of hydraulic constraints to water transport in taller trees.

About 60 to 70% of studies that have measured one or more of the components of

the hydraulic limitation hypothesis have found results that were consistent (Ryan et al.
http://en.wikipedia.org/wiki/David_Donhttp://en.wikipedia.org/wiki/Stephan_Ladislaus_Endlicherhttp://en.wikipedia.org/wiki/David_Donhttp://en.wikipedia.org/wiki/Stephan_Ladislaus_Endlicher


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2006), although several studies have provided some contradictory data (McDowell et al.

2002a; Barnard and Ryan 2003). One reason for the conflicting information could be the

complexity of most tree systems. Their complicated branching patterns mean that each

leaf on the tree will have a different path length from the ground. Also, the hydraulic

architecture has been shown to be designed in such a way that branches receiving more

sunlight are hydraulically favored over lower branches (Protz et al. 2000). Much of this

complexity could be avoided if a simpler tree species such as palms were used to study

hydraulic limitation. Palms represent a desirable tree form to use because, unlike most

trees, they lack complex branching patterns and exhibit relatively fixed crown sizes

(Zimmermann et al. 1982; Tomlinson 1990) making the path length of water flow as well

as leaf area easily measurable. Additionally, hydraulic limitations in palms become

especially important considering they lack secondary growth and may exhibit decreased

functioning of xylem and phloem tissues with age (Zimmermann, 1973). This is

especially relevant for a palm species such as Washingtonia robusta where older, taller

palms are likely to have experienced more frost episodes over their lifetime than younger,

shorter palms and may not be able to refill embolized conduits (Sperry, 1986). If vessels

are able to refill, multiple freeze-thaw episodes have been shown to negatively affect the

functioning of xylem tissues through cavitation fatigue, which may or may not be

reversible (Hacke et al. 2001; Stiller and Sperry, 2002).

We studied hydraulic limitation in Mexican fan palms, Washingtonia robusta (H.

Wendl.), a species that is naturally distributed throughout southern and central Baja

California and western Sonora, Mexico along streams and canyons or near springs (Uhl


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and Dransfield 1987; Bullock and Heath 2006). Bullock and Heath (2006) studied

Washingtonia robusta in the Baja California desert and estimate that they reach

reproductive maturation at approximately 8 m tall with the tallest palms in their study

being 32 m. They also estimate the potential longevity of these palms to exceed 500

years. We hypothesize that hydraulic constraints on leaf gas exchange will increase with

height in Washingtonia robusta with taller palms having lower sap flux per unit leaf area

and lower stomatal conductance than shorter palms. Additionally, we are interested in

any alterations in physiology or hydraulic architecture that tall palms exhibit in order to

compensate for an increased path length of water flow relative to shorter palms; including

changes in minimum leaf water potential, maximum photosynthetic rates, and leaf area to

conducting area ratios. Not only could this research shed light on the physiological costs

of increasing size in palms, but, because the biophysical variables we studied are also

shared by woody plants, it may shed light on the physiological costs and compensations

in tree species, in general, as they grow taller.

2.2 MATERIALS AND METHODS

2.2.1 SITE DESCRIPTION

This study was performed from July 23 to August 3, 2007 on 10 individuals of

Washingtonia robusta (H. Wendl.) growing at the Los Angeles County Arboretum &

Botanic Garden (34 8' 29.43"N, 118 3' 15.15"W) in Arcadia, California. The site

contained several open-grown palm individuals scattered throughout a lawn with palms

varying in height from 1 m tall to approximately 35 m tall. The site is adjacent to a

natural aquifer; therefore, the water table is elevated in this location. Also, the area was


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sprinkler irrigated approximately three times per week for one to two hours at each

interval, and plants, therefore, should not have experienced soil moisture stress during the

experimental period. Measurements of solar radiation were made using a pyranometer

(Apogee Instruments, Roseville, CA, USA) located on an open lawn. Diurnal measures

of temperature and relative humidity were made using a Campbell Scientific (CS215)

temperature and relative humidity sensor (Campbell Scientific Inc., Logan UT, USA)

located approximately 5 m above the ground in an exposed area. Atmospheric and

radiation data were captured at intervals of 30 sec and logged every 2 min using a CR10X

datalogger and an associated AM16/32A multiplexer (Campbell Scientific Inc., Logan,

UT, USA). For the 10 day measurement period, daily maximum temperatures ranged

from 29 to 34oC and nightly minimum temperature ranged from 17 to 20 oC. Daily

minimum humidity ranged from 21 to 50% and nightly maximum humidity ranged from

82 to 90%. Daily maximum vapor pressure deficits (VPD) ranged from 2 to 4 kPa.

Maximum daily solar radiative flux densities ranged from 850 to 1060 W m-2.

2.2.2 PALM HEIGHT ESTIMATION

Palm heights were estimated, with the aid of a tape measure, from the ground to

the point at which the lowest leaves of the crown attached to the bole. Tall palms were

accessed using a bucket lift. For shorter palms, height was estimated by making a mark

on the bole 1 m above the ground, then estimating height by eye based on this mark.

Because shorter palms tended to have more leaves, height was measured from the ground

to the midpoint of the crown of leaves. The palm reported to be 2 m tall was a juvenile


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that did not possess a trunk; therefore its height was measured from the top of its leaves

to the ground.

2.2.3 SAP FLUX MEASUREMENT

Sap flux was measured in a total of 10 palms in this study, ranging in height from

2 m to 34 m. Sap flux sensors were distributed as follows: Sap flux sensors were

installed in the boles of eight palms ranging from 7 m to 34 m tall using 2 cm long

Granier heat dissipation sensors (Granier 1987). Sap flux was also measured in the

petioles in five palms ranging from 2 m tall to 34 m tall, in each of which two sap flux

sensors were installed in two separate petioles. Table 2.1 summarizes tree data and

sampling details.

In boles, the leaf bases (if any) were removed from the base of the palm and two

sensors were installed on either side of the bole directly beneath the pseudobark, with this

position indicated by a color change that was indicative of wet, conductive bole material.

An additional sensor was also installed at a depth of 2 cm below the pseudobark, giving a

total of three sensors per palm bole. Sensors were connected to a CR10X datalogger and

an associated AM16/32A multiplexer that captured data at intervals of 30 sec and logged

every 2 min (Campbell Scientific Inc., Logan, UT, USA). The boles were wrapped with

reflective insulation to prevent external temperature fluctuations. Data from outer bole

sensors and the 2 cm depth sensor were pooled because statistical analysis indicated that

they were not significantly different (p-value = 0.25). Sap flux data (g m-2 s-1) were

scaled up to the whole tree level by multiplying by the cross-sectional area of the bole

minus the area of the pseudobark (approximated to be 1 cm thick) giving sap flux units of


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kg day-1 and then divided by total palm leaf area to give units of kg day-1 m-2 leaf area.

We are assuming that sap flux is more or less constant across the radius of the bole (and

indeed we found no significant differences between outer bole and 2 cm depth sensors).

This assumption is also validated by work done by Roupsard et al. (2006) who found a

constant pattern of sap flux throughout the stem of coconut palms (Cocos nucifera L.) up

to a 12 cm radius. Likewise, Sellami and Sifaoui (2003) found that the sap flow in date

palms (Phoenix dactylifera L.) did not differ significantly between the 3 cm and 6 cm

depth sensors.

For the petiole sap flux, 1 cm long Granier sensors were only used in the petioles

of the 2 m tall palm. All other petioles had 2 cm long Granier sensors. The shape of the

petiole cross-section is approximately a semi-circle and sensors were inserted in the flat

portion of the petiole (adaxial side). Petioles were then wrapped with reflective

insulation to minimize external temperature fluctuations. Sap flux data (g m-2 s-1) were

scaled-up by multiplying by the cross-sectional area of the petiole, giving sap flux units

of kg day-1 and then divided by individual frond leaf area to give units of kg day-1 m-2 leaf

area.

Stem water storage was also investigated in order to ascertain contributions of

stored water in the bole to overall sap flux. Palms that are more reliant on stored bole

water will tend to lag behind palms that have less stored bole water. To evaluate this,

diurnal time courses of bole sap fluxes (g m-2 s-1) in two short palms (7 m and 8 m) were

compared with diurnal time courses of bole sap fluxes (g m -2 s-1) in three tall palms (28 m

to 34 m) using cross-correlation analysis. In addition, two of the ten palms (8 m and 28


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m tall) had simultaneous bole and petiole sap flow data. In these individuals, stem water

storage was estimated by pairing diurnal time courses of sap flux made in the bole with

that of corresponding petioles using cross-correlation analysis. The time lag

corresponding to the maximum degree of correlation between the petiole sap flux and the

bole sap flux represented the approximate amount of water storage in the bole of the

palm.

2.2.4 STOMATAL CONDUCTANCE AND PHOTOSYNTHESIS

Stomatal conductance and photosynthetic assimilation were measured diurnally

on five palms ranging from 8 m to 34 m tall using a LiCor 6400 photosynthesis system

(Licor, Lincoln, NE, USA) fitted with a red-blue LED light source. Five palms were

chosen from among the 10 total palms to span a large range in height of reproductively

mature individuals, and to maintain reasonable sampling time intervals throughout the

day. Measurements were made over a four day period at intervals of approximately 1.5 h

(from approx. 7 am to 5 pm) with a light level of 1500 mol m-2 s-1 and a CO2 level of

400 mol mol-1. Two leaf segments from different fronds were measured per palm during

each time period. Measurements were taken when both stomatal conductance and

photosynthetic assimilation had stabilized. Measurements from the two leaf segments

were averaged per time period. Polynomial equations were then fit to the diurnal stomatal

conductance and photosynthetic assimilation data and were used to calculate maximum

daily stomatal conductance and maximum daily photosynthetic assimilation rate (both

calculated by setting the first derivative equal to zero) for each palm measured.


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Equations and r2 values are presented in Table 2.2. With few exceptions, the rankings of

maximum photosynthetic assimilation and maximum stomatal conductance across tree

height were the same whether values were obtained from the polynomial equations or

from the raw data.

2.2.5 WATERPOTENTIAL MEASUREMENT

Diurnal measures of leaf water potential were made on five palms ranging from 2

m to 28 m tall using a Scholander type pressure chamber (Soil Moisture Equipment

Corp., Santa Barbara, CA, USA)(Scholander et al. 1965). These individuals were selected

from among the ten total palms to cover a large range in palm height but to maintain a

manageable diurnal sampling schedule. Leaf water potentials were measured over a four

day period at intervals of approximately 1.5 h (from approximately 6 am to 5 pm). Water

potential measurements were made by removing two individual segments from the leaf.

Leaf segments were immediately placed in a plastic bag and covered in order to promote

stomatal closure and prevent further leaf dehydration. An approximately 1 cm length of

leaf tissue was removed from either side of the midrib, and the remainder of the segment

was placed in the pressure chamber with the midrib exposed to the outside environment.

Measurements from the two leaf segments were averaged to give one measurement per

palm at a given time period. Polynomial equations were then fit to the diurnal leaf water

potential data and were used to calculate minimum leaf water potential and the time of

minimum leaf water potential (both calculated by setting the first derivative equal to

zero) for each palm. Equations and r2 values are presented in Table 2.2. The rankings of


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minimum leaf water potential across tree height were the same whether values were

obtained from the polynomial equations or from the raw data.

2.2.6 LEAF CHARACTERISTICS

Leaf areas were estimated for the same two leaves used to measure petiole sap

flux in four of the ten study palms ranging from 8 m to 34 m tall. These palms

represented a range of heights and could be accessed using the bucket lift or ladder.

Leaves were harvested after sap flux measurement had been completed and an overhead

digital photo was taken of each leaf with a known scaling factor. Image analysis software

(Image J, Scion Image, Frederick, MD, USA) was then used to measure leaf area.

Measurements of the two leaves were pooled to give an average leaf area for each palm.

In order to measure leaf dry mass per unit area (LMA), three leaf segments from each

leaf were removed and their individual areas measured. These leaf segments were

allowed to air dry for one week, then their dry weight was obtained. Additionally, live

leaf counts were made on each individual.

Leaf epidermal cell sizes, stomatal density and guard cell length were measured

by making hand sections of leaf material. To begin, individual leaf segments were

gathered from palms 2 m to 34 m tall, and then thin sections were cut with a razor blade

from the adaxial side of two individual leaf segments tangential to the midrib. Sections

were then stained with a solution of 1% Toluidine Blue O and mounted on slides using

Permount. The slides were viewed at 200X magnification using a compound light

microscope (Nikon Alphaphot-2, Melville, NY, USA) and photographs were taken with

digital camera (Fuji FinePix F700 Valhalla, NY, USA). These photographs were


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imported into image analysis software (Image J, Scion Image, Frederick, MD, USA) for

measurement. Leaf epidermal cell areas were measured by tracing around the perimeter

of approximately 20 to 30 cells per photograph and calculating the area of this traced

section. Approximately 2 to 5 photographs per palm were used for this measurement

giving a total of approximately 50 to 150 cells measured per palm. Stomatal densities

were calculated by counting the number of stomata within a field of view, then

calculating the area of that field of view. Approximately 15 to 20 different fields of view

were used per palm. Guard cell lengths were calculated by measuring the distance

between the two points where the guard cells meet. Approximately 50 and 100 stomata

were measured per palm in order to calculate average guard cell length.

2.2.7 STATISTICAL ANALYSES

Means, standard errors and Tukey HSD tests were calculated using JMP 7.0

statistical software (SAS Institute, Cary, NC, USA). All linear and polynomial

regressions were fitted using SigmaPlot 2000 Version 6.1 (SPSS Inc. Chicago, IL, USA)

software. R2 and p-values for all regressions were also calculated using SigmaPlot 2000.

2.3 RESULTS

Palms were found to range in height from 2 m tall to 34 m tall. Due to its

significant lean, the crown of one of the palms reported to be 34 m tall was only 27 m

about the ground. However, because hydraulic limitation is concerned with friction

imposed on water travel as well as hydrostatic gradients in water potential, trunk length is

as important in this study as actual crown height above the ground. All other palms did


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not possess a significant lean. The palm reported to be 2 m tall was a juvenile that did

not possess a trunk therefore, because this individual differed so greatly in development

from the other individuals, only petiole sap flux, water potentials and leaf epidermal cell

areas were compared with taller palms, and only to make limited inferences (in

discussion).

Representative diurnal time courses of bole sap flux (g m-2 s-1) in palms ranging in

height from 7 m to 34 m tall are presented in Fig. 2.1 with corresponding VPD and solar

radiation time data. Daily minimums, maximums, means and standard errors for the

palms studied are presented in Table 2.3. There is a slight positive relationship between

palm height and daily bole sap flux per unit leaf area (Fig. 2.2a; p=0.06, r2 = 0.46) due to

the decrease in leaf area with increases in palm height, but no relationship between palm

height and daily petiolar sap flux per unit leaf area (Fig 2.2b; p-value= 0.54). Variability

in Washingtonia robusta sap flux both within and between palms was considerable with

bole sap flux per unit leaf area differing by up to a factor of about 3 and petiole sap flux

per unit leaf area differing by almost a factor of about 6. Roupsard et al. (2006) also

found large variability in coconut palms with sap flow varying by up to a factor of 3

between palms.

Investigation of stem water storage using between-palm cross-correlation analysis

indicated that bole sap flux in large palms lagged behind bole sap flux of small palms by

30 min (n= 4 palms; maximum r2 = 0.68). Similarly, within-palm cross-correlation

analysis indicated a larger petiole-bole time lag in a larger palm (44 min lag in a 28m tall

palm; maximum r2 = 0.87) than a shorter palm (28 min lag in an 8 m tall palm; maximum


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r2 = 0.8). These lags correspond to approximately 16 and 22% of daily water use

respectively according to procedures and assumptions described in Phillips et al (2003b).

Daily maximum stomatal conductances and photosynthetic assimilation rates

were calculated from the curves shown in Figs. 2.3a and b and equations with r2 values

are presented in Table 2.2. Maximum daily stomatal conductance showed no discernable

relationship with tree height (Fig. 2.4a; p-value = 0.36). Additionally, there was no

observable relationship between tree height and either stomatal density (p-value = 0.82)

or guard cell length (p-value = 0.86)(data not shown). Maximum daily photosynthetic

assimilation rate exhibited a positive linear correlation with tree height (Fig. 2.4b;

r2=0.80, p-value=0.041) with taller palms exhibiting higher photosynthetic rates than

shorter palms. There was no observable relationship between LMA and tree height (Fig

2.4c; p-value = 0.19) and individuals did not differ significantly from one another.

Minimum leaf water potentials and the timing of minimum leaf water potentials

were calculated from the curves shown in Fig. 2.3c and equations with r2 values are

presented in Table 2.2. Minimum leaf water potentials were significantly and non-

linearly correlated with tree height (Fig. 2.5a; r2=0.97, p-value=0.0234) with taller palms

generally having more negative minimum leaf water potentials than shorter palms;

although the slope of the curve decreases with increasing height. Also, the time of

minimum leaf water potential was negatively correlated with tree height (Fig 2.5b;

r2=0.61, p-value=0.12) with taller palms reaching a minimum leaf water potential earlier

in the day than shorter palms. Because all individuals were open grown, it is unlikely

that differences in the timing of minimum leaf water potential were the result of


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differences in solar radiation throughout the day. The earliest minimum leaf water

potential (in the tallest palm) was reached at approximately 13:42 PDT and the latest

minimum leaf water potential (in the shortest palm) was reached at 16:36 PDT (mean =

14:42 PDT, SE = 29 min). Although the 2 m tall palm was still a juvenile and therefore

differed developmentally from the other individuals, we feel that comparisons of leaf

water potential are still valid.

Leaf areas were significantly and negatively correlated with tree height. Taller

palms had fewer leaves (Fig. 2.6a; r2=0.6, p-value=0.026) and leaves with smaller areas

(Fig. 2.6b; r2=0.99, p-value = 0.058) than shorter palms; although both of these

relationships were non-linear with the slope of the curve decreasing as tree height

increased. Leaf epidermal cell sizes were found to remain constant in palms 2 m tall to

22 m tall with Tukey HSD tests confirming that cell sizes in these palms did not differ

significantly from one another at a 95% confidence level (Fig. 2.6c). On the other hand,

leaf epidermal cell sizes in both 28 m tall palms and 34 m tall palms were found to be

significantly smaller (p-values < 0.05) than in palms 2 m to 22 m tall. Additionally, leaf

epidermal cells from the 34 m tall palm were found to be significantly smaller than cells

from the 28 m tall palm (Fig. 2.6c; p-value < 0.001).

2.4 DISCUSSION

The hydraulic limitation hypothesis states that as trees grow taller, greater friction

to water flow causes taller trees to reach minimum leaf water potentials sooner in the day

than shorter trees causing stomatal closure that decreases carbon gain (Yoder et al. 1994;

Ryan and Yoder 1997). However, Washingtonia robusta palms showed no discernable


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decrease in daily bole sap flux per unit leaf area, daily petiolar sap flux per unit leaf area,

maximum stomatal conductance, stomatal densities or stomatal sizes with an increase in

tree height suggest that taller palms are not experiencing the effects of hydraulic

limitation. Barnard and Ryan (2003) also found that tallerEucalyptus trees had similar

sap flux per unit leaf area and whole tree stomatal conductance compared to their shorter

counterparts. McDowell et al. (2002a) found similar results in old-growth Douglas-fir

trees, in that stomatal conductance, photosynthetic assimilation and leaf-specific

hydraulic conductance were not significantly different among trees of different heights.

Although hydraulic limitation was not observed in Washingtonia robusta in terms

of lower stomatal conductance in taller palms, we found several alterations in physiology

with height including decreasing leaf area, decreasing leaf cell sizes and more negative

midday leaf water potentials suggesting that tall palms face, and in turn compensate for,

some degree of hydraulic path length constraint. For example, taller palms reached a

minimum leaf water potential sooner in the day than shorter palms. Because all

individuals were open grown, it is unlikely that differences in the timing of minimum leaf

water potential were the result of differences in solar radiation throughout the day. The

timing of minimum leaf water potential may have been even more disparate had all palms

exhibited the same minimum leaf water potential. However, as with many other studies

of trees across a height gradient (McDowell et al. 2002a; Barnard and Ryan 2003;

Woodruff et al. 2007), taller palms also exhibited more negative minimum leaf water

potentials than shorter trees. It is interesting that taller palms would be able to withstand

more negative water potentials than shorter palms and it begs the question; Why dont


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shorter palms keep their stomata open longer and reach the minimum water potentials

that tall palms do? One reason could be that the petiole xylem of taller palms is more

resistant to cavitation than petioles of shorter palms. Woodruff et al. (2007) found

minimum leaf water potentials were highly correlated with the water potential at which

leaves lost hydraulic conductance along a height gradient. Their finding implies that not

only do the leaves at the top of tall trees keep their stomata open at more negative leaf

water potentials, but stomatal closure could be correlated to a loss of leaf hydraulic

conductance.

Lower leaf water potentials were one way in which taller palms compensate for

a longer path length of water travel. However, in order to sustain these more negative

water potentials, leaf cells in taller trees may have had to increase their osmotic potential

in order to maintain turgor (Meinzer et al., 2008). This additional carbon requirement in

taller palms could negatively affect increased height growth. It is also interesting to note

that differences in minimum leaf water potential between the 8 m tall palm and the 28 m

tall palm are approximately 0.4 MPa, and we would expect the taller palm to exhibit, at

minimum, a decrease of 0.2 MPa, simply due to the hydrostatic gradientalone (i.e. ifthere were zero frictional resistance). Therefore, about half of the difference in leaf water

potential between the 8 m tall palm and the 28 m tall palm is a consequence of moving

water against gravity with the other half likely resulting from the added friction to the

water column given the increased path length.

Another compensation that trees can make to offset hydraulic limitation is to alter

their hydraulic architecture so that they have less leaf area for a given unit of sapwood


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area that needs to be supplied with water. For many palm species including Washingtonia

robusta,bole diameters remain more or less constant across various heights once their

stems reach maximum diameter and begin to elongate vertically. Likewise, palm species

do not lose conducting area of their boles through the formation of heartwood. Therefore,

any changes in hydraulic architecture in this and many other palm species occur mainly

through changes in leaf area. In our study, not only did taller palms have fewer leaves

than shorter palms, but the leaves they had were smaller in area. This is unlikely to be

influenced by light environment since all individuals were open-grown.Decreases in leaf

area to sapwood area ratios across height has been seen in several other studies (Schfer

et al. 2000; Phillips et al. 2001; McDowell et al. 2002b; Barnard and Ryan 2003),

although there are some exceptions to this trend (Phillips et al., 2003a). Specifically,

Buckley and Roberts (2006) predict that leaf area to sapwood area ratios should increase

with height growth (until height growth tapers off) in order to maximize carbon gain. Of

course, thes

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