Tree Physiology 29, 663–674
doi:10.1093/treephys/tpp006
A comparison of growth, photosynthetic capacity
and water stress in Eucalyptus globulus coppice regrowth
and seedlings during early development
PAUL L. DRAKE,1,2,3,4 DANIEL S. MENDHAM,3,4 DON A. WHITE3,4
and GARY N. OGDEN3,4
1
School of Biological Sciences and Biotechnology, Murdoch University, South Street, Murdoch, WA 6150, Australia
2
Corresponding author ([email protected])
3
CSIRO Sustainable Ecosystems, Centre for Environment and Life Sciences, Private Bag 5, Wembley, WA 6913, Australia
4
Co-operative Research Centre for Forestry, Private Bag 12, Hobart, Tasmania 7001, Australia
Received November 2, 2008; accepted January 11, 2009; published online February 19, 2009
Summary Eucalyptus globulus Labill., a globally significant plantation species, is grown commercially in a
multiple rotation framework. Second and subsequent
crops of E. globulus may be established either by allowing
the cut stumps to resprout (commonly referred to as
coppice) or by replanting a new crop of seedlings.
Currently, long-term growth data comparing coppice and
seedling productivity in second or later rotations in
southern Australia is limited. The capacity to predict
productivity using these tools is dependent on an understanding of the physiology of seedlings and coppice in
response to light, water and nutrient supply. In this study,
we compared the intrinsic (independent of the immediate
environment) and native (dependent on the immediate
environment) physiology of E. globulus coppice and
second-generation seedlings during their early development in the field. Coppice not only grew more rapidly, but
also used more water and drew on stored soil water to a
depth of at least 4.5 m during the first 2 years of growth,
whereas the seedlings only accessed the top 0.9 m of the soil
profile. During the same period, there was no significant
difference between coppice and seedlings in either their
stomatal response to leaf-to-air vapour pressure difference
(D) or intrinsic water-use efficiency; CO2- and lightsaturated rates of photosynthesis were greater in seedlings
than that in coppice as were the quantum yield of
photosynthesis and total leaf chlorophyll content. Thus,
at a leaf scale, seedlings are potentially more productive per
unit leaf area than coppice during early development, but
this is not realised under ambient conditions. The underlying cause of this inherent difference is discussed in the
context of the allocation of resources to above- and belowground organs during early development.
Keywords: carboxylation efficiency, defoliation, quantum
yield, resprout, soil water, water-use efficiency.
Introduction
Eucalyptus globulus Labill. is a globally significant plantation species and more than 450,000 ha are now planted in
Mediterranean environments across southern Australia
(Parsons et al. 2006). Most of these plantations were established after 1997 and over the past 3 years the harvested
area has increased exponentially. After harvest, plantation
managers can choose to re-establish the plantation either
with new seedlings or by allowing the cut stumps to
resprout (coppice). Process-based models of plantation
growth (e.g., CABALA – Battaglia et al. 2004) are one of
the options for simulating potential yield. Unfortunately,
such models have not been parameterized to account for
genetic differences in planting stock within species, genetic
gain or for second rotation coppice and seedlings. Realising
the potential of these tools will therefore require a quantitative understanding of key processes that affect growth in
each such system. For example, information on leaf-scale
carbon gain and water use in coppice and seedlings would
extend our understanding of the long-term trade-off
between stand productivity and drought susceptibility in
multiple rotation systems employing these growth forms.
Growth form in this context describes the structural contrast between coppice and seedlings.
After an E. globulus stem is harvested, the root system is
potentially able to supply large quantities of nutrients and
water relative to the size of the developing shoots, because
it has access to more soil resources and a large intrinsic
store of carbon in the form of starch reserves (Fleck
et al. 1996, Poorter and Nagel 2000). This imbalance
between above- and below-ground biomass may also
mean that starch reserves and current photosynthate can
be allocated to above-ground biomass components instead
of roots. Regardless of this mechanism, vigorous aboveground growth has been observed during early coppice
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664
DRAKE, MENDHAM, WHITE AND OGDEN
development in E. globulus (Antonio et al. 2007). Seedlings, in contrast, allocate assimilated carbon to biomass
components to bring the supply of nutrients and water
into balance with demand so that under resource (water
or nutrient) limiting conditions, assimilated carbon will
be directed to the growth of roots and away from the
above-ground components (Santantonio 1989, Battaglia
and Sands 1997). In addition, the disproportional amount
of live below-ground biomass in coppice could mean that
coppice and seedlings experience different soil moistures
because of contrast in the spatial distribution of roots. It
is also possible that microclimatic differences, including
atmospheric vapour pressure deficit (VPD), will occur
between coppice and seedling canopies.
The physiological behaviour of E. globulus coppice is
less well understood than seedlings, despite acknowledgement that seedlings and coppice will experience different
conditions at the same site. Several studies conducted on
northern hemisphere species have observed lower drought
tolerance in seedlings compared to that in coppice (Oechel
and Hastings 1983, Fleck et al. 1996, 1998, Williams et al.
1997, Clemente et al. 2005), but no studies have compared
the intrinsic (independent of the immediate environment)
physiological function of these growth forms. There have
been many studies of the water relations, gas exchange
and productivity of first rotation E. globulus seedlings or
saplings (Pereira et al. 1987, White et al. 1996, 1999, Pita
and Pardos 2001, Cernusak et al. 2003, Costa E Silva
et al. 2004, Macfarlane et al. 2004a, O’Grady et al.
2008), but there is no exploration of these properties in
coppiced E. globulus. Seedling-grown E. globulus has a
well-defined stomatal response to VPD (White et al.
1999) and reduces stomatal aperture with exposure to
cyclical soil water stress events (White et al. 2000). These
types of physiological responses to environment are used
in process-based growth models including CABALA
(Battaglia et al. 2004), but are poorly understood for coppice despite the opportunities for coppice regrowth to be
used in second and later rotation E. globulus plantations.
Similarly, there has been no assessment of how the early
imbalance in biomass allocation in the E. globulus coppice
growth form affects intrinsic photosynthetic function. We
hypothesize that the disproportionate amount of belowground biomass in coppice will: (a) modify the manner
by which below-ground resources are allocated to the
newly developed above-ground coppice canopy and (b)
create a different environment for coppice compared to
seedlings. Either alone or collectively, these processes
could affect both native (dependent on the immediate
environment) and intrinsic (independent of the immediate
environment) physiological function. The objectives of this
study were to: (1) evaluate seedling and coppice native
physiology under conditions of contrasting soil and atmospheric water deficit and (2) investigate differences in photosynthetic responses to CO2 and light, as indicators of
intrinsic photosynthetic capacity, under ideal conditions.
Materials and methods
Plant material and field site
All measurements were taken on field-grown E. globulus
at a second rotation plantation located in south-western
Australia (3415 0 S and 11531 0 E). The climate of the
region is Mediterranean, characterized by cool wet winters and warm dry summers (Gentilli 1972). The average
maximum and minimum air temperatures are 21 and
11 C, respectively. Air temperature and VPD can
exceed 35 C and 3.5 kPa, respectively, in summer
(December to February), and the average annual rainfall
for the site is 1040 mm. The soil has a sandy textured
A horizon (0.5–1 m deep), over a clay B horizon,
separated by a lateritic hardpan.
The first rotation was planted in 1996 and an experiment
was established at the site in 1998 to explore the effects of
nitrogen and thinning on leaf area development and response
to drought. Plot dimensions were 40 · 40 m, with an internal
measure plot of 20 · 22 m. The plantation was originally
planted at 1250 stems ha1 and was supplied with a basal
fertilizer application at age 2 comprising phosphorus (100 kg
ha1 P), potassium (125 kg ha1 K), magnesium (10 kg
ha1 MgSO4Æ7H2O), manganese (10 kg ha1 MnSO4Æ
H2O), zinc (10 kg ha1 ZnSO4Æ7H2O) and copper (5 kg
ha1 CuSO4Æ5H2O). Well-fertilized plots, which were used
for the current study (see the following paragraphs), were
supplied with nitrogen (as urea) at 250 kg ha1 N year1.
When harvested in January 2006 (summer), the trees had
accessed the full depth of the B horizon, down to 6–10 m
below the surface, and had depleted the plant available soil
stored water down to this depth (White, unpublished data).
Following the harvest of the first rotation, six second
rotation experimental plots (three coppice plots and three
seedling plots) were established by either (a) replanting in
July 2006 (winter) with 7-month-old nursery-raised seedlings at a density of 1250 stems ha1 or (b) allowing first
rotation stumps to coppice. Survival of the coppice was
82% giving a final density of 1030 stems ha1 for coppice
plots (note that a stem in this context represents a coppice
stump comprising many, up to 50, individual emerging
stems). To suppress coppicing in the seedling plots, the
stumps were painted with glyphosate soon after harvesting.
Nitrogen was applied at non-limiting rates in both treatments (250 kg N ha1 year1 from year 2). A foliar analysis during the first rotation indicated that the remaining,
potentially limiting, macro- and micronutrients were within
the normal range for eucalypts (data omitted).
At the time of planting, seedlings were 0.25 m tall and
coppice, which resprouted soon after harvest, was 0.5 m
tall. In December 2006, coppice and seedling heights were
2.0 and 0.8 m, respectively, despite a similar age in both
growth forms. This early vigour in resprout E. globulus is
typical of that commonly observed (Blake 1983), hence
comparisons made in this study are likely to be general
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COPPICE AND SEEDLING PHYSIOLOGY
for the early development of second or later rotation
E. globulus plantations.
Specific leaf area, leaf area index and stand biomass
In December 2006, fully hydrated (Garnier et al. 2001) and
expanded leaves were collected from each coppice and seedling plot (n = 5 leaves per tree from 20 coppice and 20
seedlings spread across the six measure plots). Leaf areas
(m2) were measured with a leaf area meter (model
Li-3100C Li-Cor Inc., Lincoln, NE) and leaves were then
oven dried at 70 C for 48 h and dry weight (kg) was determined. Specific leaf area (SLA) was calculated for each leaf
as m2 of leaf surface area per kg of leaf dry weight.
Also in December 2006, the seedling stem conical volume
(Mendham et al. 2003) was calculated from the stem diameter (m, at a distance of 0.05 m from the ground surface)
and height (m) of each seedling was measured from each
measure plot. Because coppice stumps comprised many
individual stems, the total stem conical volume for each
stump was calculated from a relationship derived between
three-dimensional crown dimensions (width, length and
height, m) and individual stem dimensions (height and
diameter at 0.05 m) of a subset of 60 stumps (r2 = 0.81,
P < 0.001). Stem conical volume has been shown to be a
good correlate for total above-ground biomass components
(Mendham et al. 2003).
Also in December 2006, total above-ground biomass
(t dry weight ha1) and leaf area index (LAI) in seedling
and coppice plots were estimated using allometric functions
derived from a harvest sample of coppice and seedlings comprising stems of a range of diameter and height (n = 20
seedling and n = 20 coppice stems originating from the
six coppice and seedling plots). To develop allometric functions, we measured stem height, stem diameter (0.05 m from
the soil surface for seedlings and 0.05 m from the point of
emergence for coppice), total plant dry weight (kg) and leaf
area (m2) from each harvested sample. For seedlings, the
allometric functions linking stem conical volume to total
plant dry weight and leaf area allowed us directly to calculate the above-ground biomass and LAI in the seedling
plots. For coppice, we developed allometric functions linking crown volume to total plant dry weight and leaf area
(derived from the 60 subsamples), allowing estimates of
above-ground biomass and LAI in coppice plots.
Soil water
Neutron moisture access tubes were installed within each
plot (n = 2 for each) to a depth of 8 m. The tubes were
made from polyvinyl chloride with an internal diameter
of 0.04 m installed in 0.05 m holes back-filled with kaolin
and cement (Prebble et al. 1981). Soil moisture was monitored at 0.2 m increments to 1.5 m from the soil surface,
then at 0.5 m increments for the remaining soil profile using
a neutron moisture meter (Model CPN 503, Campbell
Pacific Nuclear, Concord, CA). Soil moisture was
665
monitored through the inter-rotation period and during
coppice and seedling establishment at intervals that were
approximately the same as for native leaf water potential
and gas exchange measurements (see below). Neutron
moisture meter counts were converted to soil volumetric
water content (h*, %) using a calibration-based approach
where soil samples, taken over the entire profile, were collected simultaneously with neutron moisture meter readings
over a range of values of h* (Hingston et al. 1998). Volumetric soil water for each vertical interval was multiplied
by the respective increment (mm) and, by summing all
intervals, an estimate of total soil water content (h*tot,
mm) was obtained for each sampling period. Soil water deficit (W, mm) was then calculated for the entire soil profile
as the deviation from capacitance, which was determined
at the end of winter when soil profiles are typically
recharged by winter rainfall. In this study, September
2000 was used as the capacitance reference, this being a period early during the development of the first rotation and
when seasonal rainfall recharge was high.
Native leaf water potential and gas exchange
All measures of native and inherent properties were taken
on juvenile leaves, these being the dominant leaf type in
both growth forms throughout the experiment. The diurnal
course of leaf water potential (Wleaf, MPa) was observed on
six occasions between December 2006 and April 2008,
encompassing the wet–dry seasonal oscillations typical of
a Mediterranean-type habitat. On each measurement day,
Wleaf was measured at intervals from before dawn to just
before dusk at 19:00 h local standard time. One leaf
was collected from each of the five plants selected at random from within each plot. The leaves were wrapped in
plastic film and kept on ice until all leaves were collected.
The leaf water potential was then measured using
a Scholander-type pressure chamber (model PMS 1000,
PMS Instrument Co., Corvallis, OR). Sources of error
associated with measurement of water potential via the
pressure chamber technique, as outlined in Ritchie and
Hinkley (1975), were minimized. The maximum root-to-leaf
hydrodynamic water potential gradient (DW, MPa) was calculated for each measurement day as the difference between
the maximum and the minimum values observed.
On five of the six diurnal Wleaf measurements, in situ photosynthetic assimilation rate (A, lmol CO2 m2 leaf s1),
leaf transpiration rate (E, mmol H2O m2 leaf s1), stomatal conductance to water vapour (gw, mmol H2O m2 leaf
s1), leaf temperature (T, C) and the partial pressure of
ambient and intercellular CO2 (Pa and Pi, respectively, lmol
CO2 mol1 air) were determined with a portable photosynthesis system (model Ciras-1, PP Systems, Hitchin, Herts,
UK). These measurements were initiated after sunrise and
coincided with sampling of leaves for leaf water potential.
Briefly, in situ leaf gas exchange properties were taken
from five randomly selected plants (n = 1 fully expanded,
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DRAKE, MENDHAM, WHITE AND OGDEN
well-lit leaf per plant) from each of the measurement plots at
a Pa of 350 lmol mol1, ambient temperature, ambient
light intensity and ambient relative humidity.
Steady-state leaf gas exchange
Between September and December 2006, when the soil was
the wettest and therefore the likelihood of abscisic acid
induced stomatal closure was low, steady-state leaf gas
exchange measurements were made with a portable photosynthesis system (model Li-6400, Li-Cor Inc., Lincoln,
NE). All measurements were initiated early in the morning
( 08:00 local standard time) and were concluded within
the natural daylight photoperiod. Measurements were made
on one leaf from each of the four plants from one coppice
and one seedling plot. The leaves were sealed in the
instrument leaf chamber and allowed to reach a steady state
under the following conditions: Pa = 350 lmol mol1,
T = 25 C, photosynthetically active radiation (PAR) =
1500 lmol m2 s1 and leaf-to-air vapour pressure difference (D) = 1 kPa. The relationship between A and Pi was
then obtained by manipulating Pa over the range 50–
2000 lmol mol1 and described empirically by fitting a rectangular hyperbola of the form (Olsson and Leverenz 1994):
CEP i Amax
2 1
A lmol m s
r;
ð1Þ
¼
CEP i þ Amax
where CE is the carboxylation efficiency, Amax is the
assimilation rate at saturating CO2 and r is the combination of light and dark respiratory processes. Carboxylation efficiency was initially assessed as the slope of
the relationship in the linear phase of the relationship
(50–250 lmol mol1), with r taken as the y-intercept.
The assimilation rate at saturating CO2 was estimated
from the largest value for A. The initial estimates of
CE, r and Amax were entered in Eq. (1) and an iterative
least square fit approach was used to refine the model
over the measured values of A and Pi.
The leaf was then allowed to return to a steady state at a
Pa of 350 lmol mol1 (no hysteresis was observed between
the first and the second steady states) and the response of
A to PAR was established by controlling PAR over the
range 0–2000 lmol m2 s1. Photosynthetic light response
curves were described by fitting a non-rectangular hyperbola of the form (Marshal and Biscoe, 1980):
A lmol m2 s1
¼
2UQx Amax qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
rd ;
UQx þ Amax þ ðUQx þ Amax Þ2 4hUQx Amax ð2Þ
where Qx is the incident PAR, U is the slope of the relationship at low Qx (the apparent quantum yield), Amax*
is the light-saturated A, rd is the leaf dark respiration rate
and h is the shape of the photosynthetic light response
curve. Estimates of U were first obtained as the slope of
the linear phase of the relationship in the range of PAR
from 0 to 200 lmol m2 s1, and rd was estimated as
the y-intercept. An initial estimate of Amax* was taken
as the maximum value for A and a notional value of 0.5
was assigned to h. These initial values were then entered
in Eq. (2) and the model was refined over the data range
through an iterative least square fit approach.
Leaf chlorophyll and nitrogen content
In November 2006, one newly expanded leaf was collected
from the upper canopy of five individuals from each plot.
Taking care to avoid the midvein, 16.0 cm2 of the leaf
material was subsampled from each leaf. Each subsample
was ground (in the dark) in 80% acetone and acid-washed
sand. The mixture was then centrifuged to remove cellular
debris and the supernatant was made up to 2.5 ml. Absorbance was then immediately measured in an ultraviolet–
visible spectrophotometer (model UV-1601, Shimadzu,
Kyoto, Japan) at 652 nm. Total chlorophyll was determined according to Sestak et al. (1971) as
1000
1
;
ð3Þ
¼ A652
Total chlorophyll mg l
34:5
where A652 is the absorbance of the supernatant at
652 nm. This was converted to a leaf area basis through
Total chlorophyll mg m2 ðleafÞ
3
Total chlorophyll mg l1 2:510
1
¼
:
ð4Þ
1:6 103
The residual leaf tissue from chlorophyll determination
was analysed for nitrogen content. This tissue was oven dried
at 70 C then finely ground with a ball mill. Exactly 0.050 g of
the ground material was then weighed and the nitrogen elemental composition (%) was determined using a mass spectrometer (model 20-20 IRMS, Europa, Crewe, UK).
Data analysis
Wherever appropriate, mean values were compared via
t tests or one-way analyses of variance (ANOVAs) at the
0.05 level of significance (SPSS for Windows Version
13.0, SPSS Inc., Chicago, IL). Analyses of covariance were
used to detect any differences between coppice and seedlings in correlations among various parameters (SPSS for
Windows, Version 13.0) and the strength of the regression
analyses was quantified as correlation coefficient.
Results
Diurnal and seasonal patterns of water status
for coppice and seedlings
During the measurement period, the pattern of rainfall was
typical of a Mediterranean climate. For example, more
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than 77% of the annual total rainfall occurred between
late autumn (May) and mid-spring (October) in 2007
(Figure 1A). Soil water deficits increased during summer
for both coppice and seedlings, and this was associated with
a decrease in predawn leaf water potential (Figure 1B and C).
Although the total soil water deficit (W) was higher
under coppice than seedlings throughout the measurement
period (Figure 1B), predawn leaf water potential was generally lower for seedlings than coppice and this difference was
significant in March 2007, June 2007 and December 2007
(Figure 1C). The maximum root-to-leaf hydrodynamic
pressure gradient (DW) also fluctuated seasonally, but there
was no significant difference between coppice and seedlings
(Figure 1D).
From December 2006, when the soil was close to being
fully recharged, to March 2007 there was only 112.9 mm
of rainfall (Figure 1A) and the coppice extracted water
from the top 450 cm of soil while, in contrast, the seedlings
only used water from the top 90 cm of the soil profile
(Figure 2). Despite this disparity in effective rooting depth
between coppice and seedlings, diurnal fluctuations in Wleaf
for each growth form followed a similar pattern throughout
667
the study. In December 2006 (Figure 3A), there was no
distinction in Wleaf between coppice and seedlings at any
time of the day. Leaf water potential, with the exception
Figure 2. Volumetric soil water fraction, as a function of soil
depth for (A) coppice and (B) seedlings.
Figure 1. (A) Daily rainfall (mm), (B) soil water deficit
(W, mm), (C) mean predawn (±SE, n = 15) leaf water potential
and (D) mean (±SE, n = 15) maximum root to leaf hydrodynamic pressure gradient (DW, MPa) of coppice and seedlings
between December 2006 and April 2008.
Figure 3. Diurnal course of leaf water potential (Wleaf) for
E. globulus coppice and seedlings during (A) December 2006 and
(B) March 2007.
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DRAKE, MENDHAM, WHITE AND OGDEN
Table 1. Above-ground biomass and leaf chlorophyll content in coppice and seedlings. Mean (±SE) SLA (m2 kg1), LAI, total aboveground plant dry weight (t dry weight, kg ha1) and total leaf chlorophyll content (chlorophyll, mg m2) for coppice and seedlings. The
type of statistical analysis, corresponding P value and n for the comparison of coppice versus seedlings are indicated for each
parameter.
Parameter
2
Coppice
1
SLA (m kg )
LAI
t dry weight (kg ha1)
Chlorophyll (mg m2)
12.33
0.7696
1.6008
331.11
±
±
±
±
Seedlings
0.60
0.2639
0.5582
10.66
17.73
0.0045
0.0077
453.21
of the predawn phase, for coppice and seedlings was also
similar in March 2007 (Figure 3B) except that Wleaf for seedlings increased at around midday. At all other times the measurement of Wleaf between December 2006 and March 2007
was similar throughout the day for coppice and seedlings.
Biomass, LAI and leaf characteristics
of coppice and seedlings
In December 2006, when coppice and seedlings were 11
months old, each coppice stump had an average of 20 stems
compared to an individual seedling stem and this translated
to around 200-fold more above-ground biomass and LAI
for coppice compared to seedlings (Table 1). The SLA of
juvenile leaves was, however, 44% higher in seedlings compared to coppice (Table 1).
Leaf-scale gas exchange characteristics
of coppice and seedlings
On each diurnal sampling date, net carbon assimilation rate
(A) was linearly related to the rate of transpiration (E) and
there was no significant effect of growth form on either the
slope or the intercept of these relationships at any time of
the year (Figure 4). The slope of the relationship between
A and E, which is analogous to leaf-scale water-use efficiency (WUE) integrated over a day, was found to vary significantly with sampling date. Leaf-scale WUE (expressed
as A/E and derived as a slope) was not significantly correlated to Wpd (P > 0.05), but an exponential decay function
with daily average leaf-to-air vapour pressure difference (D)
(Figure 5) was significant and it explained 86% of the variation in A/E. Also shown in Figure 5 is the short-term
steady-state response of A/E to a step-wise change in
D from 1.0 to 2.0 kPa under well-watered conditions and
the trajectory of this response was similar to that observed
under native conditions.
Leaf-scale photosynthetic rate was positively correlated
with leaf conductance to water vapour (gw) on all measurement dates (Figure 6). Neither the y-intercept nor the slope
differed significantly between coppice and seedlings
(P > 0.05), hence data from both growth forms were
pooled for analysis. The data in Figure 6 also compare the
results in this study with the relationship published by Macfarlane et al. (2004b), and it indicates a similar relationship
for droughted E. globulus trees. The slope or the intrinsic
±
±
±
±
0.24
0.0009
0.0016
20.19
Test
P
n
ANOVA
t test
t test
ANOVA
< 0.01
0.04
0.05
< 0.01
15
3
3
15
water-use efficiency (WUEi) was twofold greater in the study
of Macfarlane et al. (2004b) (0.12 lmol mmol1) than that
for both growth forms in this study (0.06 lmol mmol1).
Seedlings had a significantly higher CE (P = 0.03), CO2saturated photosynthetic rate (Amax, P = 0.02) and foliar
respiration rate (incorporating both light and dark respiratory processes) (r, P = 0.02) than coppice (Figure 7).
Moreover, seedlings tended to have a higher light-saturated
photosynthetic rate (Amax*) and apparent quantum yield
(U) in response to light (Figure 8), but these differences
were not significant. These differences in the inherent gas
exchange characteristics between seedlings and coppice
were associated with a significantly higher total leaf chlorophyll content (expressed on a per unit leaf area basis) in
seedlings compared to coppice (t test, P < 0.05, Table 1).
Seedlings, on average, had 122.11 mg m2 (37%) more
total leaf chlorophyll than coppice. Higher total leaf chlorophyll contents in seedlings were associated with higher leaf
nitrogen and the correlation between leaf chlorophyll and
nitrogen content across growth forms could be described
by a linear model (Figure 9, P < 0.05).
Discussion
Different intrinsic gas exchange properties and chlorophyll
and nitrogen content between coppice and seedlings suggested a larger resource investment by seedlings at the leaf
scale in structures and enzyme systems associated with photosynthetic capacity. However, this did not translate to differences between coppice and seedlings in either leaf water
potential or observed rates of gas exchange in the field and
was more than offset by the higher early biomass and LAI
of coppice compared to that of seedlings. However, the
more rapid growth in coppice was also associated with a
faster depletion of soil water stores and this may lead to
earlier competition for resources and convergence of
growth rates later in the rotation. Our hypotheses were that
the disproportionate amount of below-ground biomass in
coppice would: (a) modify the manner by which belowground resources were allocated to the newly developed
above-ground coppice canopy and (b) create a different
environment for coppice compared to seedlings. The most
likely theoretical explanation for the different intrinsic
properties of coppice and seedlings is the manipulation in
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669
Figure 4. Leaf photosynthetic rate (A) as a function of leaf transpiration rate (E) on five diurnal measurements made between
December 2006 and April 2008 (A–E). For each time period r2 and P values for fitted models were as follows: (A) December 2006: 0.88
and < 0.0001; (B) March 2007: 0.81 and < 0.0001; (C) June 2007: 0.65 and < 0.0001; (D) December 2007: 0.34 and < 0.0001; and
(E) April 2008: 0.94 and < 0.0001. Note that data obtained before sunrise and after sunset have been omitted and also note that
coppice resprout and seedling data are pooled.
root-to-shoot ratio and the reallocation of resources
brought about by harvesting of the previous rotation. Thus,
our results lead us to accept hypothesis (a), but reject
hypothesis (b).
Leaf and soil properties under ambient conditions
Each coppice stump supported a higher leaf area and total
above-ground biomass than the seedling stems, and this
was reflected in a significantly higher LAI of coppice compared to seedlings. Coppice also extracted water deeper
from the soil profile and generated a much larger soil water
deficit than that which was observed for seedlings. Put
together, these observations confirm that coppice retains a
substantial amount of live below-ground biomass from
the original tree and that this temporary imbalance in biomass allocation during early development provides for a
greater supply of soil resources to above-ground biomass
components compared to seedlings.
Contrary to our original hypothesis, leaf water potential, gas exchange and WUEi did not differ markedly
between growth forms during the first 2 years of growth.
Under natural conditions, plants typically experience oscillation in irradiance, temperature, VPD and soil water
potential at various time scales. Stomatal guard cells interpret this oscillation and manipulate stomatal aperture to
optimize gas exchange such that carbon uptake is maximized for a given water use (Cowan 1977). Under water
limitation, stomata typically close. Hence, it might be anticipated that the greater soil water deficit under coppice
would be associated with a lower Wpd and depressed rates
of leaf gas exchange. The similarity in these traits suggests
that the supply of plant available soil water (PASW) was
balanced with demand for both growth forms and that,
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DRAKE, MENDHAM, WHITE AND OGDEN
Figure 5. Daily integrated leaf-scale WUE, defined as the
quotient of photosynthetic rate and transpiration rate (A/E),
as a function of the daily average leaf-to-air vapour pressure
difference (D) (note data for coppice and seedlings were pooled).
The fitted exponential decay function has the equation:
A/E = 3.12 + 10.03e((D 0.90)/1.79) (r2 = 0.86). Also shown
on the graph is a typical steady-state response of A/E to D for
field-grown seedling E. globulus (Drake, unpublished data).
Figure 6. Leaf photosynthetic rate (A) was positively correlated
with stomatal conductance to water vapour (gw) in a linear
fashion (r2 = 0.81, P < 0.0001). Also shown on the graph is the
same relationship for droughted E. globulus observed by
Macfarlane et al. (2004b).
in this case, total soil water deficit was not a good measure
of underlying water stress. Some process-based models of
plantation growth, including CABALA (Battaglia et al.
2004), use relative soil water deficit to predict water potential and in turn leaf conductance and net carbon assimilation. These results highlight the importance of expressing
PASW as a function of effective rooting depth or root density in second rotation E. globulus plantations.
The persistence of roots from the previous rotation in
coppice trees has been less studied. The depletion of deep
soil moisture layers underlying coppice in this study, to
our knowledge, is the first evidence that at least some residual deep roots are retained and are actively involved in water
uptake in second rotation E. globulus systems. Wildy et al.
(2004) made a similar observation for coppice Eucalyptus
kochii Maiden and Blakely subsp. Plenissima Gardener
(Brooker) growing in tree belts. Coppice are often characterized by vigorous growth, at least some of which must be
attributable to the presence of the existing root system.
However, the respiratory cost of such a system must prohibit retention of root biomass over and above that required
for resource capture. For example, fine roots could be lost
soon after harvest due to diminished resource demand from
the canopy. This was shown by Wildy and Pate (2002) in the
study on E. kochii. Further investigation on root biomass
turnover in multiple rotation forestry systems would be
valuable to complement our current understanding.
In our study, mean daily integrated WUE varied from 1.81
to 7.27 lmol CO2 mmol1 H2O depending on season. This
difference was driven by a mean 36% increase in E and a
mean 34% decrease in A during the first growing season.
Instantaneous leaf-scale WUE (A/E) in glasshouse grown
E. globulus seedlings has been reported from 4.71 lmol CO2
mmol1 H2O in low N status plants to 17.60 lmol CO2
mmol1 H2O in well-watered and fertilized plants (Sheriff
1992). White et al. (2000) showed that much of the variation
in E in well-watered E. globulus trees can be explained by the
positive relationship between E and VPD. Our observation
that A/E is related to D provides additional insight into the
drivers for carbon and water exchange in this species. It
remains to be seen, however, whether this strong coupling
is consistent in both growth forms under a severely depleted
soil moisture profile where the root-to-shoot water stress signal may override atmospheric drivers for gas exchange.
The WUEi, calculated as the slope of the relationship
between A and gw, did not differ between growth forms
and was about half of that reported for 4-year-old trees
of the same species in the study of Macfarlane et al.
(2004b). This is probably due to the much greater level of
water stress, induced by lower mean annual rainfall,
reported by Macfarlane et al. (2004b); the average value
for Wpd in Macfarlane et al. (2004b) was less than 1.8
MPa, whereas in this study the minimum value for Wpd
was 0.32 ± 0.02 MPa for coppice and 0.38 ± 0.03
MPa for seedlings. This suggests that gw is more sensitive
to soil moisture than A and therefore the impact of drought
on photosynthesis is likely to be mediated by stomatal
limitation. Despite this, E. globulus is anisohydric, i.e., the
combination of soil and atmospheric water deficit results
in an increase in the maximum root-to-leaf hydrodynamic
pressure gradient (Tardieu and Simonneau 1998, Franks
et al. 2007). This functional attribute implies that, despite
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671
Figure 7. Comparative plots of
leaf photosynthetic rate (A) versus the intercellular partial pressure of CO2 (Pi) for coppice (A)
and seedlings (B) (n = 4 for
each). The fitted lines are rectangular hyperbolas: r2 = 0.96
for coppice and r2 = 0.97 for
seedlings.
Figure 8. Leaf photosynthetic rate (A) as a function of PAR (or
Qx in Eq. (2)) for coppice and seedlings. The fitted lines are nonrectangular hyperbolas: r2 = 0.99 for coppice and r2 = 0.99 for
seedlings. The models yielded the following values for coppice
and seedlings, respectively: apparent quantum yield (U): 0.04
and 0.05; light-saturated photosynthetic rate (Amax*): 17.40 and
20.32 lmol m2 s1 and leaf dark respiration rate (rd): 1.86 and
1.44 lmol m2 s1.
modification to stomatal aperture with the onset of soil or
atmospheric water deficit, this species cannot necessarily
regulate water potential within a range to sustain physiological processes. This current study did not capture native
conditions that would test this possibility. Hence, we were
unable to assess the relative advantage of coppice versus
seedlings under such conditions.
Figure 9. Total leaf chlorophyll (mg m2) was positively correlated with leaf nitrogen content (%). The fitted linear model has
the equation: total chlorophyll = 74.81 104.48 · leaf nitrogen (r2 = 0.40, P < 0.01).
Inherent photosynthetic capacity
The capacity to assimilate CO2 was higher in seedling leaves
than in coppice leaves, as indicated by a higher maximum
rate of CO2-saturated photosynthesis. However, under
ambient CO2 concentrations, the observed difference
between the growth forms was not significant. The greater
photosynthetic capacity of seedlings at high Pi suggests that
the regeneration of RuBP in the C3 cycle is faster (von
Caemmerer and Farquhar 1981) in seedlings compared to
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DRAKE, MENDHAM, WHITE AND OGDEN
that in coppice. The photochemical partial processes contributing to the regeneration of RuBP are light interception,
energy funnelling, phosphorylation of ADP and reduction
of NADP. The aggregate of these processes was characterized by comparing photosynthesis as a function of irradiance. The estimated quantum yield of photosynthesis was
lower in coppice compared to seedlings, at 0.039 lmol CO2
lmol1 versus 0.045 lmol CO2 lmol1 PAR, respectively.
The quantum yield of seedlings in this study was similar to
that found in 4-year-old E. globulus (0.05; Battaglia and
Sands 1997), and is similar to the value reported more generally for C3 species by Ehleringer and Björkman (1977).
The lower quantum yield in coppice resprouts compared
to seedlings was associated with a lower light-saturated rate
of photosynthesis in coppice (17.40 lmol m2 s1) compared to seedlings (20.32 lmol m2 s1). Battaglia et al.
(1996), also using the model of Marshal and Biscoe
(1980), recorded light-saturated rates of photosynthesis in
E. globulus in the order of 14.35–15.70 lmol m2 s1.
In this study, coppice had less total leaf chlorophyll
than seedlings. The range of 330 mg m2 (coppice) to
452 mg m2 (seedlings) was within the range of total chlorophyll values of 220–600 mg m2 found by Pinkard et al.
(2006) in a range of glasshouse- and field-grown E. globulus
seedlings. Chlorophyll pigments intercept sunlight and represent the start of the photosynthetic process. In this way,
photosynthesis at light saturation and the quantum yield
of photosynthesis are linked to the quantity of chlorophyll
in leaf chloroplasts. Combined with the observed response
of photosynthesis to Pi, it would appear that the protein
available to leaves for the construction of chlorophyll pigments and electron transport capacity is lower in coppice
compared to seedlings. The linkage between leaf nitrogen,
a fundamental component for protein synthesis, and total
leaf chlorophyll implied in this study provides insight into
the contrasting mechanisms of resource allocation in
coppice and seedling forms of E. globulus during early
development.
Resource allocation
Resh et al. (2003) estimated that E. globulus allocates
1 kg dry weight m2 soil surface year1 into coarse roots
(roots > 2 mm in diameter). Although the conditions of
our study differed from that of the study of Resh et al.
(2003), it is conceivable that coppice accumulated around
10 kg roots m2 soil surface during the first rotation. This
imbalance in biomass components during early development of the second rotation increased the amount of
resources available to the young coppice canopy. With
potentially more reserves for early canopy development, it
could be expected that the leaf level concentration of
resources would be elevated in coppice, but this was not
the case in this study. Both leaf chlorophyll content and leaf
nitrogen content were lower in the coppice growth form,
compared to seedlings, resulting in a lower CE and CO2-
saturated rate of photosynthesis. In coppice-grown E. globulus, the existing root system, retained after the original tree
was harvested and during the development of regenerative
buds, represents a large pool of carbohydrate. Theoretically, this carbohydrate pool can be utilized to maintain
the living root system, and contribute to rapid early growth
of support structures. Coppice biomass rapidly increases
under favourable conditions (Schlesinger and Gill 1980,
Kruger and Reich 1993, Fleck et al. 1998), resulting in
the production of multiple stems, which are the support
structures for many leaves (evidenced as a greater LAI than
seedlings). This production of above-ground support structures to deploy leaves for photosynthesis is a rapid-start
feed-forward system that contributed to a greater aboveground biomass (quantified as dry weight) than seedlings.
Coppice also have the advantage of not requiring substantial development of a below-ground support structure to
deploy fine roots. In contrast, seedlings are initially dependent on the carbohydrate reserves of the seed. With this
they grow a few leaves, and all functional and supporting
structures above and below ground must be grown thereafter using the current photosynthate. In this way, the accumulation of leaf area, in balance with root area, is
necessarily slower in seedlings than that in coppice. Therefore, seedling development can, in the case of this study, be
considered primarily limited by a capacity to physically
deploy leaves and fine roots on support structures.
As a result of contrasts in the construction of leaf support
structures, the resource investment in leaves of coppice and
seedlings is likely to be different. Seedlings can only support
a few leaves, and are likely, therefore, to invest all of their
nutritional (mostly nitrogen) resources into them to optimize carbon assimilation per unit leaf area. Our observation of a high SLA in seedlings is consistent with the
general hypothesis that a greater concentration of nitrogen
in leaves translates to leaf lamina expansion. Coppice with
the advantage of rapid construction of leaf support structures can deploy many leaves and accordingly distribute
nutritional resources more diffusely across the leafy canopy.
We hypothesize that this is the most likely underlying reason for the different inherent photosynthetic properties
and leaf-area-scaled total chlorophyll content across the
growth forms of this study.
Conclusions
The similarity in native response to environment across
growth forms, exemplified by analogous WUEi and leaf
water potential, is suggestive that the contrast in belowground root distributions did not alter the growth conditions of E. globulus in this study. Instead, the substantial
contrast in the inherent properties of coppice and seedlings
supports the idea that a large root-to-shoot ratio and storage reserves are the drivers for early vigorous growth in
E. globulus coppice. It remains to be seen whether the initial
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advantage of coppice, i.e., the early expression of aboveand below-ground support structures and access to a significant carbohydrate store, will persist over the second rotation period.
Funding
The authors thank the Co-operative Research Centre for Forestry.
Acknowledgments
We thank Scott Walker, Tammi Short and Jessie Rutter for technical assistance and Chris Beadle, Jenny Carter and Bernie Dell for
helpful comments. We are grateful to Hansol P.I. and Great
Southern Plantations for site access.
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