Tree Physiology 00, 958–967
doi:10.1093/treephys/tps066
Research paper
Interactive effects of water supply and defoliation on
photosynthesis, plant water status and growth of Eucalyptus
globulus Labill.
A. G. Quentin1,2,4,5, A. P. O’Grady2,3,4, C. L. Beadle1,2,4, C. Mohammed1,2 and E. A. Pinkard2,4
1Tasmanian
Institute of Agriculture, University of Tasmania, Private Bag 54, Hobart, Tasmania 7001, Australia; 2Co-operative Research Centre for Forestry, Private Bag 12,
Hobart, Tasmania 7001; 3School of Plant Science, University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia; 4CSIRO Ecosystem Science, Private Bag 12,
Hobart, Tasmania, 7001; 5Corresponding author ([email protected])
Received November 7, 2011; accepted May 30, 2012; handling Editor Michael Ryan
Increased climatic variability, including extended periods of drought stress, may compromise on the health of forest ecosystems. The effects of defoliating pests on plantations may also impact on forest productivity. Interactions between climate
signals and pest activity are poorly understood. In this study, we examined the combined effects of reduced water availability
and defoliation on maximum photosynthetic rate (A sat), stomatal conductance (gs), plant water status and growth of Eucalyptus
globulus Labill. Field-grown plants were subjected to two water-availability regimes, rain-fed (W−) and irrigated (W+). In the
summer of the second year of growth, leaves from 75% of crown length removed from trees in both watering treatments and
physiological responses within the canopies were examined. We hypothesized that defoliation would result in improved plant
water status providing a mechanistic insight into leaf- and canopy-scale gas-exchange responses. Defoliated trees in the W+
treatment exhibited higher A sat and gs compared with non-defoliated trees, but these responses were not observed in the
W− treatment. In contrast, at the whole-plant scale, maximum rates of transpiration (Emax) and canopy conductance (G Cmax)
and soil-to-leaf hydraulic conductance (KP) increased in both treatments following defoliation. As a result, plant water status
was unaffected by defoliation and trees in the defoliated treatments exhibited homeostasis in this respect. Whole-plant soilto-leaf hydraulic conductance was strongly correlated with leaf scale gs and A sat following the defoliation, providing a mechanistic insight into compensatory up-regulation of photosynthesis. Above-ground height and diameter growth were unaffected
by defoliation in both water availability treatments, suggesting that plants use a range of responses to compensate for the
impacts of defoliation.
Keywords: compensatory response, leaf and canopy stomatal conductance, maximum leaf photosynthetic rate, soil-to-leaf
hydraulic conductance, transpiration.
Introduction
Individually, stressors such as water deficit, salinity, heat or
pests have been the subject of intensive research (Karban and
Baldwin 1997, Munns 2002). However in natural environments,
trees are routinely subjected to a combination of different
biotic and/or abiotic stressors; among them, insect defoliation
and low water availability are common causes of dieback
(Hogg et al. 2002, Bréda and Badeau 2008). In Australia,
these are important causes of lost production in eucalypt plantations (Nahrung 2003, Mummery and Battaglia 2004). It is
widely predicted that changing climates will affect the amount
and distribution of rainfall (Allen and Ingram 2002). Across
much of the main plantation growing regions of southern
Australia, annual rainfall has been declining since the 1970s
(www.bom.gov.au), resulting in an increased frequency of periods with low water availability and reduced tree growth.
© The Author 2012. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Interactive effects of water supply and defoliation 959
Changes in the amount and distribution of rainfall as well as
changes in the prevailing temperature regimes may also affect
pest populations and outbreaks (Ayres and Lombardero 2000,
Netherer and Schopf 2010). The response to co-occurrence of
these stressors is not predictable from single-factor analyses.
This makes the study of interactions both appropriate and
complex, as a combination of stressors can result in intensification, overlapping or amelioration of the effects of stress
(Osmond et al. 1987). Resolving these potential outcomes is
essential for predicting the impact of changing water regimes
and pest population dynamics on forest production.
Trees typically experience a range of resource limitations
that affect productivity. Defoliation associated with herbivory
or disease can also reduce productivity. However, there is little
understanding of their interactive effects on photosynthesis
and growth (McGraw et al. 1990, Kolb et al. 1999, Gieger and
Thomas 2002, 2005). In red oak (Quercus rubra L.) and
Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), there were
significant interactions between defoliation and soil moisture
such that defoliation stimulated photosynthesis in waterstressed but not non-stressed seedlings (Kolb et al. 1999,
Gieger and Thomas 2005). In English oak (Quercus robur L.),
stomatal conductance in response to drought and defoliation
was primarily regulated by the tree’s capacity to transport
water to the leaves through changes in soil-to-leaf hydraulic
conductance (KP); in sessile oaks (Quercus petraea [Matt.]
Liebl.) gs was mostly determined by the environmental variables (Gieger and Thomas 2005). To date, studies of physiological responses of Eucalyptus globulus Labill. in response to
stress have focused on a single stressor (e.g., defoliation,
Pinkard et al. 2007, Turnbull et al. 2007, Quentin et al. 2010,
Quentin et al. 2011; drought, Pereira et al. 1987, White et al.
1996, O’Grady et al. 2008).
Defoliation of E. globulus commonly induces an increase in
the rate of net photosynthesis (Pinkard et al. 2007, Turnbull
et al. 2007, Quentin et al. 2010), transpiration per unit leaf area
and hydraulic conductance (Quentin et al. 2011). In contrast,
drought has been shown to have negative impacts on E. globulus responses including reductions in transpiration and canopy
conductance (O’Grady et al. 2008), in leaf water potential
(White et al. 1996, O’Grady et al. 2008) and stomatal conductance (Pereira et al. 1987). However, there is little information
about the interactive effects of low water availability and defoliation on physiological responses of eucalypt species in a field
environment. Removal of significant amounts of leaf material
may alter the water balance within trees and forest ecosystems
significantly by influencing transpiration (Cunningham et al.
2009, Quentin et al. 2011). Water transport and photosynthesis in trees are regulated by the hydraulic conductance of the
pathway from the soil to the leaf (Mencuccini 2003, Tyree
2003). Understanding the regulation of plant water use is an
important consideration for managing plantations and their
associated impacts on water balance. Defoliating agents such
as insects can significantly affect plantation productivity but
their effect on plantation water use remains poorly
understood.
In this study, we investigated the interacting effects of low
water availability and artificial defoliation on the gas exchange,
water relations and growth of E. globulus saplings. These
stressors were selected because the combined effects of
water stress and defoliation can trigger eucalypt dieback
(Landsberg and Wylie 1983, Landsberg 1985). We investigated tree- and leaf-level responses to the separate and combined action of these stressors and asked the following
questions: Does defoliation improve the water status of plants
grown under low water availability? If so, does this response
underpin the often observed up-regulation of stomatal conductance and photosynthesis? What are the implications of the
interaction of drought and defoliation for forest productivity?
Our hypothesis was that partial defoliation offsets the negative
effects of low water availability on growth. To test these questions, we examined (i) the photosynthetic compensatory
responses of E. globulus to defoliation and two water-availability regimes, (ii) the regulation of water transport through the
tree and (iii) the responses of tree height and diameter growth
to defoliation.
Materials and methods
Site
The experiment was conducted in the Pittwater research plantation located 20 km east of Hobart, Australia (42°49.4′S,
147°30.6′E) during summer 2007/2008. Details of soil and
climatic conditions at the site are presented in O’Grady et al.
(2005). Monitoring of the climate variables at the site was carried out using an automatic weather station installed in an open
field ~100 m north-west of the plantation (Quentin et al. 2011).
Eucalypus globulus plants of 0.25 m height were planted at
the site in December 2006. During the first 2 months after
planting, seedlings were irrigated every second day using
municipal water at a rainfall equivalent of 3 mm until February
2007, when watering treatments were applied (as described
below). Fertilizers were applied 1 month after planting and then
at 3-monthly intervals (Eyles et al. 2009). Trace elements were
included in the mix (O’Grady et al. 2005). At the start of this
study in December 2007, the saplings were 12 months old. The
average height at this time was 3.1 and 2.3 m and basal diameter at 0.15 m above the soil surface was 58 and 43 mm, in the
irrigated and rain-fed treatments, respectively.
Treatments and experimental design
The experiment was established as a completely randomized
split-plot design (Eyles et al. 2009), with three replicates of
each of two plot-level treatments:
Tree Physiology Online at http://www.treephys.oxfordjournals.org
960 Quentin et al.
1. Irrigated (W+) (watered every second day with the equivalent of 3 mm daily, plus rainfall); irrigation was doubled from
the original amount in December 2007 to account for the
increased size of the saplings.
2.Rain-fed (W−) (low water availability treatment). Plants
received only rainfall.
Each plot comprised six treatment saplings surrounded by a
12-sapling buffer. The water availability regimes commenced in
February 2007, at which time total soil N in the top 10 cm
averaged 6.4 and 3.1 mg kg−1 (Pinkard et al. 2011).
Two defoliation treatments were randomly applied to each of
three saplings per plot:
1. Undefoliated (no defoliation).
2.Defoliated (remove leaves from 75% crown length on 2
January 2008 when saplings were 15 months old).
The defoliation treatments removed leaves from the crown
apex downwards, excluding apical buds. Leaves were removed
using secateurs. Approximately 60% of total leaf area, as estimated from the allometric relationships of O’Grady et al.
(2006), was removed.
Stem growth response
The height from the ground to the apical meristem and diameter over-bark at 0.15 m above ground of each tree were
measured monthly from early summer 2007 (December
2007) to mid-autumn 2008 (May 2008). Callipers were used
to measure the diameter over-bark. Mean height and diameter of all trees at the start of the experiment was 2.72 m and
50.5 mm, respectively. Analysis of variance (ANOVA) of tree
size prior to the defoliation event found no significant differences in height between the W+ (3.13 m) and W− (2.33 m)
treatments. In contrast, tree diameter was significantly lower
in the W− trees (42.3 mm) than the W+ trees (58.6 mm)
(P < 0.05).
Gas exchange
Maximum light-saturated photosynthesis (Asat) and stomatal
conductance (gs) were measured on three leaves of each sapling in all treatments, using a CIRAS-1 portable infrared gas
analyser (PP Systems, Herts, UK). Leaves were exposed to
direct sunlight at the time of the measurement. Measurements
were made between 10:00 and 14:00 h Australian Eastern
Standard Time (AEST). Leaves were enclosed in a 250 mm2
leaf chamber fitted with a light source providing a photosynthetic photon flux density of 1500 µmol m−2 s−1. The ambient
CO2 concentration was maintained at 360 ppm. Leaf temperature varied between 16 and 21 °C during the experiment. Data
were recorded when gs had stabilized. Measurements were
made 2 weeks before defoliation, 18 December 2007, and
Tree Physiology Volume 00, 2012
then on three occasions following defoliation: 3 January 2008,
24 January 2008 and 18 January 2008.
Leaf water potential
Diurnal patterns of leaf water potential (Ψ) within the canopy
were measured using a Scholander-type pressure chamber.
For each measurement, three leaves per treatment were
excised and placed into plastic bags within an insulated icecooled container, where they remained until measurement
within 10 min of excision. Pre-dawn leaf water potential (Ψpd)
was used as a surrogate of soil water potential. All leaf Ψ measurements were done between 04:30 and 22:30 h AEST, at
60–90 min intervals, on the same days as leaf gas exchange
measurements. Leaves were collected from the lower crown
zone.
Whole-tree water use
Sap flow was measured on each of the six trees using SF100
sap-flow probes and loggers (Greenspan, Warwick, Qld.,
Australia). Probesets were inserted radially into the trunk on
the northern and eastern aspects, and thermistors were stratified with depth. Heat-pulse velocity was measured at four
depths across the sapwood at 15 min intervals. Heat-pulse
velocity was converted to sap velocity (JS, g m−2 s−1) following
Edwards and Warwick (1984), and scaled to tree water-use
using the weighted averages technique (Hatton et al. 1990).
The solutions of Swanson and Whitfield (1981) were applied to
correct for the effects of wounding using a wound width of
3.1 mm (O’Grady et al. 2008). Bark depth, sapwood depth and
heartwood depth were determined from core samples taken
from each tree. No heartwood was present and this was confirmed using dimethyl orange dye. It was assumed that all sapwood was conducting. The leaf area of each tree was estimated
using allometric equations developed for trees at this site
(O’Grady et al. 2006). Water use was expressed on a per-tree
basis as EL (kg m−2 h−1, leaf area). Sap flow measurements
were conducted between 18 December 2007 and 24 February
2008.
The mean canopy stomatal conductance to water vapour
(GC; m s−1) was calculated from EL and D using the following
equation (Monteith and Unsworth 1990):
Gc =
KG(TA ) EL
(1)
D
where KG(TA) is the conductance coefficient as a function of air
temperature (115 ± 0.4236(TA); kPa m3 kg−1), which accounts
for the temperature effects on the psychrometric constant,
latent heat of vaporization, specific heat of air at constant pressure and the density of air (Phillips and Oren 1998). Canopy
conductance was converted from m s−1 to mol m−2 s−1 using
equations in Pearcy et al. (1989). Estimates of canopy conductance calculated in this manner assume that boundary-layer
Interactive effects of water supply and defoliation 961
conductance is high so that D is close to leaf-to-air vapour
pressure deficit. It also assumes that there are no vertical gradients in D throughout the canopy and negligible water stored
above the sap flux measurement point. Eucalypts, in general,
are strongly coupled to the atmosphere (Morris et al. 1998,
Mielke et al. 1999, Hutley et al. 2000, Whitehead and Beadle
2004), and this assumption was anticipated to hold for the
trees at this site (O’Grady et al. 2008). To keep the measurement errors in GC below 10%, GC was calculated only when
D ≥ 0.6 kPa (Ewers and Oren 2000).
Soil-to-leaf hydraulic conductance was calculated (KP) from
the following equation (Loustau and Granier 1993):
EL
KP =
(Ψ s − ΨL ) (2)
where ΨS is soil water potential (MPa), ΨL is the leaf water
potential taken at midday (MPa) and EL is mmol m−2 s−1. We
assumed that predawn Ψpd was equal to ΨS.
Data analysis
Effects of water stress and defoliation on tree growth
Over the period of the experiment, height growth was significantly influenced by water stress (P < 0.05) (Figure 1a),
whereas diameter growth was not affected by water stress
(Figure 1b). At the end of the experiment, the height increment
was reduced by 11.5% with W− treatment compared with the
W+ treatment. There were no significant effects of defoliation
or the water × defoliation interaction on height or diameter
growth.
Effects of water treatment and defoliation on CO2 uptake
Light-saturated photosynthesis (Asat) of saplings growing with
W+ averaged 14.5 µmol m−2 s−1. Trees in the W+ treatment displayed the highest values of Asat; the lowest values were associated with the W− treatments (Figures 2a and b). Asat of
saplings in the W− treatment declined gradually throughout the
experiment (P < 0.05; Figure 2b).
One day after the defoliation treatment was applied, Asat in the
W+ treatment significantly decreased (Figure 2a). Photosynthesis
Diameter and height increments were calculated, and split-plot
ANOVA used to examine differences between treatments in
stem growth. Repeated measures analysis of variance was
employed to test for the effects of water availability and defoliation on individual leaf gas exchange, water potential, sap flux
and plant hydraulic conductance across multiple sampling
dates. A probability level of 0.10 was considered significant,
unless otherwise reported due to the small number of replicates (n = 3) and the associated low power (30.4%). For multiple comparisons, the least significant difference t-test was
used.
Group regressions (McPherson 1990) were used to determine the effect of defoliation treatment on the slope and
intercept of the relationship between Asat and gs; between gs
and KP; and A sat and KP. This procedure tests the hypotheses
that: (i) the regression lines have a common slope allowing
for the possibility that they have different intercepts and (ii)
that the same line applies to all defoliation treatments.
GenStat, 10th edition, was used for all analyses (GenStat
1989).
Results
Predawn (Ψpd) and midday (Ψmd) leaf water potential were significantly affected by water availability (P < 0.05). Trees in the
W+ treatment had less negative Ψpd and Ψmd (−0.22 and
−1.34 MPa, respectively) than trees in the W− treatment
(−0.34 and −1.53 MPa, respectively). The time × water interaction had a significant effect on Ψpd on only two occasions following the defoliation treatment (data not shown; P < 0.05).
Neither defoliation nor its interaction with time and/or water
treatment affected Ψpd and Ψmd (P > 0.1).
Figure 1. ​The rain-fed (W−) treatment reduced the height growth of
E. globulus compared with the irrigated (W+) treatment, whereas the
stem diameter growth was not altered by the water treatment. The
defoliation did not affect height and diameter growth in this study.
Mean (a) height and (b) diameter growth increment of E. globulus in
W+ or W− water treatment. Values are the means of three saplings
(±SE). Error bars indicate standard errors of the mean (P < 0.05).
Tree Physiology Online at http://www.treephys.oxfordjournals.org
962 Quentin et al.
Figure 2. ​Following the defoliation, only trees in the W+ treatment showed up-regulation of photosynthesis. Maximum (a, b) light-saturated net CO2
uptake (Asat), (c, d) leaf stomatal conductance (gs), and (e, f) whole-plant hydraulic conductance (KP) of defoliated and undefoliated E. globulus
growing in (a, c, e) irrigated (W+) or (b, d, f) rain-fed (W−) water treatment. Trees were defoliated 1 day before the second measurement. They
were 15 months of age at the time of the defoliation. Error bars indicate the standard errors of the difference between means (P < 0.05).
(Asat) and gs were significantly higher in the W+ treatment on 24
January 2008 (Figures 2a and c). However, this photosynthetic
up-regulation was not maintained, and there were no differences
in Asat or gs 3 weeks later. There were no effects of defoliation
on Asat or gs in the W− treatment (Figures 2b and d).
There were strong linear relationships between Asat and gs 22
days after the defoliation event (Figure 3). The relationship
between gs and Asat differed between the W+ and W− treatments
(P < 0.001). The relationship between Asat and gs was stronger in
the W− treatment (Asat = 5.05 + 40.34gs; R2 = 0.93; P < 0.001)
compared with the W+ treatment (Asat = 8.25 + 24.68gs;
R2 = 0.82; P < 0.001). Defoliation did not affect the slope and/or
intercept of these relationships (P > 0.1). Thus, although Asat and
gs were higher in the defoliated trees of the W+ treatment, defoliated and undefoliated trees could be explained by a single common relationship.
Tree Physiology Volume 00, 2012
Effects of water treatment and defoliation on tree
water use
There were no significant differences in total daily water use
between defoliation treatments, although trees in the defoliated treatment tended to have lower total daily water use and
higher transpiration rates. As a result, there were no differences in total daily canopy conductance between defoliation
treatments. In contrast, there were significant differences in
maximum daily canopy conductance (GCmax) and transpiration
(Emax) between defoliation treatments.
Over the experimental period, trees in all treatments exhibited similar temporal variation in Emax and GCmax. Before the
defoliation event, there were no significant differences in daily
Emax and GCmax between W+ and W− treatments (P > 0.05).
Similarly, post-defoliation, there were no significant effects
of water treatment on Emax and GCmax, although the
Interactive effects of water supply and defoliation 963
Figure 3. ​Relationship between leaf conductance (gs) and maximum
light-saturated CO2 uptake (Asat) for defoliated and undefoliated E. globulus growing in irrigated (W+) and rain-fed (W−) water treatment. Data
were collected 22 days after the defoliation event. The water availability
treatment significantly affected the slope of the relationship, with higher
gs per unit of A in W+ treatment than W− treatment. Although defoliation
treatment did not affect the relationship in either water treatment, defoliated trees in the W+ treatment showed higher gs for higher Asat.
Figure 4. ​The defoliation treatment significantly increased the maximum transpiration and canopy conductance although the increase was
only short-lived. Daily mean variation of (a) maximum transpiration
rates (Emax) and (b) maximum canopy conductance (G Cmax) of undefoliated and defoliated E. globulus. Error bars show standard error differences (α = 0.05). The arrows indicate the application of the defoliation
treatment.
time × ­defoliation interaction had a significant effect on Emax
(P < 0.001) and GCmax (P = 0.077). However, over the postdefoliation period of the experiment, defoliated trees displayed
higher Emax and GCmax than undefoliated trees across water
treatment (Figures 4a and b). These responses to the defoliation treatment were apparent for a short period between
4 January 2008 and 1 February 2008, and reached their maximum around 6 January 2008 for Emax (Figure 4a) and around
24 January 2008 for GCmax (Figure 4b), coinciding with the
peak in leaf-level photosynthesis.
Soil-to-leaf hydraulic conductance (KP) was not significantly
affected by water availability or defoliation (P > 0.1), although
strong trends were apparent in the data (Figures 2e and f). On
24 January 2008, KP was higher in defoliated trees than undefoliated trees (P < 0.05). There was a positive correlation
between KP and gs (Figure 5a), and KP and Asat. (Figure 5b) 22
days after the defoliation event (P < 0.001). Water availability
affected the intercept of the relationships (P < 0.05; Figures
5a and b). There was a stronger coupling of gs and KP in
­treatments with W+ (gs = 0.19 + 0.005 KP; R2 = 0.60; P < 0.05)
Figure 5. ​
Relationship between whole-plant hydraulic conductance
(KP) and (a) leaf conductance (gs) and (b) maximum light-saturated
net CO2 uptake (Asat) for E. globulus growing in irrigated (W+) or rainfed (W−) water treatment. Data were collected 22 days after the defoliation event. Trees in the W+ treatment displayed higher KP per unit gs
and A than trees in the W− treatment. The defoliation treatment had no
effect on the relationship.
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964 Quentin et al.
Figure 6. ​
Relationship between whole-plant hydraulic conductance
(KP) and maximum light-saturated net CO2 uptake (Asat) for E. globulus
growing in irrigated (W+) or rain-fed (W−) water treatment. Data were
averaged per treatment for each measuring date. The whole-plant
hydraulic conductance (KP) and maximum light-saturated net CO2
uptake (Asat) were strongly coupled. Although the water treatment had
no significant effect on the relationship, trees in the W+ treatment displayed higher KP per unit A than trees in the W− treatment. The defoliation treatment had no effect on the relationship.
compared with the W− treatment (gs = 0.02 + 0.008 KP;
R2 = 0.40; P < 0.05). Although the coupling between KP and
Asat was not affected by water availability treatment
(Asat = 8.55 + 0.31 KP; R2 = 0.45; P < 0.05; when data were
pooled together), trees in W+ treatment had higher KP per unit
A than in W− treatment (Figure 5b). There was a strong coupling between Asat and KP when all data across all dates were
pooled together (Asat = 4.15 + 0.56 KP; R2 = 0.75; P < 0.005;
Figure 6). There was a common relationship that could describe
responses in defoliated and undefoliated trees.
Discussion
Responses of plants to interactive stresses are complex (Mittler
2006) and can be difficult to interpret. We hypothesized that
improved water status as a result of defoliation could provide
mechanistic insights into underpinning compensatory physiological responses to defoliation. In this study, compensatory
leaf-level photosynthetic up-regulation in response to defoliation was short-lived and only observed in the W+ treatment.
However, although there was no evidence of photosynthetic
up-regulation to defoliation at the leaf level in the W− treatments, defoliation did not result in a significant growth reductions, suggesting that some compensatory responses were
occurring.
Leaf-level responses to defoliation is influenced by water
availability
Photosynthetic and stomatal conductance responses to defoliation are thought to depend primarily on water availability
Tree Physiology Volume 00, 2012
(Stoneman et al. 1994, Gieger and Thomas 2005, Pinkard
et al. 2011). Previous studies under well-watered conditions
have reported that defoliation of E. globulus resulted in a transient increase in foliar photosynthesis (see Pinkard et al. 2007
Turnbull et al. 2007, Quentin et al. 2010; including at the site:
Pinkard et al. 2011). In the present study, defoliated trees in the
W+ treatment also exhibited a transient ~30% increase in Asat
following defoliation with the peak in up-regulation occurring
about 22 days after the defoliation event. For these trees, the
peak in photosynthetic up-regulation also corresponded with a
peak in leaf-scale gs. The increase in Asat of defoliated trees in
W+ treatment was strongly correlated with the increase in gs
(see Figure 3), suggesting that changes in gs were important in
governing A.
In contrast, for defoliated trees in the W− treatment, there
was no observed up-regulation of photosynthesis. Up-regulation
of photosynthesis in response to defoliation is often interpreted
as a compensatory mechanism (Quentin et al. 2010, Eyles
et al. 2011, Pinkard et al. 2011), such that increased assimilation of the remaining leaf tissue can at least partially compensate for the loss of photosynthetically active tissue and lost
assimilates. However, despite this lack of photosynthetic upregulation in the W− treatment, defoliation did not result in a
loss of growth in defoliated trees. Furthermore, there was a
common relationship between gs and Asat for both defoliated
and undefoliated plants in both W+ and W− treatments, suggesting that changes in gas exchange could underpin the
observed photosynthetic responses between treatments.
The capacity and evidence for up-regulation of Asat and gs
vary between species and experimental treatments. For example, an increase in Asat was observed following defoliation in
seedlings grown under low water availability conditions but not
in well-watered conditions (McGraw et al. 1990, Pinkard et al.
2011). In contrast, Suwa and Maherali (2008) found no evidence of photosynthetic up-regulation following defoliation in
the Mediterranean grass Avena barbata grown under limiting
and non-limiting conditions of nutrient availability. In the
McGraw et al. (1990) study, seedlings were completely defoliated; however, the expression of photosynthetic up-regulation
can depend on the degree of defoliation (Pinkard et al. 2007).
A potential explanation for the lack of photosynthetic response
to defoliation in W− treatment may be that the lower water
availability in the rain-fed treatment prevented up-regulation of
photosynthesis. In this study, trees in the W− treatment had
lower leaf water potentials than plants in the W+ treatment.
Eucalyptus globulus is particularly sensitive to soil water deficits, and stomatal conductance may be significantly reduced at
leaf water potentials as high as −0.4 to −0.5 MPa (this study;
White et al. 1999, O’Grady et al. 2008). An alternative explanation may be that the sampling strategy employed here may
have missed the peak in Asat, as photosynthesis was typically
measured from late morning to early afternoon. However,
Interactive effects of water supply and defoliation 965
under water-limited periods the peak in gs and canopy conductance may occur earlier in the morning (O’Grady et al. 2008,
Quentin et al. 2011) and this may have impacted on our results.
Canopy-level responses to defoliation
In contrast to the results observed at the leaf scale, we
observed significant increases in whole-plant estimates of
GCmax, Emax and Kp in both the W− and W+ treatments following
defoliation. Removal of the transpiring surface area via defoliation of the plant canopy may act to reduce water loss and conserve soil moisture (McNaughton 1983, Brown 1995) and this
response has been used to explain observed increases in leaf
water potential, transpiration rate and canopy conductance following defoliation (Stephens et al. 1972, Welker and Menke
1990, Reich et al. 1993, Vanderklein and Reich 2000, Gieger
and Thomas 2002, Quentin et al. 2011). However in this study,
neither the defoliation treatment nor its interaction with time
and/or water availability significantly affected plant water status, as measured by leaf water potential, total plant water use
or total daily canopy conductance. However, we observed
increases in Emax, GCmax and Kp following defoliation in both
water availability treatments, which implies an improvement in
tree water transport capacity following defoliation, irrespective
of the water treatment. Thus despite defoliation, total canopy
conductance was similar between defoliated and non-defoliated trees in both the W+ and W− treatments.
Canopy conductance is a key variable in the regulation of
plant water use. Whitehead and Jarvis (1981) proposed that
plants maintain a homeostasis in plant water status that is regulated via stomatal conductance of the canopy. They proposed a
simple model that predicts how GC at a given value of D should
vary with KP, sapwood-to-leaf area ratio (AS/AL) and the water
potential gradient between the soil and the leaf (ΔΨpd–md):
GC = K P
AS 1
(∆Ψpd − md − hη) (3)
AL D
where η is the viscosity of water at 20 °C and h is the tree
height. Our findings provide support for this model. Defoliation
in both water availability treatments resulted in increased AS/AL
(data not shown), GCmax, Emax and as a result an increase in KP.
These observations suggest that the observed GC responses in
defoliated trees were hydraulically mediated. Previous defoliation experiments (Meinzer and Grantz 1990, Pataki et al.
1998) have also shown that stomata respond rapidly to defoliation and it has been argued that this response is related to
the associated increases in KP. In further support of this argument, Asat and gs were linearly related to KP across treatments
following the defoliation treatment application (Figures 5a, b
and 6), an observation that is consistent with hydraulic regulation of plant water status and gas exchange (Meinzer 2002,
Mencuccini 2002). Strong relationships between photosynthesis and hydraulic conductance at the leaf and branch scale
have been observed before (e.g., Hubbard et al. 1999, Brodribb
and Feild 2000, Brodribb et al. 2007), but we believe that this
is the first time that this relationship between KP and Asat has
been demonstrated at the whole-plant scale.
Evidence for compensatory growth in response to
defoliation
Previous studies of defoliation on the growth of young E. globulus trees have shown reduced diameter and height growth
(Pinkard et al. 2007, 2011), or no significant change in diameter
following partial defoliation (Eyles et al. 2009). In this study,
there were no significant changes in height or diameter increments following defoliation in either water-availability treatment,
despite removal of ~60% of the total leaf area. This suggests
that some compensatory responses following defoliation were
occurring. The lack of differences in tree growth between defoliated and undefoliated trees in the W+ treatment may be attributable to the up-regulation of photosynthesis. However, the lack of
growth responses in the W− treatment were more difficult to
resolve as there was no observed increase in Asat following defoliation; however, total canopy conductance for defoliated and
undefoliated trees were similar in the W− treatment. An alternate
strategy to mitigate the negative effects of defoliation on growth
is alteration of carbon allocation patterns and the mobilization of
stored resources to accelerate the replacement of damaged tissue. Studies on the interactive effects of defoliation with abiotic
factors such as nutrient and/or water supply in E. globulus have
demonstrated that adequate resource availability favours the
compensatory response (Pinkard et al. 2007, 2011), as observed
in this study. Eyles et al. (2009) found that starch concentrations in coarse roots declined in response to defoliation in the
W− treatments suggesting that above-ground growth may have
been maintained by the depletion of below-ground carbohydrate
reserves. Pinkard et al. (2011) suggest that as carbohydrates
have a role in osmoregulation under drought conditions,
increases in foliar carbohydrates in the W− treatments may
inhibit a compensatory leaf photosynthetic response.
Conclusions
Leaf- and tree-level gas exchange responses to the interacting
effects of and defoliation depended upon water availability. Plants
maintained a homeostasis in plant water status between undefoliated and defoliated plants that was mediated through changes
in sapwood area to leaf area ratios, increased maximum canopy
conductance and soil-to-leaf hydraulic conductance of defoliated
trees. As a result, there were no differences in the daily integral
of canopy conductance between defoliation treatments. The
strong linear relationship between Asat, gs and KP at the leaf and
whole-tree scale across treatments ­
provides a mechanistic
insight into the observed up-regulation of photosynthesis in
response to defoliation, although reduced water a­vailability
Tree Physiology Online at http://www.treephys.oxfordjournals.org
966 Quentin et al.
played an important role in mediating plant responses to defoliation. Up-regulation of Asat was observed in irrigated but not in
rain-fed trees. Despite this, above-ground growth in both the irrigated and rain-fed treatments was unaffected by defoliation, suggesting that E. globulus trees are able to use a number of
compensatory mechanisms to maintain above-ground growth
increments in response to defoliation. At the stand scale, single
defoliation events in young fast-growing plantation species such
as E. globulus may have relatively small impacts on stand productivity. However, the impacts of changes in the frequency of these
stochastic events require further investigation.
Acknowledgments
Thanks to Dr Sebastian Leuzinger and an anonymous reviewer
for their valuable comments and suggestions on the earlier
manuscript. We thank Dale Worledge, Malcolm Hall, Stephen
Paterson, Alieta Eyles, Maria Ottenschlaeger and Craig Baillie
for assistance with field work. The authors also thank Charles
and Robin Lewis of Milford farm for their support of the Pittwater
Research Plantation. We greatly appreciate the editorial inputs
of Dr Michael Ryan on an earlier version of this manuscript.
Conflict of interest
None declared.
Funding
Funding was provided by CRC for Forestry. O’Grady was supported by an ARC linkage grant (LP0454287).
References
Allen, M.R. and W.J. Ingram. 2002. Constraints on future changes in
climate and the hydrologic cycle. Nature 419:224–232.
Ayres, M.P. and M.A.J. Lombardero. 2000. Assessing the consequences of global change for forest disturbance from herbivores
and pathogens. Sci. Total Environ. 262:263–286.
Bréda, N. and V. Badeau. 2008. Forest tree responses to extreme
drought and some biotic events: towards a selection according to
hazard tolerance? C. R. Géosci. 340:651–662.
Brodribb, T.J. and T.S. Feild. 2000. Stem hydraulic supply is linked to
leaf photosynthetic capacity: evidence from New Caledonian and
Tasmanian rainforests. Plant Cell Environ. 23:1381–1388.
Brodribb, T.J., T.S. Field and G.J. Jordan. 2007. Leaf maximum photosynthetic rate and venation are linked by hydraulics. Plant Physiol.
144:1890–1898.
Brown, R.W. 1995. The water relations of range plants: adaptations to
water deficits. In Wildland Plants: Physiological Ecology and
Developmental Morphology. Eds. D. J. Bedunah and R. E. Sosebee.
Society for Range Management, Denver, CO, pp 291–413.
Cunningham, S., K. Pullen and M. Colloff. 2009. Whole-tree sap flow is
substantially diminished by leaf herbivory. Oecologia 158:633–640.
Edwards, W.R.N. and N.W.M. Warwick. 1984. Transpiration from a kiwifruit vine as estimated by the heat pulse technique and the PenmanMonteith equation. N. Z. J. Agric. Res. 27:537–543.
Tree Physiology Volume 00, 2012
Ewers, B.E. and R. Oren. 2000. Analyses of assumptions and errors in
the calculation of stomatal conductance from sap flux measurements. Tree Physiol. 20:579–589.
Eyles, A., E.A. Pinkard and C.L. Mohammed. 2009. Shifts in biomass
and resource allocation patterns following defoliation in Eucalyptus
globulus growing with varying water and nutrient supplies. Tree
Physiol. 29:753–764.
Eyles, A., D. Smith, E.A. Pinkard, I. Smith, R. Corkrey, S. Elms, C.L.
Beadle and C.L. Mohammed. 2011. Photosynthetic responses of
field-grown Pinus radiata trees to artificial and aphid-induced defoliation. Tree Physiol. 31:592–603.
GENSTAT. 1989. Genstat 5 reference manual. Oxford, UK.
Gieger, T. and F.M. Thomas. 2002. Effects of defoliation and drought
stress on biomass partitioning and water relations of Quercus robur
and Quercus petraea. Basic Appl. Ecol. 3:171–181.
Gieger, T. and F.M. Thomas. 2005. Differential response of two
Central-European oak species to single and combined stress factors. Trees 19:607–618.
Hatton, T.J., E.A. Catchpole and R.A. Vertessy. 1990. Integration of sapflow velocity to estimate plant water use. Tree Physiol. 6:201–209.
Hogg, E.H., J.P. Brandt and B. Kochtubajda. 2002. Growth and dieback
of Aspen forests in Northwestern Alberta, Canada, in relation to climate and insects. Can. J. For. Res. 32:823–832.
Hubbard, R.M., B.J. Bond and M.G. Ryan. 1999. Evidence that hydraulic
conductance limits photosynthesis in old Pinus ponderosa trees.
Tree Physiol. 19:165–172.
Hutley, L.B., A.P. O’Grady and D. Eamus. 2000. Evapotranspiration
from eucalypt open-forest savanna of Northern Australia. Funct.
Ecol. 14:183–194.
Karban, R. and I.T. Baldwin. 1997. Induced responses to herbivory.
University of Chicago, Chicago, IL, 330 p.
Kolb, T.E., K.A. Dodds and K.M. Clancy. 1999. Effect of western spruce
budworm defoliation on the physiology and growth of potted
Douglas-fir seedlings. For. Sci. 45:280–291.
Landsberg, J. 1985. Drought and dieback of rural eucalypts. Aust. J.
Ecol. 10:87–90.
Landsberg, J. and F.R. Wylie. 1983. Water stress, leaf nutrients and
defoliation: a model of dieback of rural eucalypts. Aust. J. Ecol.
8:27–41.
Loustau, D. and A. Granier. 1993. Environmental control of water flux
through maritime pine (Pinus pinaster Ait.). In Water Transport in
Plants Under Climatic Stress. Eds. M. Borghetti, J. Grace and A. Raschi.
Cambridge University Press, Cambridge, UK. pp 205–218.
McGraw, J.B., K.W. Gottschalk, M.C. Vavrek and A.L. Chester. 1990.
Interactive effects of resource availabilities and defoliation on photosynthesis, growth, and mortality of red oak seedlings. Tree Physiol.
7:247–254.
McNaughton, S.J. 1983. Compensatory plant growth as a response to
herbivory. Oikos 40:329–336.
McPherson, G. 1990. Statistics in scientific investigation. Its basis,
application and interpretation. Springer-Verlag, London, UK.
Meinzer, F.C. 2002. Co-ordination of vapour and liquid phase water
transport properties in plants. Plant Cell Environ. 25:265–274.
Meinzer, F.C. and D.A. Grantz. 1990. Stomatal and hydraulic conductance in growing sugarcane: stomatal adjustment to water transport
capacity. Plant Cell Environ. 13:383–388.
Mencuccini, M. 2002. Hydraulic constraints in the functional scaling of
trees. Tree Physiol. 22:553–565.
Mencuccini, M. 2003. The ecological significance of long-distance water
transport: short-term regulation, long-term acclimation and the hydraulic costs of stature across plant life forms. Plant Cell Environ.
26:163–182.
Mielke, M.S., M.A. Oliva, N.F. de Barros, R.M. Penchel, C.A. Martinez
and A.C. de Almeida. 1999. Stomatal control of transpiration in the
Interactive effects of water supply and defoliation 967
canopy of a clonal Eucalyptus grandis plantation. Trees
13:152–160.
Mittler, R. 2006. Abiotic stresses, the field environment and stress
combination. Trends Plant Sci. 11:15–19.
Monteith, J.L. and M.H. Unsworth. 1990. Principles of environmental
physics. E.A. Arnold, 291 p.
Morris, J., L. Mann and J. Collopy. 1998. Transpiration and canopy conductance in a eucalypt plantation using shallow saline groundwater.
Tree Physiol. 18:547–555.
Mummery, D. and M. Battaglia. 2004. Significance of rainfall distribution in predicting eucalypt plantation growth, management options,
and risk assessment using the process-based model CABALA. For.
Ecol. Manag. 193:283–296.
Munns, R. 2002. Comparative physiology of salt and water stress.
Plant Cell Environ. 25:239–250.
Nahrung, H.F. 2003. Reproductive ecology of Chrysophtharta agricola
(Chapuis) (Coleoptera: Chrysomelidae). School of Agricultural
Science, University of Tasmania, Hobart, Tasmania, Australia, 349 p.
Netherer, S. and A. Schopf. 2010. Potential effects of climate change
on insect herbivores in European forests—general aspects and the
pine processionary moth as specific example. For. Ecol. Manag.
259:831–838.
O’Grady, A.P., D. Worledge and M. Battaglia. 2005. Temporal and spatial changes in fine root distributions in a young Eucalyptus globulus
stand in southern Tasmania. For. Ecol. Manag. 214:373–383.
O’Grady, A.P., D. Worledge and M. Battaglia. 2006. Above- and belowground relationships, with particular reference to fine roots, in a
young Eucalyptus globulus (Labill.) stand in southern Tasmania.
Trees 20:531–538.
O’Grady, A.P., D. Worledge and M. Battaglia. 2008. Constraints on
transpiration of Eucalyptus globulus in southern Tasmania, Australia.
Agric. For. Meteorol. 148:453–465.
Osmond, C.B., M.P. Austin, J.A. Berry, W.D. Billings, J.S. Boyer, J.W.H.
Dacey, P.S. Nobel, S.D. Smith and W.E. Winner. 1987. Stress physiology and the distribution of plants. BioScience 37:38–48.
Pataki, D.E., R. Oren and N. Phillips. 1998. Responses of sap flux and
stomatal conductance of Pinus taeda L. trees to stepwise reductions
in leaf area. J. Exp. Bot. 49:871–878.
Pearcy, R.W., E.D. Schulze and R. Zimmermann. 1989. Measurement of
transpiration and leaf conductance. In Plant Physiological Ecology:
Field Methods and Instrumentation. Eds. R.W. Pearcy, J.R.
Ehleringer, H. Mooney and P.W. Rundel. Chapman & Hall, New York,
pp 137–160.
Pereira, J.S., J.D. Tenhunen and O.L. Lange. 1987. Stomatal control of
photosynthesis of Eucalyptus globulus Labill. trees under field conditions in Portugal. J. Exp. Bot. 38:1678–1688.
Phillips, N. and R. Oren. 1998. A comparison of daily representations
of canopy conductance based on two conditional time-averaging
methods and the dependence of daily conductance on environmental factors. Ann. For. Sci. 55:217–235.
Pinkard, E.A., M. Battaglia and C.L. Mohammed. 2007. Defoliation and
nitrogen effects on photosynthesis and growth of Eucalyptus globulus. Tree Physiol. 27:1053–1063.
Pinkard, E.A., A. Eyles and A.P. O’Grady. 2011. Are gas exchange
responses to resource limitation and defoliation linked to source:sink
relationships? Plant Cell Environ. 34:1652–1665.
Quentin, A.G., E.A. Pinkard, C.L. Beadle, T.J. Wardlaw, A.P. O’Grady,
S. Paterson and C.L. Mohammed. 2010. Do artificial and natural
defoliation have similar effects on physiology of Eucalyptus globulus
Labill. seedlings? Ann. For. Sci. 67:203–203.
Quentin, A.G., A.P. O’Grady, C.L. Beadle, D. Worledge and E.A. Pinkard.
2011. Responses of transpiration and canopy conductance to partial
defoliation of Eucalyptus globulus trees. Agric. For. Meteorol.
151:356–364.
Reich, P.B., M.B. Walters, S.C. Krause, D.W. Vanderklein, K.F. Raffs and
T. Tabone. 1993. Growth, nutrition and gas exchange of Pinus resinosa following artificial defoliation. Trees 7:67–77.
Stephens, G.R., N.C. Turner and H.C. De Roo. 1972. Some effects of
defoliation by gypsy moth (Porthetria dispar L.) and elm spanworm
(Ennomos subsignarius Hbn.) on water balance and growth of deciduous forest trees. For. Sci. 18:326–330.
Stoneman, G. L., N. C. Turner and B. Dell. 1994. Leaf growth, photosynthesis and tissue water relations of greenhouse-grown Eucalyptus
marginata seedlings in response to water deficits. Tree Physiol
14:633–646.
Suwa, T. and H. Maherali. 2008. Influence of nutrient availability on the
mechanisms of tolerance to herbivory in an annual grass, Avena
barbata (Poaceae). Am. J. Bot. 95:434–440.
Swanson, R.H. and D.W.A. Whitfield. 1981. A numerical analysis of heat
pulse velocity theory and practice. J. Exp. Bot. 32:221–239.
Turnbull, T.L., M.A. Adams and C.R. Warren. 2007. Increased photosynthesis following partial defoliation of field-grown Eucalyptus globulus
seedlings is not caused by increased leaf nitrogen. Tree Physiol.
27:1481–1492.
Tyree, M. 2003. Hydraulic limits on tree performance: transpiration,
carbon gain and growth of trees. Trees 17:95–100.
Vanderklein, D.W. and P.B. Reich. 2000. European larch and eastern
white pine respond similarly during three years of partial defoliation.
Tree Physiol. 20:283–287.
Welker, J.M. and J.W. Menke. 1990. The influence of simulated browsing on tissue water relations, growth and survival of Quercus
­douglasii (Hook and Arn.) seedlings under slow and rapid rates of
soil drought. Funct. Ecol. 4:807–817.
White, D.A., C.L. Beadle and D. Worledge. 1996. Leaf water relations
of Eucalyptus globulus ssp. globulus and E. nitens: seasonal, drought
and species effects. Tree Physiol. 16:469–476.
White, D.A., C.L. Beadle, P.J. Sands, D. Worledge and J.L. Honeysett.
1999. Quantifying the effect of cumulative water stress on stomatal
conductance of Eucalyptus globulus and Eucalyptus nitens: a phenomenological approach. Funct. Plant Biol. 26:17–27.
Whitehead, D. and P.G. Jarvis. 1981. Coniferous forests and plantations. In Water Deficits and Plant Growth. Ed. T. T. Kozlowski.
Academic Press, New York. pp 49–152.
Whitehead, D. and C.L. Beadle. 2004. Physiological regulation of productivity and water use in Eucalyptus: a review. For. Ecol. Manag.
193:113–140.
Tree Physiology Online at http://www.treephys.oxfordjournals.org