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Tree Physiology 32, 146–160
Variation in water potential, hydraulic characteristics and water
source use in montane Douglas-fir and lodgepole pine trees
in southwestern Alberta and consequences for seasonal changes
in photosynthetic capacity
Shilo F. Andrews, Lawrence B. Flanagan1, Eric J. Sharp and Tiebo Cai
Department of Biological Sciences, Water & Environmental Sciences Building, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta, Canada T1K 3M4;
1Corresponding author (firstname.lastname@example.org)
Received July 4, 2011; accepted December 2, 2011; published online February 8, 2012; handling Editor Frederick Meinzer
Tree species response to climate change-induced shifts in the hydrological cycle depends on many physiological traits, par-
ticularly variation in water relations characteristics. We evaluated differences in shoot water potential, vulnerability of branches
to reductions in hydraulic conductivity, and water source use between Pinus contorta Dougl. ex Loud. var. latifolia Engelm.
(lodgepole pine) and Pseudotsuga menziesii (Mirb.) Franco (interior Douglas-fir), and determined the consequences for sea-
sonal changes in photosynthetic capacity. The Douglas-fir site had soil with greater depth, finer texture and higher organic
matter content than soil at the lodgepole pine site, all factors that increased the storage of soil moisture. While the measured
xylem vulnerability curves were quite similar for the two species, Douglas-fir had lower average midday shoot water potentials
than did lodgepole pine. This implied that lodgepole pine exhibited stronger stomatal control of transpiration than Douglas-fir,
which helped to reduce the magnitude of the water potential gradient required to access water from drying soil. Stable hydro-
gen isotope measurements indicated that Douglas-fir increased the use of groundwater during mid-summer when precipita-
tion inputs were low, while lodgepole pine did not. There was a greater reduction of photosynthetic carbon gain in lodgepole
pine compared with Douglas-fir when the two tree species were exposed to seasonal declines in soil water content. The con-
trasting patterns of seasonal variation in photosynthetic capacity observed for the two species were a combined result of
differences in soil characteristics at the separate sites and the inherent physiological differences between the species.
Keywords: drought, hydrogen stable isotope ratio, oxygen evolution, xylem.
Climate change has contributed to shifts in the hydrological
cycle in northern ecosystems such as a lower winter snow
pack and earlier spring snow melt and runoff (Barnett et al.
2005, Rood et al. 2005). Further increases in atmospheric
CO2 concentration and associated warmer temperatures are
projected to increase the frequency and intensity of summer
drought (Meehl et al. 2007). There is concern that widespread
forest decline will result from environmentally imposed physi-
ological stress in trees and other climate-induced processes
like insect outbreaks and forest fire (Allen et al. 2010, van der
Molen et al. 2011). There is evidence that some forested eco-
systems are already being negatively impacted by climate
change, with the potential to disrupt important ecosystem ser-
vices provided by forests and cause a positive feedback to
further increases in atmospheric CO2 concentrations (Allen and
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Breshears 1998, Breshears et al. 2005, Hogg et al. 2008,
Kurz et al. 2008, van Mantgem et al. 2009, Allen et al. 2010,
Matyas 2010, Michaelian et al. 2011).
There is variation among tree species in their ability to accli-
mate to, and survive, environmental change that is particularly
dependent on variation in hydraulic characteristics and pat-
terns of water use (McDowell et al. 2008, Allen et al. 2010).
Under conditions of steady state, water loss in transpiration is
balanced by water movement through stem xylem as repre-
sented in Eq. (1) (Whitehead et al. 1984, Martínez-Vilalta et al.
K ? ?
where gwv is the canopy-averaged conductance to water vapor
between leaves and the atmosphere, AL/AS is the ratio of leaf
area to sapwood area, D is the vapor pressure gradient
between leaves and the atmosphere, KS is the sapwood-spe-
cific hydraulic conductivity and ΔΨ is the water potential gradi-
ent through the xylem. Equation (1) illustrates how variation in
plant physiological characteristics could help to acclimate or
adapt to conditions of low soil moisture availability (Martínez-
Vilalta et al. 2004, Creese et al. 2011). For example, higher KS
or lower AL/AS would reduce the magnitude of the water poten-
tial gradient required to access water from drying soil or under
conditions of increased D (Tyree and Ewers 1991, DeLucia
et al. 2000). Alternatively, plants with reduced vulnerability to
water stress-induced xylem cavitation would suffer less loss of
hydraulic conductivity as soil water stress caused declines in
xylem water potential (Pockman and Sperry 2000, Maherali
et al. 2004). Some tree species have relatively high stem water
storage that can buffer or slow changes in xylem water poten-
tial following increases in transpiration and xylem water flux
(Meinzer et al. 2009). Finally, strong stomatal control of water
loss could minimize ΔΨ values as soil water content declined,
but this would come at the expense of restricting CO2 uptake
and plant carbon gain in photosynthesis (Brodribb and Feild
2000, Sperry 2000, Maherali et al. 2006, Brodribb et al.
2007). While Eq. (1) provides a useful context in which to
compare various plant characteristics among species for their
potential contribution to acclimation and survival of summer
drought, it does not consider soil water uptake or water trans-
port in roots (Creese et al. 2011).
The availability of water to trees depends on the amount and
timing of precipitation, the ability of the soil to store water, and
the tree’s ability to access resident soil moisture which is
dependent on rooting pattern and other root characteristics
that affect the efficiency of water uptake (Weltzin et al. 2003,
Meinzer et al. 2007). Stable isotope measurements can pro-
vide useful information about tree water source use (White
et al. 1985, Flanagan et al. 1992, Ehleringer and Dawson
1992, Ehleringer et al. 2000, Dawson et al. 2002). Since there
is no fractionation during the bulk flow of water into the xylem,
the isotope composition of xylem water can be used to trace
the uptake of different sources of water, if the sources have
naturally different isotope compositions (Ehleringer et al.
2000). In temperate locations, the isotopic composition of pre-
cipitation varies seasonally so that summer rain often consists
of water relatively enriched in heavy isotopes, while ground-
water (a weighted average of year-round precipitation inputs)
is relatively depleted in heavy isotopes (Dansgaard 1964, Gat
1996). A variety of other processes can also contribute to dif-
ferences in the stable isotope composition of water in shallow
and deeper soil layers (Ehleringer et al. 2000, Brooks et al.
2010). Measurements of the stable isotope composition of
xylem water, in comparison with possible water sources, can
indicate differences between tree species in functional rooting
patterns that access contrasting soil layers, or variation
between sites in the habitat’s capacity for soil water storage
and the availability of groundwater reserves.
Pinus contorta Dougl. ex Loud. var. latifolia Engelm. (lodge-
pole pine) and Pseudotsuga menziesii (Mirb.) Franco (interior
Douglas-fir) are important dominant tree species on the eastern
slopes of the Rocky Mountains in southwestern Alberta at the
sharp transition zone that occurs between grassland and mon-
tane forest ecosystems in this region (Farr and Andrews 2003,
Chen et al. 2010, Chhin et al. 2010). This transition zone should
be very susceptible to anticipated climate changes, including
warmer temperatures and shifts in availability of water sources
(altered snow melt patterns, change in spring/summer precipi-
tation and increased drought), with important ecological and
economic consequences of forest decline in this region of
national parks and watersheds that supply the prairie provinces
(Schindler and Donahue 2006, Schneider et al. 2009,
Boisvenue and Running 2010). In southwestern Alberta, the
two tree species normally form almost pure stands on sites that
differ greatly in soil characteristics. In general, soil at sites domi-
nated by Douglas-fir have greater depth, finer soil texture and
higher organic matter content than soil at sites dominated by
lodgepole pine, all factors that would increase the ability to
store moisture (Andrews 2009). Previous studies have indi-
cated that Douglas-fir and lodgepole pine also differ in physio-
logical characteristics, with lodgepole pine considered to have
the greater tolerance to drought (Smith 1985, Piñol and Sala
2000, Martínez-Vilalta et al. 2004). Despite a greater drought
tolerance, lodgepole pine was shown to be more susceptible
than Douglas-fir to drought-induced loss of xylem hydraulic
conductivity (Piñol and Sala 2000). This implies that other
physiological and structural characteristics of lodgepole pine
may compensate to prevent shoot and stem water potentials
from reaching values low enough to cause significant cavitation
and loss of hydraulic conductivity under field conditions (Stout
and Sala 2003, Martínez-Vilalta et al. 2004). In particular, we
Eco-physiology of Douglas-fir and lodgepole pine 147
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Tree Physiology Volume 32, 2012
hypothesize that stronger stomatal control of transpiration in
lodgepole pine could help reduce the magnitude of the water
potential gradient required to access water from drying soil. If
this was true, we would also predict a greater reduction of pho-
tosynthetic carbon gain in lodgepole pine compared with
Douglas-fir when the two tree species were exposed to sea-
sonal decreases in soil water content. The expected contrasting
patterns of seasonal variation in photosynthetic carbon gain
between lodgepole pine and Douglas-fir would be a combined
result of differences in their typical site soil characteristics and
the inherent physiological differences between the species.
The objective of this study was to measure shoot water
potential, vulnerability of branches to reductions in hydraulic
conductivity, and water source use between lodgepole pine
and Douglas-fir, and to determine the consequences for sea-
sonal changes in photosynthetic capacity and carbon isotope
discrimination in order to test the hypotheses proposed above.
Materials and methods
Fieldwork for the study was primarily conducted at two sites in
southwestern Alberta, Canada in the front ranges of the Rocky
Mountains during the summer growing season (May–October)
of 2008. The Douglas-fir site (49.69901 °N, 114.02244 °W,
elevation 1530 m) was located ~25 km north of Pincher Creek,
Alberta on private ranch land. The town of Claresholm, Alberta
is the closest (47 km) Environment Canada weather station
with long-term temperature records, and Cowley, Alberta is the
closest (15 km) Environment Canada weather station with
long-term precipitation records. The mean annual temperature
(1971–2000) was 5.2 °C, and the average annual precipitation
was 494 mm, with 61% rain and 39% snow. The lodgepole
pine site (49.83208 °N, 114.42068 °W, elevation 1550 m)
was located ~20 km north of Coleman, Alberta. The closest
Environment Canada weather station was at Coleman, where
the mean annual temperature (1971–2000) was 3.5 °C and
the average annual precipitation was 576 mm, with 69% rain
and 31% snow. The Douglas-fir site was located ~30 km
southeast of the lodgepole pine site. A comparison of the
monthly average air temperature and the monthly total precipi-
tation in May–October 2008 at the two study sites relative to
the long-term (30-year) climate records is shown in Figure 1.
These two primary study sites were typical of areas dominated
by the two species in southern Alberta. Lodgepole pine usually
forms pure stands in the montane and sub-alpine zones of the
Rocky Mountains, while relatively pure Douglas-fir forests
extend slightly further east at lower elevations where the
mountains and foothills transition into fescue grassland. A third
study site that included both tree species (subsequently
referred to as the ‘combined site’) was located ~24 km directly
north of the Douglas-fir site, in similar terrain and with the
same general forest features and same soil type as the
Douglas-fir site. We used the combined site to evaluate inher-
ent differences in (i) shoot water potential and (ii) the vulner-
ability of branches to loss of hydraulic conductivity between
the two species while they were growing adjacently at the
same site during August 2009.
Tree and forest characteristics
Tree and forest characteristics (tree diameter at breast height
(DBH, 1.35 m above ground), tree height (measured with a cli-
nometer), tree density and basal area) were sampled in quad-
rats positioned along three 100 m long transect lines at the two
primary sites. At the Douglas-fir site, five 25 m2 quadrats were
sampled on each transect with the data gathered in all quadrats
averaged for the entire transect (i.e., the transect was consid-
ered the basic sampling unit or replicate). At the lodgepole pine
site, 5 or 10 quadrats of 25 or 100 m2 were sampled on each
transect depending on tree density. Sapwood area was also
measured at all study sites by coring 10 randomly chosen trees
and immediately marking the transition point between heart-
wood and sapwood. The transition point was distinguished by
examining the core in direct sunlight and observing the transi-
tion between translucent and non-translucent tissue, sapwood
being the translucent portion of the core. Sapwood area was
calculated using the measured basal area of the tree and the
depth of the sapwood (Hall et al. 2003, West et al. 2008).
Meteorological instruments were mounted on 3 m tall triangular
masts that were installed in open areas at both primary study
sites. Air temperature and relative humidity were measured
using an air temperature and relative humidity probe (Vaisala
HMP45C, Campbell Scientific, Edmonton, Alberta, Canada)
mounted in a ventilated radiation shield at a height of 1.5 m. Soil
temperature was measured at three soil depths (5, 15 and
30 cm) using thermistor soil temperature probes (107B,
Campbell Scientific). Soil water content reflectometers (CS616,
Campbell Scientific) were installed at an angle to sample the
0–15 cm depth in three locations at each primary study site. The
reflectometer period measurements were calibrated and con-
verted to volumetric water content measurements based on
manual measurements of volumetric soil moisture content that
were collected at intervals throughout the growing season. The
manual soil moisture samples were collected using a soil corer
(0–15 cm depth) from five randomly chosen locations at each
primary study site at ~2-week intervals during May–October
2008. Total rainfall was measured in 30-min intervals with a tip-
ping bucket rain gauge (CS700, Campbell Scientific). All meteo-
rological sensors were connected to a data logger (CR23X,
Campbell Scientific) and, with the exception of the rain gauge,
were scanned at 5-s intervals and stored as 30-min averages.
148 Andrews et al.
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lodgepole pine’s physiological characteristics and the shallow,
coarse-textured soil conditions of its typical habitat, it is more
susceptible than Douglas-fir to climate change-induced forest
decline, at least in the region of southwestern Alberta.
We thank David Pearce for help with the xylem vulnerability
curve measurements. Stewart Rood kindly allowed us to use his
Cavitron apparatus, which was built by Mel Tyree. Helpful com-
ments were provided on an earlier version of the manuscript by
David Pearce, Rick Meinzer and two anonymous reviewers.
This study was funded by the Natural Sciences and Engineering
Research Council of Canada and the Alberta-Israel Water
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