Directory UMM :Data Elmu:jurnal:T:Tree Physiology:vol17.1997:
Tree Physiology 17, 151--160
© 1997 Heron Publishing----Victoria, Canada
Boxelder water sources and physiology at perennial and ephemeral
stream sites in Arizona
THOMAS E. KOLB,1 STEPHEN C. HART1 and RONALD AMUNDSON2
1
School of Forestry, College of Ecosystem Science and Management, Northern Arizona University, Flagstaff, AZ 86011-5018, USA
2
Division of Ecosystem Sciences, University of California, Berkeley, CA 94720, USA
Received March 15, 1996
Summary To assess the influence of stream water on leaf gas
exchange and water potential in different sized boxelder trees
(Acer negundo L.), we compared these characteristics in trees
growing beside a perennial stream and a nearby ephemeral
stream in a montane--riparian forest in northern Arizona. Patterns of tree water use were quantified by stable isotope analysis (δ18O). Physiological characteristics were similar for large
and small trees. Similarity between sites in predawn and daytime water potentials and xylem δ18O indicated that stream
water was not a physiologically important water source. Seasonal and site variations in light-saturated net photosynthetic
rate were significantly related to leaf-to-air vapor pressure
deficit (r = --0.691) and foliar nitrogen concentration (r =
0.388). Although deep water was the dominant water source,
surface soil water was utilized following precipitation, especially by small trees. We conclude that net carbon gain and
severity of water stress are only weakly coupled to stream water
availability.
Keywords: Acer negundo, nitrogen, photosynthesis, riparian
forests, stable isotopes, stomatal conductance, water relations.
with detrimental physiological changes and reduced growth of
riparian trees in western North America (Stromberg and Patten
1990, Smith et al. 1991, Tyree et al. 1994). The effects of
stream diversion on leaf physiology can differ among trees of
different sizes, with greater detrimental effects reported for
juvenile trees than for mature trees (Smith et al. 1991). Juvenile trees may be especially sensitive to water stress because
shallow root systems limit their water source to the surface
soil, whereas more reliable water sources in deep soil and
ground water are only available to deeply rooted, larger trees
(Dawson and Ehleringer 1991, Dawson 1993).
In this study, we compared water sources, leaf gas exchange
and water relations of different-sized boxelder (Acer negundo L.) trees growing beside a perennial stream and a nearby
ephemeral stream in a montane--riparian forest in northern
Arizona. Specifically, we assessed: (1) the effect of stream
water accessibility on water-source utilization, water relations
and photosynthesis of different-sized trees; (2) if tree size and
dependability of the water source influence shifts in watersource use during the growing season; and (3) the roles of
environmental and physiological factors in limiting photosynthesis of boxelder under field conditions.
Introduction
Riparian forests are a threatened ecosystem in the American
Southwest (Lowe and Brown 1982, Johnson and Haigt 1984).
Decline of native riparian trees may result from modification
of surface and ground water flows by impoundments and
diversions, as well as by changes in associated abiotic and
biotic factors, such as increased alluvium salinity and invasion
by nonnative species (e.g., Stromberg and Patten 1990, Smith
et al. 1991, Busch et al. 1992, Busch and Smith 1993, Busch
and Smith 1995).
Linkages between water sources, water use, and productivity of riparian trees in the American Southwest are poorly
understood. Many riparian trees of western North America are
assumed to be phreatophytes that avoid water stress by tapping
reliable water sources with deep roots. However, riparian trees
can be subject to water stress, especially in mid to late summer
when water availability is reduced ( Dina and Klikoff 1973,
Foster and Smith 1991, Smith et al. 1991, Dawson and Ehleringer 1993a). Stream flow diversions have been associated
Materials and methods
Study sites
The study area was in upper Oak Creek Canyon in north-central Arizona (35°00′ N, 111°44′ W). Oak Creek is a perennial
stream that drains the southern edge of the Colorado Plateau
south of Flagstaff, Arizona. Annual precipitation in the region
averages 57 cm (Schubert 1974), occurring as snow and rain
during winter (November--March) and as monsoonal thundershowers during late summer (July--September). Upper Oak
Creek Canyon contains a cold-temperate, montane--riparian
forest type (Brown 1982). Dominant tree species include:
white fir (Abies concolor (Gord.) Lindl. ex Hildebr.), boxelder
(Acer negundo), bigtooth maple (Acer grandidentatum
Marsh.), Arizona alder (Alnus oblongifolia Torr.), velvet ash
(Fraxinus velutina Torr.), Arizona walnut (Juglans major
(Torr.) A. Heller), Gambel oak (Quercus gambelii Nutt.), ponderosa pine (Pinus ponderosa Dougl. ex P. Laws & C. Laws),
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KOLB, HART AND AMUNDSON
Arizona sycamore (Platanus wrightii P. Wats.), black cherry
(Prunus serotina J.F. Ehrh.) and narrowleaf cottonwood
(Populus angustifolia James).
We measured water-source use and physiological characteristics of boxelder trees at two upper Oak Creek Canyon sites
differing in proximity to perennial stream water. The perennial
stream site is located adjacent to Oak Creek, and the ephemeral
stream site is located adjacent to Ritter Wash, an intermittent
stream that drains into Oak Creek. The ephemeral stream site
is 0.4 km from the perennial stream site, and 0.2 km upstream
from the intersection of Ritter Wash and Oak Creek. Both sites
have similar elevation (1,650 m), slope and aspect. Although
stream water can occur at the ephemeral stream site in early
spring following snowmelt, surface water at the ephemeral
stream site was not observed during the study (April--November 1994). This observation, coupled with the distance between the ephemeral stream site and perennial water in Oak
Creek, strongly suggest that there was no direct uptake of
stream water by vegetation at the ephemeral stream site.
Soils from both sites are previously unclassified types in the
Soil Taxonomy Family of sandy-skeletal, mixed, mesic Typic
Udifluvents (Soil Survey Staff 1990, S.C. Hart, personal observation 1994). Despite depths of several meters, these soils have
high rates of water infiltration and low capacities for water
storage because of their coarse texture, and because ≥ 35% of
their volume is occupied by rock fragments (particles greater
than 2 mm in diameter).
Site environment
At both sites, we measured volumetric soil water content
weekly between late April and early October 1994, with a
time-domain reflectometry system (TRASE System 1, Soilmoisture Corp., Santa Barbara, CA). We established sample
areas at each site every 10 m along a linear transect in the
riparian forest at a distance of 2.3 m from the stream bank edge.
A pair of stainless steel rods was inserted into the mineral soil
to a depth of 30 cm at each of three sample areas at both sites.
A regression relationship between volumetric soil water content and the dielectric constant of soil between the pair of rods
was applied to predict soil water content at each site. The
regression relationship was determined for soils from each site
in a laboratory experiment in which soils wetted to field capacity were air dried (sensu, Gray and Spies 1995). The same
linear regression equation provided a good fit to data for soils
from both sites (r = 0.982).
At one sample location for each site, we measured soil (15
and 30 cm depths) and air temperatures (1 m above ground)
hourly over the same timespan described for soil water content
measurements. Temperatures were measured with thermocouples and recorded with dataloggers (Campbell Scientific Inc.,
Logan, UT). A tipping-bucket gauge was used to measure daily
bulk precipitation from a clearing near the perennial stream
site.
Isotopic analysis of tree water sources
At both sites, we sampled ground water, stream water, soil
water, and xylem water monthly during the 1994 growing
season. Ground water was sampled from springs adjacent to
the perennial stream site (Cave Spring) and in the upper drainage of the ephemeral stream site (Ritter Spring). Stream water
was sampled from well-mixed sections of the stream. Ground
and stream water samples were collected in glass vials fitted
with air-tight polyethylene caps and stored at 4 °C. Because the
ephemeral stream did not flow during the study period, no
stream water samples were obtained from this site.
Soil was sampled along the same 30-m transects described
above. Samples were pooled over the 0--30 cm soil layer at the
first sampling date (May 2). Thereafter, the 0--5 cm and 5--30 cm
soil layers were sampled separately. Soil samples were placed
in air-tight jars and stored at --18 °C until water was extracted.
Several 5- to 7-cm long suberized stem sections (Dawson
and Ehleringer 1993b) were sampled from the same three large
and three small boxelder trees at each site on each sampling
date. All of these trees were growing within 3 m of the stream
bank, and were chosen from the population of trees sampled
for physiological measurements (Table 1). Suberized stems
were placed in air-tight jars and stored at --18 °C. Water from
soil and xylem samples was extracted by a cryogenic vacuum
distillation technique (Sternberg et al. 1986, Ehleringer and
Osmond 1989, Smith et al. 1991).
Precipitation samples from significant individual storm
events (> 5 mm) were collected in 10 polyethylene bottles in
an open area adjacent to the perennial stream site. Precipitation
collections represent over 95% of the precipitation volume that
occurred during the study period. A small amount of mineral
oil was contained in each bottle to prevent evaporation of
collected water between sample bottle placement and retrieval
(0.5 to 3 days, depending on storm duration). Precipitation
water was stored at --18 °C.
All water samples were analyzed for 18O/16O isotope ratio at
Lawrence Berkeley National Laboratory, Berkeley, CA, on a
V6 Prism Isotope Ratio Mass Spectrometer by an automated
CO2--H2O equilibration technique (Epstein and Mayeda 1953).
Oxygen isotope ratios (δ18O values) were expressed as:
δ18 O = (Rsample ⁄ Rstandard −1)1000 ‰,
where R refers to the 18O/16O ratios of sample and standard
(standard mean ocean water, SMOW), respectively. Duplicate
isotopic analyses of randomly selected water samples (run
about every 25 samples) always differed by ≤ 0.1‰.
We applied a two-member mixing model to calculate the
relative proportion of surface soil (0--30 cm) to deep soil or
ground water utilized by each tree size at each site (White
1989). Because ground and stream water were isotopically
similar (see Results), we could not distinguish between these
two water sources at the perennial-stream site. We assumed
that when tree xylem and ground and stream water δ18O were
statistically similar (two-tailed Student’s t-Test, P > 0.05), the
percentage of xylem water derived from the soil was zero. The
soil water δ18O value applied in the mixing model was a
depth-weighted average of the δ18O values from the 0--5 and
5--30 cm soil layers on a given sampling date, and was calculated assuming that soil water within the upper 30 cm of soil
WATER SOURCE VARIATION AND PHYSIOLOGY OF BOXELDER
was uniformly utilized by tree roots. This assumption was not
rigorously tested; however, we did observe living fine roots at
all depths between 1 and 30 cm.
Tree water potential and gas exchange
We measured xylem water potential and leaf gas exchange on
both large and small boxelder trees (Table 1) at each site. All
sample trees grew in a closed-canopy forest within 12 m of the
stream bank. The light environment for large and small trees at
both sites was heterogeneous as a result of shading by other
trees. Most small trees at both sites likely originated as sprouts
from older stems that had been damaged by flooding. Although
we are aware of potential differences in leaf physiology between male and female boxelder trees (Dawson and Ehleringer
1993a), sex determination of sample trees was not possible
because neither flowers nor seeds were present.
Xylem water potential and light-saturated leaf gas exchange
characteristics were measured approximately every two weeks
from time of full-leaf expansion in spring (mid May) to onset
of leaf senescence in fall (early September). Net photosynthetic rate and stomatal conductance to water vapor were
measured midmorning (0900--1100 h) on all dates, and early
afternoon (1300--1500 h) on the first five dates. After mid July,
afternoon measurements of gas exchange were not made because of frequent precipitation and cloudy conditions. At each
date and sampling time within a date, a different sample of
three to five trees from each site and tree size class was
measured to minimize defoliation. At each sampling time
within a date, the order of measurement between sites and tree
sizes within sites was determined randomly.
We measured gas exchange on one representative leaf per
tree in the lower or middle canopy exposed to photosynthetically active radiant flux that was more than saturating for
boxelder (> 600 µmol m −2 s −1, Foster 1992). Gas exchange
was measured over 30 s on a fixed leaf area (8.2 cm2) with an
LI-6200 portable photosynthesis system (Li-Cor, Inc., Lincoln, NE) equipped with a 250-ml cuvette. During measurements, water vapor pressure and temperature in the cuvette
were within 10% and 3 °C of ambient values, respectively. For
large trees, we made approximately 90% of measurements in
situ on attached leaves, and about 10% of measurements were
made on leaves from twigs detached with pole pruners because
of difficulty in accessing the leaves for in situ measurements.
Leaves on detached twigs were measured within 30 s follow-
153
ing detachment in a light environment similar to that in the
canopy. Preliminary studies showed that net photosynthetic
rates of leaves on detached twigs do not differ from those of
leaves attached to the tree under our experimental conditions.
Similar methods have been used successfully in studies with
other species (e.g., Gower et al. 1993, Meng and Arp 1993).
Leaf-to-air vapor pressure deficit was calculated as the difference between atmospheric vapor pressure measured in the
cuvette and saturated vapor pressure calculated for leaf mesophyll. Leaf temperature used in calculating saturated vapor
pressure of leaf mesophyll was measured with a fine-wire
thermocouple placed on the leaf underside during the gas
exchange measurement. Short-term measurements of leaf temperature with an external thermocouple may have underestimated leaf temperature, and consequently vapor pressure
deficit, compared with estimates made by permanently attached in situ thermocouples (Tyree and Wilmot 1990).
Mid-morning and early afternoon xylem water potentials
were measured with a pressure chamber (Model 1000 PMS
Instruments, Corvallis, OR), as described by Ritchie and
Hinckley (1975), on the trees sampled for gas exchange assessments. Predawn xylem water potential (0500--0600 h) was
measured on all dates in five trees from each site and size class.
Preliminary studies showed that a more precise endpoint assessment was obtained with terminal twig segments than with
leaf rachises because the large twig cross-sectional area facilitated visual observation, and because there was less phloem
flow from twigs compared with leaf rachises. Therefore, midmorning and afternoon assessments of xylem water potential
were made on the terminal portion of the twig supporting the
leaf sampled for the gas exchange measurement. Predawn
assessments were made on a similar twig portion.
Foliar nitrogen
We measured total nitrogen concentration of leaves from five
trees sampled for gas exchange measurements, for each combination of date, site and tree size class. Leaf tissue was ground
(< 0.85 mm) and digested by a micro-Kjeldahl method (Issac
and Johnson 1976), which oxidizes organic nitrogen to ammonium. Ammonium concentration was measured with a Lachat
flow-injection analyzer (QuikChem Systems 1992). Nitrogen
concentration was calculated on a leaf area basis (g cm −2) to
facilitate comparison with leaf area-based gas exchange data.
Table 1. Observed mean and range of stem diameter (1 cm above soil level), stem height, and distance from stream bank of large and small boxelder
trees.
Diameter (cm)
Mean
Range
Height (m)
Mean
Range
Distance to the stream (m)
Mean
Range
Perennial-stream site
Large trees (n = 10)
Small trees (n = 8)
35.7
6.1
26.9--49.8
4.6--9.7
6.8
2.4
3.7--8.1
1.5--3.6
6.8
5.2
1.7--11.9
1.6--10.6
Ephemeral-stream site
Large trees (n = 7)
Small trees (n = 7)
30.0
6.1
22.4--48.8
4.8--7.9
7.9
2.5
6.3--9.0
2.2--3.1
3.7
4.5
0.1--9.7
1.0--8.7
154
KOLB, HART AND AMUNDSON
Statistical analysis
We compared differences in soil water content between sites at
each date by a one-way ANOVA. Mid-morning and afternoon
net photosynthetic rates, predawn, mid-morning, and afternoon water potentials, leaf nitrogen concentration, and δ18O of
stem water were compared between sites and tree sizes for
each date with a fixed effects, two-factor ANOVA. Factors in
the ANOVA were site, tree size class, and their interaction. A
fixed effects, two-factor ANOVA was also conducted for each
sampling date to test for significant differences in δ18O values
of soil water extracted from each site at 0--5 cm and 5--30 cm
soil depths (except on May 2 when soil was collected from the
0--30-cm depth), with soil depth, site, and their interaction as
factors. Ground and stream waters sampled on a given date
were generally within the analytical precision of oxygen isotopic measurements (about 0.2‰), so δ18O values of ground
water and stream water were pooled by date and tested for
significant variation over the growing season by a one-factor
ANOVA. Broad-scale relationships between net photosynthetic rate and environmental and physiological variables were
examined with scatter plots and Pearson-product correlations
from data pooled over sites and tree sizes. Statistical analyses
were performed with the SAS software package (SAS Institute, Cary, NC) and Statgraphics Statistical Software (STSC,
Rockville, MD).
Results
Site environment
At both sites, soil water content decreased from late April to
mid June, remained relatively constant until mid August and
increased for several weeks in September (Figure 1). Soil
water content was significantly higher at the ephemeral stream
site than at the perennial-stream site during the wet conditions
prevailing in April and early May, but there were no significant
differences between sites under drier conditions. High soil
water content was associated with periods of frequent, heavy
precipitation (Figure 1). Daily maximum air temperature generally increased between mid April and mid June, and de-
Figure 1. (A) Daily precipitation at study
sites. (B) Daily maximum and minimum
air temperatures at the perennial-stream
site. Temperatures at the ephemeralstream study site were typically within
1 °C of perennial-stream site temperatures. (C) Mean volumetric soil water content 0--30 cm soil depth for perennialand ephemeral-stream sites. Bars are
equal to ± 1 standard error of the mean.
Sites differed significantly (P ≤ 0.05) on
the first three measurement dates.
WATER SOURCE VARIATION AND PHYSIOLOGY OF BOXELDER
creased between mid August and late September (Figure 1).
During the study, freezing temperatures occurred occasionally
until mid May, but not thereafter (Figure 1).
Tree water sources
The δ18O of ground and stream water averaged --12.0‰ between April and October, with no significant monthly variation
(Figure 2). The δ18O of precipitation increased from --13.6‰
in late April to about --3.3‰ in mid June. Precipitation δ18O
remained fairly constant between mid June and late August,
and then decreased to --7.6‰ at the end of the growing season
in September (Figure 2).
At both sites, δ18O of soil water at both soil depths exhibited
a temporal trend similar to that of precipitation (Figure 2). In
early May, δ18O of soil water in the 5--30 cm depth was similar
to that of ground and stream water. Subsequently, δ18O of soil
water in this layer gradually increased, reaching a maximum
value of about --3.5‰ at both sites in early September. Soil
155
water δ18O generally decreased at the end of the growing
season following several large precipitation events (Figure 1),
because of relatively low δ18O precipitation values (Figure 2).
The δ18O of soil water from the 0--5 cm soil depth was
significantly greater than from the 5--30 cm depth on all
sampling dates except September 9. Soil water δ18O at both
soil depths differed significantly between the perennial- and
ephemeral-stream sites only on May 15 and August 10. Soil
water δ18O was higher at the perennial-stream site than at the
ephemeral-stream site on May 15, whereas the pattern was the
opposite on August 10. There were no significant soil depth by
site interactions on any sampling date.
On all sampling dates, δ18O of xylem water did not differ
significantly between large and small trees or between sites,
and there were no significant tree size by site interactions.
Xylem water δ18O was similar to ground and stream water
early in the growing season to early August (Figure 2). From
early August until the end of the growing season, xylem water
δ18O increased steadily to reach values between those of
ground/stream water and precipitation.
Based on the mixing model (Table 2), large trees at the
perennial-stream site were exclusively dependent on stream or
ground water, or both, until fall (September and October) when
up to 20% of water uptake was from surface soil. Small trees
at the perennial-stream site used surface soil water in both
early summer and fall. Both large and small trees at the ephemeral-stream site used surface soil water early and late in the
growing season. At both sites, small trees depended on surface
soil water more frequently and in greater proportion than large
trees (Table 2). When surface soil water was used, it generally
represented < 40% of total water uptake.
Tree water potential
On average, predawn water potential varied from about --0.3 to
--0.5 MPa, with the highest values in May (Figure 3). Predawn
Table 2. Estimated percentage of xylem water of large and small
boxelder trees derived from surface soil (0--30 cm) at perennial- and
ephemeral-stream sites. Values were calculated by a two end-member
mixing model, assuming that surface soil water (0--30-cm depth) and
deep soil or ground water were the only water sources. Where xylem
and ground water δ18O values were not statistically different (twotailed Student’s t-test, P > 0.05), the percentage of xylem water derived
from soil was assumed to be zero.
Sampling date Percentage of xylem water derived from surface soil
Perennial-stream site
Large
Small
Figure 2. Mean δ18O values of potential water sources and xylem
water from large and small boxelder trees at perennial- and ephemeralstream sites. Bars are ± 1 standard error of the mean for xylem water
(n = 3). Mean δ18O values of ground (spring) and stream water were
not statistically different (P > 0.05), and did not vary more than 0.2‰
throughout the study period. Stream water was not present at the
ephemeral-stream site during the study period.
Ephemeral-stream site
Large
Small
May 2
0
0
28.2
43.6
May 18
0
0
17.3
55.4
June 15
0
10.4
0
0
July 14
0
8.4
0
0
August 10
0
0
0
0
Sept 9
15.6
16.8
24.0
31.8
Oct 6
20.0
38.6
34.1
40.0
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KOLB, HART AND AMUNDSON
water potentials did not differ significantly between large and
small trees on any date. Likewise, there was no significant
interaction between site and tree size on predawn water potential. Predawn water potential was similar at each site on five
dates (Figure 3). On the other four measurement dates
(May 18, June 15, July 27, August 10), predawn water potentials were significantly lower at the perennial-stream site than
at the ephemeral-stream site; however, these differences were
small (approximately 0.1 MPa) (Figure 3).
On average, mid-morning water potentials varied from
about --0.7 MPa in May to about --1.5 MPa from July through
September (Figure 3). There was no significant interaction
between site and tree size on mid-morning water potential on
any date. Mid-morning water potentials were significantly
lower at the perennial- versus the ephemeral-stream site only
on June 29 and July 13, and the opposite pattern was observed
on July 27. There was no significant effect of tree size on
mid-morning water potential, except on August 10 when large
trees had lower water potentials than small trees. On all dates,
afternoon water potentials were within 0.2 MPa of mid-morning water potentials and there were no significant site or size
effects (data not shown).
Net photosynthetic rate
A significant interaction between site and tree size on net
photosynthetic rate was only observed at mid-morning on
July 13. Mid-morning photosynthetic rates did not differ significantly between sites or tree sizes between May 18 and June
15 (Figure 4); however, between June 29 and September 8,
mid-morning photosynthetic rates were generally greater at the
perennial stream site than the ephemeral-stream site, with
significant differences observed on July 27 and September 8.
Large trees had significantly greater mid-morning photosynthetic rates than small trees only on August 10 and September 8. Generally, afternoon photosynthetic rates (Figure 4) did
not differ significantly between sites or tree sizes, except on
June 15 when photosynthetic rates of trees were greater at the
perennial-stream site than at the ephemeral-stream site.
Associations between leaf gas exchange and environmental
or physiological variables
Both net photosynthetic rate and stomatal conductance were
significantly and negatively correlated with vapor pressure
deficit (Figure 5 and Table 3). Foliar nitrogen concentration
was significantly and positively correlated only with net pho-
Figure 3. Mean predawn and midmorning xylem water potentials of
boxelder for nine dates, two stream
sites (perennial, ephemeral) and
two tree size classes (large, small).
Bars are equal to 1 standard error
of the mean. Predawn water potential differed significantly (P ≤ 0.05)
between sites on May 18, June 15,
July 27 and August 10. Mid-morning water potential differed significantly between sites on June 29,
July 13 and July 27, and between
tree sizes on August 10.
WATER SOURCE VARIATION AND PHYSIOLOGY OF BOXELDER
157
Figure 4. Mean mid-morning and afternoon net photosynthetic rates (Pn)
of boxelder trees for several dates,
two stream sites (perennial, ephemeral) and two tree size classes (large,
small). Bars are equal to 1 standard
error of the mean. Mid-morning net
photosynthetic rate differed significantly (P ≤ 0.05) between sites on
July 27 and September 8, and between tree sizes on August 10 and
September 8. Afternoon net photosynthetic rate differed significantly between sites on June 15.
tosynthetic rate (Table 3). Site was an important source of
variation for foliar nitrogen concentration. On several dates
(June 1 and 29, July 13 and 27, August 10), foliar nitrogen
concentration was significantly higher at the perennial-stream
site than at the ephemeral-stream site (Figure 6). Foliar nitrogen concentration differed significantly between large and
small trees only on August 24 (Figure 6). Net photosynthetic
rate was significantly correlated with water potential measured
at the time of the gas exchange measurement, and minimum
air and soil temperatures for the week preceding the gas exchange measurement (Table 3).
Discussion
We predicted that small boxelder trees growing along the
perennial stream would utilize stream water (cf. Dawson and
Ehleringer 1991, Smith et al. 1991). Based on our δ18O data,
we were unable to determine directly if this prediction was
correct because ground and stream water had similar δ18O
values. However, differences in both predawn and mid-morning water potentials between ephemeral- and perennial-stream
sites were generally small and inconsistent among dates, implying that stream water is not a physiologically important
Figure 5. Net photosynthetic rate (Pn) versus leaf-to-air vapor pressure
deficit of boxelder trees from two stream sites (perennial, ephemeral)
and two tree size classes (large, small). Data for each site × tree size
combination are mid-morning and afternoon means from different
dates. The correlation coefficient (r) is for data pooled over sites and
tree sizes (n = 56).
158
KOLB, HART AND AMUNDSON
Table 3. Pearson product correlation coefficients (r) between leaf gas
exchange and selected environmental and physiological variables for
means pooled over measurement times, dates, sites, and tree sizes
(n = 56 unless otherwise noted). (*; P = 0.05 and **; P = 0.01)
Variable
Net photosynthetic
rate (µmol m −2 s −1)
Stomatal conductance
(mol m −2 s −1)
Net photosynthetic
rate (µmol m −2 s −1)
--
+0.470**
Stomatal conductance +0.470**
(mol m −2 s −1)
--
Predawn xylem water
potential (MPa)
+0.1253
+0.2024
+0.1213
+0.1604
Daytime xylem water
potential1 (MPa)
+0.361**
+0.146
Leaf-to-air vapor
pressure deficit (kPa)
--0.691**
--0.270*
Foliar nitrogen
+0.388*3
−2
concentration (g m )
+0.0753
Minimum air
temperature (°C)2
--0.470**3
+0.1713
Minimum soil
temperature (°C)2
--0.442**3
+0.2593
1
2
3
4
Measured at the time of gas exchange measurement.
Measured the week preceding the gas exchange measurement.
Mid-morning (n = 36).
Afternoon (n = 20).
water source for either large or small boxelder trees. Further,
the poor correlation of variations in water potential with lightsaturated net photosynthetic rate and stomatal conductance,
imply that the small differences in water potential measured
between sites on some dates were of little physiological sig-
nificance. In contrast to our results, stream water has been
shown to be an important water source that can influence water
relations and carbon gain of trees in other riparian ecosystems
in western North America (Dawson and Ehleringer 1991,
Smith et al. 1991).
We also expected water uptake by both small and large trees
at each site to be primarily from ground- or stream water, or
both, during the early summer drought, with recent precipitation becoming an increasingly important water source following the onset of monsoonal rains in late summer. Overall, the
mixing model results support this hypothesis, although trees at
the ephemeral-stream site, especially small trees, also utilized
soil water in May. The source of this soil water was either
winter precipitation (soil recharge), rains that occurred in late
April, or both. It is unlikely that the high δ18O values of xylem
water observed in early May at the ephemeral-stream site were
a result of residual stem water subjected to evaporative enrichment throughout the previous winter (Phillips and Ehleringer
1995), because xylem water was measured only after full leaf
expansion when residual water should be depleted by transpiration. Even on days following recent precipitation when surface soil water was utilized by large and small trees at both
sites, this water source typically constituted no more than 40%
of total water uptake.
Net photosynthetic rates differed between sites on several
dates, with generally higher rates in trees at the perennialcompared with the ephemeral-stream site. Site differences in
foliar nitrogen concentration seem to provide the best explanation of site differences in net photosynthetic rate, as foliar
nitrogen concentration at the perennial-stream site was typically greater than at the ephemeral-stream site. Our results
showing a positive relation between foliar nitrogen concentration and net photosynthetic rate reflect those reported for this
species in a comparative study of male and female trees in a
riparian forest in the Wasatch Mountains, Utah (Dawson and
Ehleringer 1993a).
Figure 6. Mean foliar nitrogen concentration of boxelder trees based on
leaf area for nine dates, two stream
sites (perennial, ephemeral) and two
tree size classes (large, small). Bars
are equal to standard error of the
mean. Foliar nitrogen concentration
differed significantly (P ≤ 0.05) between sites on June 1, June 29,
July 13, July 27 and August 10; and
between tree sizes on June 15 and
August 24.
WATER SOURCE VARIATION AND PHYSIOLOGY OF BOXELDER
At our field site, the most important limitation to light-saturated net photosynthesis and stomatal conductance of boxelder
was high leaf-to-air vapor pressure deficit that developed as a
result of high leaf temperatures combined with low atmospheric humidity. The maximum temperature of sunlit leaves in
our study (approximately 40 °C) was greater than that reported
for this species in a riparian forest in the midwestern United
States (30--35 °C, Foster 1992), and for Salix and Populus
species growing in riparian forests in intermountain basins
of the Rocky Mountains (25--30 °C, Foster and Smith 1991).
Based on these regional differences in leaf temperature and
associated vapor pressure deficit for riparian trees, we speculate that limitations on net photosynthesis by high vapor pressure deficit are stronger for riparian trees in the American
Southwest than for riparian trees in cooler regions. For example, Foster (1992) reported no limitation of leaf gas exchange
by vapor pressure deficit for boxelder in a riparian forest in the
midwestern United States.
The significant relationship between daytime water potential and net photosynthetic rate was likely the result of a strong
negative correlation between water potential and vapor pressure deficit (r = --0.706, n = 56, P < 0.01). Lack of a strong
correlation between stomatal conductance and daytime water
potential further implies that net photosynthesis was not seriously limited by stomatal closure induced by low water potential (cf. Foster 1992). Significant negative relationships
between net photosynthetic rate and minimim soil and air
temperatures observed the week preceding gas exchange
measurements likely resulted from limitation of photosynthesis by high vapor pressure deficits during warm periods, and
not from an effect of these variables on gas exchange. In our
study, low night temperatures did not strongly limit leaf gas
exchange, as has been reported for high-elevation riparian trees
and shrubs in more northern locations (Young et al. 1985,
Foster and Smith 1991).
Based on previous studies of size-related differences in
physiology for riparian trees in western North America (Donovan and Ehleringer 1991, Smith et al. 1991, Dawson and
Ehleringer 1993a), we also expected greater water stress and
lower net photosynthetic rates for small trees than for large
trees. Contrary to this hypothesis, we found that, on most dates,
both large and small trees had similar water potentials, net
photosynthetic rates and other physiological characteristics. At
the study sites, all small trees appeared to be sprouts that
originated from larger trees following flood damage. These
sprouts apparently have relatively large, deep root systems
capable of tapping the same deep water sources as larger trees.
In summary, we hypothesized a tight coupling of net carbon
gain and water stress severity to stream water availability for
boxelder in a montane-riparian forest in northern Arizona. The
data did not support this hypothesis because the trees appeared
to rely on deep water sources regardless of stream water
availability. Similar leaf water potentials and gas exchange
rates were recorded for both large and small trees, because
small trees were vegetative sprouts from older stems that
tapped the same deep water sources as large trees. Following
recent precipitation, surface soil water was also utilized by
159
trees, especially small trees, indicating flexibility in water
source use.
Acknowledgments
This research was supported by USDA Competitive Grant No. 9337106-9485 and by a grant from the Northern Arizona University
School of Forestry Mission Research Program. The authors acknowledge the efforts of Karin L. Doerr in field and laboratory measurements, A. Hope Jahren for assistance in water extraction, and Gale
Eaton for conducting the mass spectrometer analyses.
References
Brown, D.E. 1982. Biotic communities of the American Southwest---United States and Mexico. Desert Plants 4, Univ. Arizona Press,
Tucson, AZ, 342 p.
Busch, D.E., N.L. Ingraham and S.D. Smith. 1992. Water uptake in
woody riparian phreatophytes of the southwestern United States:
A stable isotope study. Ecol. Appl. 2:450--459.
Busch, D.E. and S.D. Smith. 1993. Effects of fire on water and salinity
relations of riparian woody taxa. Oecologia 94:186--194.
Busch, D.E. and S.D. Smith. 1995. Mechanisms associated with decline of woody species in riparian ecosystems of the southwestern
United States. Ecol. Monogr. 65:347--370.
Dawson, T.E. 1993. Water sources of plants as determined from
xylem-water isotopic composition: perspectives on plant competition, distribution, and water relations. In Stable Isotopes and Plant
Carbon--Water Relations. Eds. J.R. Ehleringer, A.E. Hall and
G.D. Farquhar. Academic Press, San Diego, CA, pp 465--496.
Dawson, T.E. and J.R. Ehleringer. 1991. Streamside trees that do not
use streamside water. Nature 350:335--337.
Dawson, T.E. and J.R. Ehleringer. 1993a. Gender-specific physiology,
carbon isotope discrimination, and habitat distribution in boxelder,
Acer negundo. Ecology 74:798--815.
Dawson, T.E. and J.R. Ehleringer. 1993b. Isotopic enrichment of water
in the ‘‘woody’’ tissues of plants: Implications for plant water
source, water uptake, and other studies which use the stable isotopic
composition of cellulose. Geochim. Cosmochim. Acta 57:3487-3492.
Dina, S.J. and L.G. Klikoff. 1973. Carbon dioxide exchange by several
streamside and scrub community species of Red Butte Canyon,
Utah. Am. Midl. Nat. 89:70--80.
Donovan, L.A. and J.R. Ehleringer. 1991. Ecophysiological differences among juvenile and reproductive plants of several woody
species. Oecologia 86:594--597.
Ehleringer, J.R. and C.B. Osmond. 1989. Stable isotopes. In Plant
Physiological Ecology. Field Methods and Instrumentation. Eds.
R.W. Pearcy, J.R. Ehleringer, H.A. Mooney and P.W. Rundel. Chapman Hall, London, pp 281--300.
Epstein, S. and T. Mayeda. 1953. Variations in the 18O content of waters
from natural sources. Geochim. Cosmochim. Acta 4:213--224.
Foster, J.R. 1992. Photosynthesis and water relations of the floodplain
tree, boxelder (Acer negundo L.). Tree Physiol. 11:133--149.
Foster, J.R. and W.K. Smith. 1991. Stomatal conductance patterns and
environment in high elevation phreatophytes of Wyoming. Can.
J. Bot. 69:647--655.
Gower, S.T., P.B. Reich and Y. Son. 1993. Canopy dynamics and
aboveground production of five tree species with different leaf
longevities. Tree Physiol. 12:327--345.
160
KOLB, HART AND AMUNDSON
Gray, A.N. and T.A. Spies. 1995. Water content measurement in forest
soils and decayed wood using time domain reflectometry. Can. J.
For. Res. 25:376--385.
Issac, R.A. and W.C. Johnson. 1976. Determination of total nitrogen
in plant tissue, using a block digestor. J. Assoc. Off. Anal. Chem.
59:98--100.
Johnson, R.R. and L.T. Haigt. 1984. Riparian problems and initiatives
in the American Southwest: a regional prespective. In California
Riparian Systems: Ecology, Conservation and Productive Management. Eds. R. Warner and C. Henricks. Univ. California Press,
Berkeley, CA, pp 404--412.
Lowe, C.H. and D.E. Brown. 1982. Introduction. In Biotic Communities of the American Southwest----United States and Mexico. Ed.
D.E. Brown. Desert Plants 4, Univ. Arizona Press, Tucson, AZ,
pp 8--16.
Meng, F.R. and P.A. Arp. 1993. Net photosynthesis and stomatal
conductance of red spruce twigs before and after twig detachment.
Can. J. For. Res. 23:716--721.
Phillips, S.L. and J.R. Ehleringer. 1995. Limited uptake of summer
precipitation by bigtooth maple (Acer grandidentatum Nutt) and
Gambel’s oak (Quercus gambelii Nutt). Trees 9:214--219.
QuikChem Systems. 1992. QuikChem Method No. 13-107-06-2-D.
Lachat Instruments, Milwaukee, WI.
Ritchie, G.A. and T.M. Hinckley. 1975. The pressure chamber as an
instrument for ecological research. Adv. Ecol. Res. 9:165--254.
Schubert, G.H. 1974. Silviculture of southwestern ponderosa pine.
USDA For. Ser. Res. Pap. RM-123, 71 p.
Smith, S.D., A.B. Wellington, J.L. Nachlinger and C.A. Fox. 1991.
Functional responses of riparian vegetation to streamflow diversion
in the eastern Sierra Nevada. Ecol. Appl. 1:89--97.
Soil Survey Staff. 1990. Keys to soil taxonomy, 4th Edn. SMSS
Technical Monograph No. 6, Blacksburg, Virginia, 422 p.
Sternberg L.S.L., M.J. DeNiro and H.B. Johnson. 1986. Oxygen and
hydrogen isotope ratios of water from photosynthetic tissues of
CAM and C3 plants. Plant Physiol. 82:428--431.
Stromberg, J.C. and D.T. Patten. 1990. Riparian vegetation instream
flow requirements: A case study from a diverted stream in the
eastern Sierra Nevada, California, USA. Environ. Manage. 14:185-194.
Tyree, M.T., K.J. Kolb, S.B. Rood and S. Patino. 1994. Vulnerability
to drought-induced cavitation of riparian cottonwoods in Alberta--a
possible factor in the decline of the ecosystem. Tree Physiol.
14:455--466.
Tyree, M.T. and T.R. Wilmot. 1990. Errors in the calculation of
evaporation and leaf conductance in steady-state porometry: The
importance of accurate measurement of leaf temperature. Can.
J. For. Res. 20:1031--1035.
White, J.W.C. 1989. Stable isotope ratios in plants: a review of the
current theory and some potential applications. In Stable Isotopes
in Ecological Research. Eds. P.W. Rundel, J.R. Ehleringer and
K.A. Nagy. Springer-Verlag, Berlin, pp 142--162.
Young, D.R., I.C. Burke and D.H. Knight. 1985. Water relations of
high-elevation phreatophytes in Wyoming. Am. Midl. Nat.
114:384--392.
© 1997 Heron Publishing----Victoria, Canada
Boxelder water sources and physiology at perennial and ephemeral
stream sites in Arizona
THOMAS E. KOLB,1 STEPHEN C. HART1 and RONALD AMUNDSON2
1
School of Forestry, College of Ecosystem Science and Management, Northern Arizona University, Flagstaff, AZ 86011-5018, USA
2
Division of Ecosystem Sciences, University of California, Berkeley, CA 94720, USA
Received March 15, 1996
Summary To assess the influence of stream water on leaf gas
exchange and water potential in different sized boxelder trees
(Acer negundo L.), we compared these characteristics in trees
growing beside a perennial stream and a nearby ephemeral
stream in a montane--riparian forest in northern Arizona. Patterns of tree water use were quantified by stable isotope analysis (δ18O). Physiological characteristics were similar for large
and small trees. Similarity between sites in predawn and daytime water potentials and xylem δ18O indicated that stream
water was not a physiologically important water source. Seasonal and site variations in light-saturated net photosynthetic
rate were significantly related to leaf-to-air vapor pressure
deficit (r = --0.691) and foliar nitrogen concentration (r =
0.388). Although deep water was the dominant water source,
surface soil water was utilized following precipitation, especially by small trees. We conclude that net carbon gain and
severity of water stress are only weakly coupled to stream water
availability.
Keywords: Acer negundo, nitrogen, photosynthesis, riparian
forests, stable isotopes, stomatal conductance, water relations.
with detrimental physiological changes and reduced growth of
riparian trees in western North America (Stromberg and Patten
1990, Smith et al. 1991, Tyree et al. 1994). The effects of
stream diversion on leaf physiology can differ among trees of
different sizes, with greater detrimental effects reported for
juvenile trees than for mature trees (Smith et al. 1991). Juvenile trees may be especially sensitive to water stress because
shallow root systems limit their water source to the surface
soil, whereas more reliable water sources in deep soil and
ground water are only available to deeply rooted, larger trees
(Dawson and Ehleringer 1991, Dawson 1993).
In this study, we compared water sources, leaf gas exchange
and water relations of different-sized boxelder (Acer negundo L.) trees growing beside a perennial stream and a nearby
ephemeral stream in a montane--riparian forest in northern
Arizona. Specifically, we assessed: (1) the effect of stream
water accessibility on water-source utilization, water relations
and photosynthesis of different-sized trees; (2) if tree size and
dependability of the water source influence shifts in watersource use during the growing season; and (3) the roles of
environmental and physiological factors in limiting photosynthesis of boxelder under field conditions.
Introduction
Riparian forests are a threatened ecosystem in the American
Southwest (Lowe and Brown 1982, Johnson and Haigt 1984).
Decline of native riparian trees may result from modification
of surface and ground water flows by impoundments and
diversions, as well as by changes in associated abiotic and
biotic factors, such as increased alluvium salinity and invasion
by nonnative species (e.g., Stromberg and Patten 1990, Smith
et al. 1991, Busch et al. 1992, Busch and Smith 1993, Busch
and Smith 1995).
Linkages between water sources, water use, and productivity of riparian trees in the American Southwest are poorly
understood. Many riparian trees of western North America are
assumed to be phreatophytes that avoid water stress by tapping
reliable water sources with deep roots. However, riparian trees
can be subject to water stress, especially in mid to late summer
when water availability is reduced ( Dina and Klikoff 1973,
Foster and Smith 1991, Smith et al. 1991, Dawson and Ehleringer 1993a). Stream flow diversions have been associated
Materials and methods
Study sites
The study area was in upper Oak Creek Canyon in north-central Arizona (35°00′ N, 111°44′ W). Oak Creek is a perennial
stream that drains the southern edge of the Colorado Plateau
south of Flagstaff, Arizona. Annual precipitation in the region
averages 57 cm (Schubert 1974), occurring as snow and rain
during winter (November--March) and as monsoonal thundershowers during late summer (July--September). Upper Oak
Creek Canyon contains a cold-temperate, montane--riparian
forest type (Brown 1982). Dominant tree species include:
white fir (Abies concolor (Gord.) Lindl. ex Hildebr.), boxelder
(Acer negundo), bigtooth maple (Acer grandidentatum
Marsh.), Arizona alder (Alnus oblongifolia Torr.), velvet ash
(Fraxinus velutina Torr.), Arizona walnut (Juglans major
(Torr.) A. Heller), Gambel oak (Quercus gambelii Nutt.), ponderosa pine (Pinus ponderosa Dougl. ex P. Laws & C. Laws),
152
KOLB, HART AND AMUNDSON
Arizona sycamore (Platanus wrightii P. Wats.), black cherry
(Prunus serotina J.F. Ehrh.) and narrowleaf cottonwood
(Populus angustifolia James).
We measured water-source use and physiological characteristics of boxelder trees at two upper Oak Creek Canyon sites
differing in proximity to perennial stream water. The perennial
stream site is located adjacent to Oak Creek, and the ephemeral
stream site is located adjacent to Ritter Wash, an intermittent
stream that drains into Oak Creek. The ephemeral stream site
is 0.4 km from the perennial stream site, and 0.2 km upstream
from the intersection of Ritter Wash and Oak Creek. Both sites
have similar elevation (1,650 m), slope and aspect. Although
stream water can occur at the ephemeral stream site in early
spring following snowmelt, surface water at the ephemeral
stream site was not observed during the study (April--November 1994). This observation, coupled with the distance between the ephemeral stream site and perennial water in Oak
Creek, strongly suggest that there was no direct uptake of
stream water by vegetation at the ephemeral stream site.
Soils from both sites are previously unclassified types in the
Soil Taxonomy Family of sandy-skeletal, mixed, mesic Typic
Udifluvents (Soil Survey Staff 1990, S.C. Hart, personal observation 1994). Despite depths of several meters, these soils have
high rates of water infiltration and low capacities for water
storage because of their coarse texture, and because ≥ 35% of
their volume is occupied by rock fragments (particles greater
than 2 mm in diameter).
Site environment
At both sites, we measured volumetric soil water content
weekly between late April and early October 1994, with a
time-domain reflectometry system (TRASE System 1, Soilmoisture Corp., Santa Barbara, CA). We established sample
areas at each site every 10 m along a linear transect in the
riparian forest at a distance of 2.3 m from the stream bank edge.
A pair of stainless steel rods was inserted into the mineral soil
to a depth of 30 cm at each of three sample areas at both sites.
A regression relationship between volumetric soil water content and the dielectric constant of soil between the pair of rods
was applied to predict soil water content at each site. The
regression relationship was determined for soils from each site
in a laboratory experiment in which soils wetted to field capacity were air dried (sensu, Gray and Spies 1995). The same
linear regression equation provided a good fit to data for soils
from both sites (r = 0.982).
At one sample location for each site, we measured soil (15
and 30 cm depths) and air temperatures (1 m above ground)
hourly over the same timespan described for soil water content
measurements. Temperatures were measured with thermocouples and recorded with dataloggers (Campbell Scientific Inc.,
Logan, UT). A tipping-bucket gauge was used to measure daily
bulk precipitation from a clearing near the perennial stream
site.
Isotopic analysis of tree water sources
At both sites, we sampled ground water, stream water, soil
water, and xylem water monthly during the 1994 growing
season. Ground water was sampled from springs adjacent to
the perennial stream site (Cave Spring) and in the upper drainage of the ephemeral stream site (Ritter Spring). Stream water
was sampled from well-mixed sections of the stream. Ground
and stream water samples were collected in glass vials fitted
with air-tight polyethylene caps and stored at 4 °C. Because the
ephemeral stream did not flow during the study period, no
stream water samples were obtained from this site.
Soil was sampled along the same 30-m transects described
above. Samples were pooled over the 0--30 cm soil layer at the
first sampling date (May 2). Thereafter, the 0--5 cm and 5--30 cm
soil layers were sampled separately. Soil samples were placed
in air-tight jars and stored at --18 °C until water was extracted.
Several 5- to 7-cm long suberized stem sections (Dawson
and Ehleringer 1993b) were sampled from the same three large
and three small boxelder trees at each site on each sampling
date. All of these trees were growing within 3 m of the stream
bank, and were chosen from the population of trees sampled
for physiological measurements (Table 1). Suberized stems
were placed in air-tight jars and stored at --18 °C. Water from
soil and xylem samples was extracted by a cryogenic vacuum
distillation technique (Sternberg et al. 1986, Ehleringer and
Osmond 1989, Smith et al. 1991).
Precipitation samples from significant individual storm
events (> 5 mm) were collected in 10 polyethylene bottles in
an open area adjacent to the perennial stream site. Precipitation
collections represent over 95% of the precipitation volume that
occurred during the study period. A small amount of mineral
oil was contained in each bottle to prevent evaporation of
collected water between sample bottle placement and retrieval
(0.5 to 3 days, depending on storm duration). Precipitation
water was stored at --18 °C.
All water samples were analyzed for 18O/16O isotope ratio at
Lawrence Berkeley National Laboratory, Berkeley, CA, on a
V6 Prism Isotope Ratio Mass Spectrometer by an automated
CO2--H2O equilibration technique (Epstein and Mayeda 1953).
Oxygen isotope ratios (δ18O values) were expressed as:
δ18 O = (Rsample ⁄ Rstandard −1)1000 ‰,
where R refers to the 18O/16O ratios of sample and standard
(standard mean ocean water, SMOW), respectively. Duplicate
isotopic analyses of randomly selected water samples (run
about every 25 samples) always differed by ≤ 0.1‰.
We applied a two-member mixing model to calculate the
relative proportion of surface soil (0--30 cm) to deep soil or
ground water utilized by each tree size at each site (White
1989). Because ground and stream water were isotopically
similar (see Results), we could not distinguish between these
two water sources at the perennial-stream site. We assumed
that when tree xylem and ground and stream water δ18O were
statistically similar (two-tailed Student’s t-Test, P > 0.05), the
percentage of xylem water derived from the soil was zero. The
soil water δ18O value applied in the mixing model was a
depth-weighted average of the δ18O values from the 0--5 and
5--30 cm soil layers on a given sampling date, and was calculated assuming that soil water within the upper 30 cm of soil
WATER SOURCE VARIATION AND PHYSIOLOGY OF BOXELDER
was uniformly utilized by tree roots. This assumption was not
rigorously tested; however, we did observe living fine roots at
all depths between 1 and 30 cm.
Tree water potential and gas exchange
We measured xylem water potential and leaf gas exchange on
both large and small boxelder trees (Table 1) at each site. All
sample trees grew in a closed-canopy forest within 12 m of the
stream bank. The light environment for large and small trees at
both sites was heterogeneous as a result of shading by other
trees. Most small trees at both sites likely originated as sprouts
from older stems that had been damaged by flooding. Although
we are aware of potential differences in leaf physiology between male and female boxelder trees (Dawson and Ehleringer
1993a), sex determination of sample trees was not possible
because neither flowers nor seeds were present.
Xylem water potential and light-saturated leaf gas exchange
characteristics were measured approximately every two weeks
from time of full-leaf expansion in spring (mid May) to onset
of leaf senescence in fall (early September). Net photosynthetic rate and stomatal conductance to water vapor were
measured midmorning (0900--1100 h) on all dates, and early
afternoon (1300--1500 h) on the first five dates. After mid July,
afternoon measurements of gas exchange were not made because of frequent precipitation and cloudy conditions. At each
date and sampling time within a date, a different sample of
three to five trees from each site and tree size class was
measured to minimize defoliation. At each sampling time
within a date, the order of measurement between sites and tree
sizes within sites was determined randomly.
We measured gas exchange on one representative leaf per
tree in the lower or middle canopy exposed to photosynthetically active radiant flux that was more than saturating for
boxelder (> 600 µmol m −2 s −1, Foster 1992). Gas exchange
was measured over 30 s on a fixed leaf area (8.2 cm2) with an
LI-6200 portable photosynthesis system (Li-Cor, Inc., Lincoln, NE) equipped with a 250-ml cuvette. During measurements, water vapor pressure and temperature in the cuvette
were within 10% and 3 °C of ambient values, respectively. For
large trees, we made approximately 90% of measurements in
situ on attached leaves, and about 10% of measurements were
made on leaves from twigs detached with pole pruners because
of difficulty in accessing the leaves for in situ measurements.
Leaves on detached twigs were measured within 30 s follow-
153
ing detachment in a light environment similar to that in the
canopy. Preliminary studies showed that net photosynthetic
rates of leaves on detached twigs do not differ from those of
leaves attached to the tree under our experimental conditions.
Similar methods have been used successfully in studies with
other species (e.g., Gower et al. 1993, Meng and Arp 1993).
Leaf-to-air vapor pressure deficit was calculated as the difference between atmospheric vapor pressure measured in the
cuvette and saturated vapor pressure calculated for leaf mesophyll. Leaf temperature used in calculating saturated vapor
pressure of leaf mesophyll was measured with a fine-wire
thermocouple placed on the leaf underside during the gas
exchange measurement. Short-term measurements of leaf temperature with an external thermocouple may have underestimated leaf temperature, and consequently vapor pressure
deficit, compared with estimates made by permanently attached in situ thermocouples (Tyree and Wilmot 1990).
Mid-morning and early afternoon xylem water potentials
were measured with a pressure chamber (Model 1000 PMS
Instruments, Corvallis, OR), as described by Ritchie and
Hinckley (1975), on the trees sampled for gas exchange assessments. Predawn xylem water potential (0500--0600 h) was
measured on all dates in five trees from each site and size class.
Preliminary studies showed that a more precise endpoint assessment was obtained with terminal twig segments than with
leaf rachises because the large twig cross-sectional area facilitated visual observation, and because there was less phloem
flow from twigs compared with leaf rachises. Therefore, midmorning and afternoon assessments of xylem water potential
were made on the terminal portion of the twig supporting the
leaf sampled for the gas exchange measurement. Predawn
assessments were made on a similar twig portion.
Foliar nitrogen
We measured total nitrogen concentration of leaves from five
trees sampled for gas exchange measurements, for each combination of date, site and tree size class. Leaf tissue was ground
(< 0.85 mm) and digested by a micro-Kjeldahl method (Issac
and Johnson 1976), which oxidizes organic nitrogen to ammonium. Ammonium concentration was measured with a Lachat
flow-injection analyzer (QuikChem Systems 1992). Nitrogen
concentration was calculated on a leaf area basis (g cm −2) to
facilitate comparison with leaf area-based gas exchange data.
Table 1. Observed mean and range of stem diameter (1 cm above soil level), stem height, and distance from stream bank of large and small boxelder
trees.
Diameter (cm)
Mean
Range
Height (m)
Mean
Range
Distance to the stream (m)
Mean
Range
Perennial-stream site
Large trees (n = 10)
Small trees (n = 8)
35.7
6.1
26.9--49.8
4.6--9.7
6.8
2.4
3.7--8.1
1.5--3.6
6.8
5.2
1.7--11.9
1.6--10.6
Ephemeral-stream site
Large trees (n = 7)
Small trees (n = 7)
30.0
6.1
22.4--48.8
4.8--7.9
7.9
2.5
6.3--9.0
2.2--3.1
3.7
4.5
0.1--9.7
1.0--8.7
154
KOLB, HART AND AMUNDSON
Statistical analysis
We compared differences in soil water content between sites at
each date by a one-way ANOVA. Mid-morning and afternoon
net photosynthetic rates, predawn, mid-morning, and afternoon water potentials, leaf nitrogen concentration, and δ18O of
stem water were compared between sites and tree sizes for
each date with a fixed effects, two-factor ANOVA. Factors in
the ANOVA were site, tree size class, and their interaction. A
fixed effects, two-factor ANOVA was also conducted for each
sampling date to test for significant differences in δ18O values
of soil water extracted from each site at 0--5 cm and 5--30 cm
soil depths (except on May 2 when soil was collected from the
0--30-cm depth), with soil depth, site, and their interaction as
factors. Ground and stream waters sampled on a given date
were generally within the analytical precision of oxygen isotopic measurements (about 0.2‰), so δ18O values of ground
water and stream water were pooled by date and tested for
significant variation over the growing season by a one-factor
ANOVA. Broad-scale relationships between net photosynthetic rate and environmental and physiological variables were
examined with scatter plots and Pearson-product correlations
from data pooled over sites and tree sizes. Statistical analyses
were performed with the SAS software package (SAS Institute, Cary, NC) and Statgraphics Statistical Software (STSC,
Rockville, MD).
Results
Site environment
At both sites, soil water content decreased from late April to
mid June, remained relatively constant until mid August and
increased for several weeks in September (Figure 1). Soil
water content was significantly higher at the ephemeral stream
site than at the perennial-stream site during the wet conditions
prevailing in April and early May, but there were no significant
differences between sites under drier conditions. High soil
water content was associated with periods of frequent, heavy
precipitation (Figure 1). Daily maximum air temperature generally increased between mid April and mid June, and de-
Figure 1. (A) Daily precipitation at study
sites. (B) Daily maximum and minimum
air temperatures at the perennial-stream
site. Temperatures at the ephemeralstream study site were typically within
1 °C of perennial-stream site temperatures. (C) Mean volumetric soil water content 0--30 cm soil depth for perennialand ephemeral-stream sites. Bars are
equal to ± 1 standard error of the mean.
Sites differed significantly (P ≤ 0.05) on
the first three measurement dates.
WATER SOURCE VARIATION AND PHYSIOLOGY OF BOXELDER
creased between mid August and late September (Figure 1).
During the study, freezing temperatures occurred occasionally
until mid May, but not thereafter (Figure 1).
Tree water sources
The δ18O of ground and stream water averaged --12.0‰ between April and October, with no significant monthly variation
(Figure 2). The δ18O of precipitation increased from --13.6‰
in late April to about --3.3‰ in mid June. Precipitation δ18O
remained fairly constant between mid June and late August,
and then decreased to --7.6‰ at the end of the growing season
in September (Figure 2).
At both sites, δ18O of soil water at both soil depths exhibited
a temporal trend similar to that of precipitation (Figure 2). In
early May, δ18O of soil water in the 5--30 cm depth was similar
to that of ground and stream water. Subsequently, δ18O of soil
water in this layer gradually increased, reaching a maximum
value of about --3.5‰ at both sites in early September. Soil
155
water δ18O generally decreased at the end of the growing
season following several large precipitation events (Figure 1),
because of relatively low δ18O precipitation values (Figure 2).
The δ18O of soil water from the 0--5 cm soil depth was
significantly greater than from the 5--30 cm depth on all
sampling dates except September 9. Soil water δ18O at both
soil depths differed significantly between the perennial- and
ephemeral-stream sites only on May 15 and August 10. Soil
water δ18O was higher at the perennial-stream site than at the
ephemeral-stream site on May 15, whereas the pattern was the
opposite on August 10. There were no significant soil depth by
site interactions on any sampling date.
On all sampling dates, δ18O of xylem water did not differ
significantly between large and small trees or between sites,
and there were no significant tree size by site interactions.
Xylem water δ18O was similar to ground and stream water
early in the growing season to early August (Figure 2). From
early August until the end of the growing season, xylem water
δ18O increased steadily to reach values between those of
ground/stream water and precipitation.
Based on the mixing model (Table 2), large trees at the
perennial-stream site were exclusively dependent on stream or
ground water, or both, until fall (September and October) when
up to 20% of water uptake was from surface soil. Small trees
at the perennial-stream site used surface soil water in both
early summer and fall. Both large and small trees at the ephemeral-stream site used surface soil water early and late in the
growing season. At both sites, small trees depended on surface
soil water more frequently and in greater proportion than large
trees (Table 2). When surface soil water was used, it generally
represented < 40% of total water uptake.
Tree water potential
On average, predawn water potential varied from about --0.3 to
--0.5 MPa, with the highest values in May (Figure 3). Predawn
Table 2. Estimated percentage of xylem water of large and small
boxelder trees derived from surface soil (0--30 cm) at perennial- and
ephemeral-stream sites. Values were calculated by a two end-member
mixing model, assuming that surface soil water (0--30-cm depth) and
deep soil or ground water were the only water sources. Where xylem
and ground water δ18O values were not statistically different (twotailed Student’s t-test, P > 0.05), the percentage of xylem water derived
from soil was assumed to be zero.
Sampling date Percentage of xylem water derived from surface soil
Perennial-stream site
Large
Small
Figure 2. Mean δ18O values of potential water sources and xylem
water from large and small boxelder trees at perennial- and ephemeralstream sites. Bars are ± 1 standard error of the mean for xylem water
(n = 3). Mean δ18O values of ground (spring) and stream water were
not statistically different (P > 0.05), and did not vary more than 0.2‰
throughout the study period. Stream water was not present at the
ephemeral-stream site during the study period.
Ephemeral-stream site
Large
Small
May 2
0
0
28.2
43.6
May 18
0
0
17.3
55.4
June 15
0
10.4
0
0
July 14
0
8.4
0
0
August 10
0
0
0
0
Sept 9
15.6
16.8
24.0
31.8
Oct 6
20.0
38.6
34.1
40.0
156
KOLB, HART AND AMUNDSON
water potentials did not differ significantly between large and
small trees on any date. Likewise, there was no significant
interaction between site and tree size on predawn water potential. Predawn water potential was similar at each site on five
dates (Figure 3). On the other four measurement dates
(May 18, June 15, July 27, August 10), predawn water potentials were significantly lower at the perennial-stream site than
at the ephemeral-stream site; however, these differences were
small (approximately 0.1 MPa) (Figure 3).
On average, mid-morning water potentials varied from
about --0.7 MPa in May to about --1.5 MPa from July through
September (Figure 3). There was no significant interaction
between site and tree size on mid-morning water potential on
any date. Mid-morning water potentials were significantly
lower at the perennial- versus the ephemeral-stream site only
on June 29 and July 13, and the opposite pattern was observed
on July 27. There was no significant effect of tree size on
mid-morning water potential, except on August 10 when large
trees had lower water potentials than small trees. On all dates,
afternoon water potentials were within 0.2 MPa of mid-morning water potentials and there were no significant site or size
effects (data not shown).
Net photosynthetic rate
A significant interaction between site and tree size on net
photosynthetic rate was only observed at mid-morning on
July 13. Mid-morning photosynthetic rates did not differ significantly between sites or tree sizes between May 18 and June
15 (Figure 4); however, between June 29 and September 8,
mid-morning photosynthetic rates were generally greater at the
perennial stream site than the ephemeral-stream site, with
significant differences observed on July 27 and September 8.
Large trees had significantly greater mid-morning photosynthetic rates than small trees only on August 10 and September 8. Generally, afternoon photosynthetic rates (Figure 4) did
not differ significantly between sites or tree sizes, except on
June 15 when photosynthetic rates of trees were greater at the
perennial-stream site than at the ephemeral-stream site.
Associations between leaf gas exchange and environmental
or physiological variables
Both net photosynthetic rate and stomatal conductance were
significantly and negatively correlated with vapor pressure
deficit (Figure 5 and Table 3). Foliar nitrogen concentration
was significantly and positively correlated only with net pho-
Figure 3. Mean predawn and midmorning xylem water potentials of
boxelder for nine dates, two stream
sites (perennial, ephemeral) and
two tree size classes (large, small).
Bars are equal to 1 standard error
of the mean. Predawn water potential differed significantly (P ≤ 0.05)
between sites on May 18, June 15,
July 27 and August 10. Mid-morning water potential differed significantly between sites on June 29,
July 13 and July 27, and between
tree sizes on August 10.
WATER SOURCE VARIATION AND PHYSIOLOGY OF BOXELDER
157
Figure 4. Mean mid-morning and afternoon net photosynthetic rates (Pn)
of boxelder trees for several dates,
two stream sites (perennial, ephemeral) and two tree size classes (large,
small). Bars are equal to 1 standard
error of the mean. Mid-morning net
photosynthetic rate differed significantly (P ≤ 0.05) between sites on
July 27 and September 8, and between tree sizes on August 10 and
September 8. Afternoon net photosynthetic rate differed significantly between sites on June 15.
tosynthetic rate (Table 3). Site was an important source of
variation for foliar nitrogen concentration. On several dates
(June 1 and 29, July 13 and 27, August 10), foliar nitrogen
concentration was significantly higher at the perennial-stream
site than at the ephemeral-stream site (Figure 6). Foliar nitrogen concentration differed significantly between large and
small trees only on August 24 (Figure 6). Net photosynthetic
rate was significantly correlated with water potential measured
at the time of the gas exchange measurement, and minimum
air and soil temperatures for the week preceding the gas exchange measurement (Table 3).
Discussion
We predicted that small boxelder trees growing along the
perennial stream would utilize stream water (cf. Dawson and
Ehleringer 1991, Smith et al. 1991). Based on our δ18O data,
we were unable to determine directly if this prediction was
correct because ground and stream water had similar δ18O
values. However, differences in both predawn and mid-morning water potentials between ephemeral- and perennial-stream
sites were generally small and inconsistent among dates, implying that stream water is not a physiologically important
Figure 5. Net photosynthetic rate (Pn) versus leaf-to-air vapor pressure
deficit of boxelder trees from two stream sites (perennial, ephemeral)
and two tree size classes (large, small). Data for each site × tree size
combination are mid-morning and afternoon means from different
dates. The correlation coefficient (r) is for data pooled over sites and
tree sizes (n = 56).
158
KOLB, HART AND AMUNDSON
Table 3. Pearson product correlation coefficients (r) between leaf gas
exchange and selected environmental and physiological variables for
means pooled over measurement times, dates, sites, and tree sizes
(n = 56 unless otherwise noted). (*; P = 0.05 and **; P = 0.01)
Variable
Net photosynthetic
rate (µmol m −2 s −1)
Stomatal conductance
(mol m −2 s −1)
Net photosynthetic
rate (µmol m −2 s −1)
--
+0.470**
Stomatal conductance +0.470**
(mol m −2 s −1)
--
Predawn xylem water
potential (MPa)
+0.1253
+0.2024
+0.1213
+0.1604
Daytime xylem water
potential1 (MPa)
+0.361**
+0.146
Leaf-to-air vapor
pressure deficit (kPa)
--0.691**
--0.270*
Foliar nitrogen
+0.388*3
−2
concentration (g m )
+0.0753
Minimum air
temperature (°C)2
--0.470**3
+0.1713
Minimum soil
temperature (°C)2
--0.442**3
+0.2593
1
2
3
4
Measured at the time of gas exchange measurement.
Measured the week preceding the gas exchange measurement.
Mid-morning (n = 36).
Afternoon (n = 20).
water source for either large or small boxelder trees. Further,
the poor correlation of variations in water potential with lightsaturated net photosynthetic rate and stomatal conductance,
imply that the small differences in water potential measured
between sites on some dates were of little physiological sig-
nificance. In contrast to our results, stream water has been
shown to be an important water source that can influence water
relations and carbon gain of trees in other riparian ecosystems
in western North America (Dawson and Ehleringer 1991,
Smith et al. 1991).
We also expected water uptake by both small and large trees
at each site to be primarily from ground- or stream water, or
both, during the early summer drought, with recent precipitation becoming an increasingly important water source following the onset of monsoonal rains in late summer. Overall, the
mixing model results support this hypothesis, although trees at
the ephemeral-stream site, especially small trees, also utilized
soil water in May. The source of this soil water was either
winter precipitation (soil recharge), rains that occurred in late
April, or both. It is unlikely that the high δ18O values of xylem
water observed in early May at the ephemeral-stream site were
a result of residual stem water subjected to evaporative enrichment throughout the previous winter (Phillips and Ehleringer
1995), because xylem water was measured only after full leaf
expansion when residual water should be depleted by transpiration. Even on days following recent precipitation when surface soil water was utilized by large and small trees at both
sites, this water source typically constituted no more than 40%
of total water uptake.
Net photosynthetic rates differed between sites on several
dates, with generally higher rates in trees at the perennialcompared with the ephemeral-stream site. Site differences in
foliar nitrogen concentration seem to provide the best explanation of site differences in net photosynthetic rate, as foliar
nitrogen concentration at the perennial-stream site was typically greater than at the ephemeral-stream site. Our results
showing a positive relation between foliar nitrogen concentration and net photosynthetic rate reflect those reported for this
species in a comparative study of male and female trees in a
riparian forest in the Wasatch Mountains, Utah (Dawson and
Ehleringer 1993a).
Figure 6. Mean foliar nitrogen concentration of boxelder trees based on
leaf area for nine dates, two stream
sites (perennial, ephemeral) and two
tree size classes (large, small). Bars
are equal to standard error of the
mean. Foliar nitrogen concentration
differed significantly (P ≤ 0.05) between sites on June 1, June 29,
July 13, July 27 and August 10; and
between tree sizes on June 15 and
August 24.
WATER SOURCE VARIATION AND PHYSIOLOGY OF BOXELDER
At our field site, the most important limitation to light-saturated net photosynthesis and stomatal conductance of boxelder
was high leaf-to-air vapor pressure deficit that developed as a
result of high leaf temperatures combined with low atmospheric humidity. The maximum temperature of sunlit leaves in
our study (approximately 40 °C) was greater than that reported
for this species in a riparian forest in the midwestern United
States (30--35 °C, Foster 1992), and for Salix and Populus
species growing in riparian forests in intermountain basins
of the Rocky Mountains (25--30 °C, Foster and Smith 1991).
Based on these regional differences in leaf temperature and
associated vapor pressure deficit for riparian trees, we speculate that limitations on net photosynthesis by high vapor pressure deficit are stronger for riparian trees in the American
Southwest than for riparian trees in cooler regions. For example, Foster (1992) reported no limitation of leaf gas exchange
by vapor pressure deficit for boxelder in a riparian forest in the
midwestern United States.
The significant relationship between daytime water potential and net photosynthetic rate was likely the result of a strong
negative correlation between water potential and vapor pressure deficit (r = --0.706, n = 56, P < 0.01). Lack of a strong
correlation between stomatal conductance and daytime water
potential further implies that net photosynthesis was not seriously limited by stomatal closure induced by low water potential (cf. Foster 1992). Significant negative relationships
between net photosynthetic rate and minimim soil and air
temperatures observed the week preceding gas exchange
measurements likely resulted from limitation of photosynthesis by high vapor pressure deficits during warm periods, and
not from an effect of these variables on gas exchange. In our
study, low night temperatures did not strongly limit leaf gas
exchange, as has been reported for high-elevation riparian trees
and shrubs in more northern locations (Young et al. 1985,
Foster and Smith 1991).
Based on previous studies of size-related differences in
physiology for riparian trees in western North America (Donovan and Ehleringer 1991, Smith et al. 1991, Dawson and
Ehleringer 1993a), we also expected greater water stress and
lower net photosynthetic rates for small trees than for large
trees. Contrary to this hypothesis, we found that, on most dates,
both large and small trees had similar water potentials, net
photosynthetic rates and other physiological characteristics. At
the study sites, all small trees appeared to be sprouts that
originated from larger trees following flood damage. These
sprouts apparently have relatively large, deep root systems
capable of tapping the same deep water sources as larger trees.
In summary, we hypothesized a tight coupling of net carbon
gain and water stress severity to stream water availability for
boxelder in a montane-riparian forest in northern Arizona. The
data did not support this hypothesis because the trees appeared
to rely on deep water sources regardless of stream water
availability. Similar leaf water potentials and gas exchange
rates were recorded for both large and small trees, because
small trees were vegetative sprouts from older stems that
tapped the same deep water sources as large trees. Following
recent precipitation, surface soil water was also utilized by
159
trees, especially small trees, indicating flexibility in water
source use.
Acknowledgments
This research was supported by USDA Competitive Grant No. 9337106-9485 and by a grant from the Northern Arizona University
School of Forestry Mission Research Program. The authors acknowledge the efforts of Karin L. Doerr in field and laboratory measurements, A. Hope Jahren for assistance in water extraction, and Gale
Eaton for conducting the mass spectrometer analyses.
References
Brown, D.E. 1982. Biotic communities of the American Southwest---United States and Mexico. Desert Plants 4, Univ. Arizona Press,
Tucson, AZ, 342 p.
Busch, D.E., N.L. Ingraham and S.D. Smith. 1992. Water uptake in
woody riparian phreatophytes of the southwestern United States:
A stable isotope study. Ecol. Appl. 2:450--459.
Busch, D.E. and S.D. Smith. 1993. Effects of fire on water and salinity
relations of riparian woody taxa. Oecologia 94:186--194.
Busch, D.E. and S.D. Smith. 1995. Mechanisms associated with decline of woody species in riparian ecosystems of the southwestern
United States. Ecol. Monogr. 65:347--370.
Dawson, T.E. 1993. Water sources of plants as determined from
xylem-water isotopic composition: perspectives on plant competition, distribution, and water relations. In Stable Isotopes and Plant
Carbon--Water Relations. Eds. J.R. Ehleringer, A.E. Hall and
G.D. Farquhar. Academic Press, San Diego, CA, pp 465--496.
Dawson, T.E. and J.R. Ehleringer. 1991. Streamside trees that do not
use streamside water. Nature 350:335--337.
Dawson, T.E. and J.R. Ehleringer. 1993a. Gender-specific physiology,
carbon isotope discrimination, and habitat distribution in boxelder,
Acer negundo. Ecology 74:798--815.
Dawson, T.E. and J.R. Ehleringer. 1993b. Isotopic enrichment of water
in the ‘‘woody’’ tissues of plants: Implications for plant water
source, water uptake, and other studies which use the stable isotopic
composition of cellulose. Geochim. Cosmochim. Acta 57:3487-3492.
Dina, S.J. and L.G. Klikoff. 1973. Carbon dioxide exchange by several
streamside and scrub community species of Red Butte Canyon,
Utah. Am. Midl. Nat. 89:70--80.
Donovan, L.A. and J.R. Ehleringer. 1991. Ecophysiological differences among juvenile and reproductive plants of several woody
species. Oecologia 86:594--597.
Ehleringer, J.R. and C.B. Osmond. 1989. Stable isotopes. In Plant
Physiological Ecology. Field Methods and Instrumentation. Eds.
R.W. Pearcy, J.R. Ehleringer, H.A. Mooney and P.W. Rundel. Chapman Hall, London, pp 281--300.
Epstein, S. and T. Mayeda. 1953. Variations in the 18O content of waters
from natural sources. Geochim. Cosmochim. Acta 4:213--224.
Foster, J.R. 1992. Photosynthesis and water relations of the floodplain
tree, boxelder (Acer negundo L.). Tree Physiol. 11:133--149.
Foster, J.R. and W.K. Smith. 1991. Stomatal conductance patterns and
environment in high elevation phreatophytes of Wyoming. Can.
J. Bot. 69:647--655.
Gower, S.T., P.B. Reich and Y. Son. 1993. Canopy dynamics and
aboveground production of five tree species with different leaf
longevities. Tree Physiol. 12:327--345.
160
KOLB, HART AND AMUNDSON
Gray, A.N. and T.A. Spies. 1995. Water content measurement in forest
soils and decayed wood using time domain reflectometry. Can. J.
For. Res. 25:376--385.
Issac, R.A. and W.C. Johnson. 1976. Determination of total nitrogen
in plant tissue, using a block digestor. J. Assoc. Off. Anal. Chem.
59:98--100.
Johnson, R.R. and L.T. Haigt. 1984. Riparian problems and initiatives
in the American Southwest: a regional prespective. In California
Riparian Systems: Ecology, Conservation and Productive Management. Eds. R. Warner and C. Henricks. Univ. California Press,
Berkeley, CA, pp 404--412.
Lowe, C.H. and D.E. Brown. 1982. Introduction. In Biotic Communities of the American Southwest----United States and Mexico. Ed.
D.E. Brown. Desert Plants 4, Univ. Arizona Press, Tucson, AZ,
pp 8--16.
Meng, F.R. and P.A. Arp. 1993. Net photosynthesis and stomatal
conductance of red spruce twigs before and after twig detachment.
Can. J. For. Res. 23:716--721.
Phillips, S.L. and J.R. Ehleringer. 1995. Limited uptake of summer
precipitation by bigtooth maple (Acer grandidentatum Nutt) and
Gambel’s oak (Quercus gambelii Nutt). Trees 9:214--219.
QuikChem Systems. 1992. QuikChem Method No. 13-107-06-2-D.
Lachat Instruments, Milwaukee, WI.
Ritchie, G.A. and T.M. Hinckley. 1975. The pressure chamber as an
instrument for ecological research. Adv. Ecol. Res. 9:165--254.
Schubert, G.H. 1974. Silviculture of southwestern ponderosa pine.
USDA For. Ser. Res. Pap. RM-123, 71 p.
Smith, S.D., A.B. Wellington, J.L. Nachlinger and C.A. Fox. 1991.
Functional responses of riparian vegetation to streamflow diversion
in the eastern Sierra Nevada. Ecol. Appl. 1:89--97.
Soil Survey Staff. 1990. Keys to soil taxonomy, 4th Edn. SMSS
Technical Monograph No. 6, Blacksburg, Virginia, 422 p.
Sternberg L.S.L., M.J. DeNiro and H.B. Johnson. 1986. Oxygen and
hydrogen isotope ratios of water from photosynthetic tissues of
CAM and C3 plants. Plant Physiol. 82:428--431.
Stromberg, J.C. and D.T. Patten. 1990. Riparian vegetation instream
flow requirements: A case study from a diverted stream in the
eastern Sierra Nevada, California, USA. Environ. Manage. 14:185-194.
Tyree, M.T., K.J. Kolb, S.B. Rood and S. Patino. 1994. Vulnerability
to drought-induced cavitation of riparian cottonwoods in Alberta--a
possible factor in the decline of the ecosystem. Tree Physiol.
14:455--466.
Tyree, M.T. and T.R. Wilmot. 1990. Errors in the calculation of
evaporation and leaf conductance in steady-state porometry: The
importance of accurate measurement of leaf temperature. Can.
J. For. Res. 20:1031--1035.
White, J.W.C. 1989. Stable isotope ratios in plants: a review of the
current theory and some potential applications. In Stable Isotopes
in Ecological Research. Eds. P.W. Rundel, J.R. Ehleringer and
K.A. Nagy. Springer-Verlag, Berlin, pp 142--162.
Young, D.R., I.C. Burke and D.H. Knight. 1985. Water relations of
high-elevation phreatophytes in Wyoming. Am. Midl. Nat.
114:384--392.