Special Section:
Coupling Soil Science and
Hydrology with Ecology
Ronald J. Ryel*
A. Joshua Leffler
Carolyn Ivans
Michael S. Peek
Martyn M. Caldwell
Functional Differences in WaterUse Patterns of Contrasting Life
Forms in Great Basin Steppelands
The temporal patterns of evapotranspiration were monitored for 2 yr for four species of differing life form that currently form near monoculture communities in the Great Basin, USA,
a region with a growing season spanning early spring to autumn and predictable overwinter
water accumulation in the vadose zone. Species included an annual grass (Bromus tectorum
L.), a perennial grass [Agropyron desertorum (Fisch. ex Link) Schult.], a shrub (Artemisia
tridentata Nutt. ssp. wyomingensis Beetle and Young), and a tree [Juniperus osteosperma
(Torr.) Little]. The two grasses and shrub were growing on the same soil type with uniform
texture and subject to near surface percolation of the vadose zone only, while J. osteosperma was growing on soils with a petrocalcic layer below which water was near field
capacity. These monotypic stands were found to differ in quantity and timing of vadose
zone water use, in use pattern of shallow and deeper water resource pools, and in depth
and quantity of rainwater hydraulically redistributed. All species rapidly utilized shallow
vadose zone water in the spring when growth was observed, but use of deep vadose zone
water varied by life form and was not linked with the period of growth for any species.
Water in the vadose zone of the grass species increased between years, with evapotranspiration less than precipitation inputs and contrasted to water use in A. tridentata where
water use approximately equaled precipitation inputs. Juniperus osteosperma used water
below the petrocalcic zone, particularly in late summer. Water use by all species was consistent with the concept of a shallow vadose zone “growth pool” of water and a deeper
vadose zone “maintenance pool” used during the summer drought period. The patterns of
water use suggest that water per se is not a limited resource for survival, but influences the
availability of nutrients necessary for plant growth that are associated with shallow vadose
zone water. We postulate that cold-adapted plants in the Great Basin have converged on
a general pattern of rapidly utilizing soil moisture in shallow depths, in part, to influence
nutrient availability. Our results strongly suggest describing the pool dynamics of vadose
zone water will be necessary to further our understanding of plant fitness, interactions
among species for resources, and species coexistence in arid and semiarid ecosystems.
Abbreviations: ET, evapotranspiration; LAI, leaf area index.
The emerging science of ecohydrology explicitly recognizes the fundamental
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role that vegetation plays in vadose zone hydrology at scales from the community to the
watershed. These effects modify, and in part control, water inputs, movement, and losses
from the vadose zone. For example, plants modify water input to soils and ultimately watersheds through processes including precipitation interception (Bosch and Hewlett, 1982;
Brown et al., 2005; LaMalfa and Ryel, 2008), fog condensation (Azevedo, 1974; Ewing et
al., 2009), alteration of surface movement by slowing flow paths (Schlesinger et al., 1990;
Wilcox et al., 2003; Ludwig et al., 2005), spatial distribution of vadose zone water (Loik
et al., 2004; Breshears et al., 2009a; Newman et al., 2010), or altered soil hydrophobicity
(Lebron et al., 2007; Madsen et al., 2008; Robinson et al., 2010). Plants influence vadose
zone movement of water by hydraulic redistribution (Caldwell et al., 1998; Burgess et
al., 2001; Hultine et al., 2003; Ryel, 2004; Scott et al., 2008) and preferential flow along
root channels (Beven, 1982; Li and Ghodrati, 1994). Plants largely control loss of water
from the vadose zone and recharge to the phreatic zone through transpiration and affects
on surface evaporation (Peck, 1978; Le Maitre et al., 1999; Jobbagy and Jackson, 2004;
Seyfried et al., 2005). The role plants play in ecohydrology is a function of their morphology,
phenology, and physiology, all of which are species specific. Furthermore, complex feedbacks among physical, chemical, and biological processes are responsible for structuring
vegetation communities and, in turn, vadose zone and watershed hydrology. If we are to
address contemporary management concerns and grasp the potential role of climate change,
these processes and feedbacks need to be more fully understood (Caylor et al., 2009).
2010, Vol. 9
www.VadoseZoneJournal.org | 1
AUTHOR: NEED EXECUTIVE
SUMMARY. FINALIZE WITH JOHN
NIEBER ([email protected])
R.J. Ryel and M.M. Caldwell, Dep. of Wildland
Resources and the Ecology Center, Utah State
Univ., Logan, UT 84322-5230. A.J. Leffler,
USDA-ARS, Forage and Range Research Lab.,
Logan, UT 84322-6300. C. Ivans, Campbell
Scientific, Inc., 815 West 1800 North, Logan,
UT 84321. M.S. Peek, Dep. of Biology, William
Paterson Univ., Wayne, NJ 07470. *Corresponding author ([email protected]).
Vadose Zone J. 9:1–13
doi:10.2136/vzj2010.0022
Received 9 Feb. 2010.
Published online.
© Soil Science Society of America
5585 Guilford Rd. Madison, WI 53711 USA.
Plant use of vadose zone water clearly has a profound influence
on hydrologic function, yet our understanding of the role of vegetation is not complete. This shortcoming is especially critical for
many arid and semiarid lands where percolation through vegetated
communities rarely makes it through the entire vadose zone to
recharge phreatic zones (Andraski, 1997; Scanlon et al., 1999;
Seyfried et al., 2005), and plant performance is dictated by the
fate of water delivered to the surface. Water movement within the
vadose zone is initiated by precipitation inputs to surface soils and
is driven by the interaction of physical processes of vertical and
horizontal water movement and plant effects on evapotranspiration and hydraulic redistribution. However, conceptual models of
water use by plants in arid and semiarid communities have been
rather simplistic, as we discuss below, and rested on several assumptions that recent studies have begun to question.
Potential evapotranspiration greatly exceeds annual precipitation
in arid lands, and sharing of this limited resource among coexisting plant species has led to postulation of niche partitioning. The
classic expression of this idea is the “Two-Layer” model (Walter,
1971, 1973), where water is partitioned among species that rely
on transient shallow vadose zone water and those that use more
dependable deep-water sources. Related is the concept that plant
morphology largely determines plant strategy with respect to water
use (Dodd et al., 1998; Breshears and Barnes, 1999). In addition,
the functional importance of the intermittency of precipitation
events in aridlands is captured in the “Pulse-Reserve” model that
links periods of physiological activity to pulses of water availability
(Noy-Meir, 1973), and the “Threshold-Delay,” which additionally
accounts for pulses of resource availability that fail to elicit a direct
physiological response (Ogle and Reynolds, 2004).
Recently, Ryel et al. (2008) proposed a new conceptual model for
plant water use that is relevant to arid lands. This “resource-pool”
model suggests that shallow vadose zone water is used primarily for
resource-intensive physiological functions such as new tissue and
seed production (“growth pool”), while deep vadose zone water (or
water at low water potentials) is used primarily to ensure survival
during unfavorable drought conditions (“maintenance pool”). The
model approaches water use from the perspective of the resource
rather than the plant strategy and was proposed because (i) most
plant species have roots concentrated in the uppermost soil, (ii)
nutrients are contained primarily in the upper soil and availability is largely linked to associated water, and (iii) water tables can
rise when arid and semiarid shrublands, woodlands, and forests
are converted to annual grasslands or crop systems as plants transpiration is less than the available vadose zone water (Schofield,
1992; Pierce et al., 1993; Cramer and Hobbs, 2002). Most important for hydrologic function, however, is the prediction from the
model that all species will rely on the growth pool of water, and
that growth limitations will incur when this pool becomes inadequate for physiological processes related to growth and nutrient
accessibility. The use and dynamics of the maintenance pool, on
the other hand, should be more linked to life history strategy than
growth. That is, plant species that maintain physiological activity
during droughts will depend on and use this pool, while species
that largely become dormant during drought will have reduced
need for this resource. Consequently, alterations in these pools
from vegetation change due to management, invasive species, or
possibly climate change will feedback into vadose zone processes
and ultimately, watershed function.
Current plant communities in the sagebrush-steppe portions of the
Great Basin can be dominated by species of different life-history
strategy and form, ranging from annual species to perennial shrubs
and trees that often occur in nearly monospecific stands (Billings,
1949; Smith and Nowak, 1990). In our study, we ascertained
temporal water-use patterns for four species that currently form
near monoculture communities in the Great Basin: an annual
grass Bromus tectorum (cheatgrass), a perennial grass Agropyron
desertorum (crested wheatgrass), a shrub Artemisia tridentata (big
sagebrush), and a tree Juniperus osteosperma (Utah juniper). We
then evaluated the effect of each species on the timing, depth,
and quantity of vadose zone water use, storage across years, and
hydraulic redistribution of precipitation. These results are used to
assess the relevance of different conceptual models of plant water
dynamics in these communities to assess how differences in water
dynamics in the vadose zone may affect plant–plant interactions
and perhaps community stability.
66Materials and Methods
Study Area
The study was conducted in nearly monotypic stands (>90% cover)
of the neophytic annual grass (cheatgrass), the neophytic perennial
grass (crested wheatgrass), the native evergreen shrub (Wyoming
big sagebrush), and the evergreen tree (Utah juniper), in Rush
Valley Utah, USA (40° 17¢ N, 112° 28¢ W; elevation: 1607–1664
m). The stands of grasses were both established in 1992 on lands
formerly dominated by stands of Ar. tridentata (Hooker et al., 2008).
Agropyron desertorum was established for range improvement by
drill-seeding (30-cm row spacing) following disking, while B. tectorum was established following a summer wildfire from seeds in
the shrub understory. The stands of Ar. tridentata and J. osteosperma
were dominated by mature individuals, and were likely established
for 30 or more years before the study, although the dominance of
these woody species is likely recent. It is likely none of the plant communities have had sufficient time to come into complete equilibrium
with the soil and vadose zone, and differences in vadose zone water
dynamics may still be in transition to equilibrium (Newman and
Graham 2008). All species, except B. tectorum, are adapted to extract
water for transpiration from soils well below the matric potential
“wilting point” of many species of −1.5 MPa.
Soils at the grass and shrub sites (Hooker et al., 2008) are a silt
loam (Erda silt loam, very deep, well-drained, mixed, superactive,
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mesic Aridic Calcixerolls) derived from lacustrine and alluvial
sediments with low shrink–swell capacity and largely uniform
texture. Soils at the J. osteosperma site (Leffler et al., 2002) are
rocky, silt loam, mixed Xerollic Calciorthid of the Taylorsflat
series (fine-loamy, mixed, superactive, mesic xeric Haplocalcids;
Tooele County, Utah Soil Survey, 1993; see http://websoilsurvey.
nrcs.usda.gov/app/WebSoilSurvey.aspx), with a petrocalcic layer
(hardpan) at a depth of 1.1 m. The water table in the grass and
shrub communities is at 15 to 20 m, with very dry soils (less than
−3 MPa) existing between the rooting zone and the water table. In
the J. ostersperma community, soils were near field capacity (0.25
cm3 cm−3; Ryel et al., 2002) for water below the hardpan, likely
due to lateral subsurface flow, although this was not measured. A
few J. ostersperma roots were observed during installation of soil
psychrometers to penetrate this hardpan.
Climate at the study site is typical of Great Basin cold deserts
(Caldwell 1985), characterized by cold winters and hot summers.
Mean temperatures at Vernon, UT (20 km south of study site; NWS
Cooperative Network station no. 429133; elevation: 1676 m; years:
1953–2005) are lowest in January (min. −10.2°C, max. 4.3°C) and
highest in July (min. 12.1°C, max. 32.3°C). Precipitation at the
study site is expected to be on average relatively constant across
months, as indicated by monthly averages for Vernon (Fig. 1, upper,
inset). Average annual precipitation for Vernon is 268 mm.
Soil Moisture Measurements
Soil matric potential at nine depths (0.3, 0.45, 0.6, 0.9, 1.2, 1.5,
1.8, 2.4, 3.0 m) was recorded hourly from mid April through mid
October, 1999 and mid March through late December, 2000 using
individually calibrated screen-cage thermocouple psychrometers
(J.R.D. Merrill Specialty Equipment, Logan, UT; Wescor, Logan,
UT) attached to individual data loggers (CR7, Campbell Scientific,
Inc., Logan, UT). Specifications from the manufacturers list the
range (and accuracy) of these psychrometers from −0.1 (±7%) to
−7.5 (±1%) MPa. Depths less than 0.3 m were not monitored because
psychrometers are sensitive to diurnal temperature variation (Rundel
and Jarrell 1989). Psychrometers were installed in November 1998,
150 mm horizontally into the walls of three trenches; psychrometers
were installed in vertical arrays, three per trench per depth, except at
2.4 and 3.0 m, where one was installed per depth. Following installation, trenches were backfilled to original grade.
Soil matric potential (Y, MPa) was converted to volumetric water
content (q, cm3 cm−3) using the soil water retention relationship
from van Genuchten (1980) as implemented in Ryel et al. (2002)
for the same location:
q=
(
qs - qr
1 + aY
n m
)
[1]
q r and q s are residual (0.02 cm3 cm−3) and saturated (0.5 cm3
cm−3) volumetric water contents, and a (0.2818 cm−1) and n (1.4)
Fig. 1. (a) Cumulative monthly precipitation at the study site for
water years (WY) 1999 and 2000 (1 Oct. 1998 to 30 Sept. 1999 and
1 Oct. 1999 to 30 Sept. 2000, respectively) and for Vernon, UT for
the period 1953 through 2005. Inset: average monthly precipitation
(mm) for Vernon, UT, 1953–2005. (b) Shallow (0.0–0.3 m depth;
dashed line) and total (solid line) vadose zone water pools for Artemisia tridentata community during the 2000 water year. Total water
and water at the 0- to 0.3-m depth were estimated from soil psychrometers at nine depths (0.3, 0.45, 0.6, 0.9, 1.2, 1.5, 1.8, 2.4, and 3.0 m)
during the period beginning 15 March. The total vadose zone water
pool during the prior period (1 October to 14 March) was estimated
from precipitation events (vertical bars) recorded at the site. The
approximate period when air and soil temperatures were suitable for
plant transpiration (15 March to 30 September), water content (39.4
mm, horizontal dotted line) in the near surface vadose zone where
nutrient diffusion becomes limited by tortuosity (see Jackson and
Caldwell, 1996; Ryel and Caldwell, 1998) and the period when nutrients would be most available (short horizontal gray bar) are shown.
are parameters based on soil characteristics, where m = 1 − 1/n.
Changes in q were used to calculate an approximate evapotranspiration (ET; mm d−1) within vadose zone layers measured with
psychrometers. Vadose zone layers were defined by the midpoints
between depths containing psychrometers. The psychrometer at
0.3 m was assumed to represent 0.0 to 0.375 m. Evapotranspiration
was estimated as the sum of differences in mean θ for each layer
between sequential days. These calculations are only approximate
estimates of ET given the potential for lateral flow and deep percolation. Percolation was not measured below approximately 2 m,
indicating the psychrometer array bounded all vertical flow. Lateral
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flow may occur, but root distributions for these communities are
relatively uniform horizontally, even with clumped vegetation (e.g.,
Dobrowolski et al., 1990; Ryel et al., 1996), which would result in
relatively uniform water use horizontally. In addition, as soils dry,
lateral flow would be minimal due to low hydraulic conductivity.
Hydraulic redistribution is the movement of water by plant roots
from soil regions of high matric potential to regions of low matric
potential. Hydraulic redistribution was estimated for all species in the days immediately following two rain events, 20 Aug.
1999 (13.7 mm) and 30 Aug. 2000 (11.7 mm) when vadose zone
matric potentials were low and piston flow would be very limited.
Increases in water measured at soil depths below 0.30 m in the days
immediately following these rain events were assumed to be due
to hydraulic redistribution based on measurements and modeling
of vadose zone water (using on a bare plot at the same study sites)
by Ryel et al. (2003). These measurements did not indicate measureable water movement to the 0.30-m sensor depth for at least
10 d following 10- and 16-mm rain events. Simulations in Ryel et
al. (2003) using the soil water model of Ryel et al. (2002) found
no percolation of water to 0.30 m for a 10-mm event, and for rain
events of 15 mm, less than 10% of the rainwater was simulated
to percolate below 0.30 m. Given these results, it was assumed
here that any increase in vadose zone water below the 0.30-m
depth within 10 d of a rain event was due primarily to hydraulic
redistribution. The one-dimensional (vertical) model of Ryel et al.
(2002) was based on Buckingham–Darcy’s Law as implemented by
Campbell (1985) and was parameterized for soils at site used in the
current study for simulations presented in Ryel et al. (2002, 2003).
communities was estimated from clipped plots (80–300 cm2) also
in areas adjacent to the measured stands. All plants within the
plots were clipped to the ground, placed in plastic bags, and then
measured in the laboratory with a leaf area meter (LI-3100, LiCor,
Inc., Lincoln, NE).
66Results
Weather
Total precipitation at the study site measured in the 1999 (278
mm) and 2000 (266 mm) water years were close to the average for
Vernon, UT (268 mm). In 1999, precipitation was more evenly
spread over the year than in 2000 (Fig. 1, upper) and reflected a
wetter fall and summer than in 2000. Temperatures during the
plant growing season (16 March to 30 September) at the study
site in 1999 and 2000 were nearly identical (14.9 vs. 15.0°C,
respectively) as were maximum (25.5, 24.4°C) and minimum
temperatures (5.2, 5.5°C). However, average maximum daytime
temperatures in 1999 were much cooler in April, May, and June
than in 2000 (1999: 12.8, 20.1, 27.4°C; 2000: 19.6, 24.1, 30.0°C,
respectively).
Evapotranspiration and Vapor Pressure Deficit
Daily water loss from the vadose zone during the growing season
from ET (15 March to 30 September) was linearly related to VPD
(ANCOVA, F = 10.35, p = 0.003) in the first part of the growing
season (2 April to 1 June) for all communities, but became disconnected from VPD (ANCOVA, F = 0.047, p = 0.50) for the rest of
the period (Fig. 2). The estimated slope of ET vs. VPD from the
Weather Data
Precipitation, air temperature, and relative humidity at the study
site were measured with a weather station (Campbell Scientific)
attached to a data logger (CR10x, Campbell Scientific). The station was located within 250 m of all psychrometer installations,
except for the J. osteosperma array that was located approximately
2.8 km to the south; sensors were mounted at ~2-m height. Vapor
pressure deficit (VPD) was calculated from air temperature and
relative humidity as the difference between saturation and actual
vapor pressures. Missing values due to equipment malfunction
were replaced by the Vernon, UT weather station.
Plant Growth
Plant growth was monitored for the Ar. tridentata and both grass
communities. Repeated measurements and phenological observations were made on 11 to 22 individuals of each species starting
in late March or early April until plant senescence. To minimize
soil compaction within areas monitored for soil matric potential,
these individuals were selected outside of the monitored stands.
For the grass species, height was measured and phenological
stage (portion alive, flowering, seedhead development) recorded.
For Ar. tridentata, the length of upper branches was measured
to determine leader growth. Leaf area index (LAI) for the grass
Fig. 2. Estimated vadose zone water loss due to evapotranspiration
(5-d averages) during the growing season (15 Mar. to 30 Sept. 2000)
for the four plant communities at the study site. Filled symbols are for
the period 2 April to 1 June, and open symbols for the period 6 June to
30 September. No line is shown for Bromus tectorum during (6 June to
30 September) because there were only two measured times when these
plants were active in the period. Individual correlations for regression
lines were (early and later periods, respectively): B. tectorum (0.57, insufficient data); Agropyron desertorum (0.54, −0.12); Artemisia tridentata
(0.58, −0.27); and Juniperus osteosperma (0.40, −0.09).
www.VadoseZoneJournal.org | 4
B. tectorum community (6.8 mm kPa−1
d−1) was greater than for the other three
communities (2.9 mm kPa−1 d−1) for
the early period, but was not significant
(ANCOVA, F = 0.03, p = 0.85).
Vadose Zone
Water Dynamics
Quantities and dynamics of vadose zone
water above 3 m varied greatly among
the plant communities (Fig. 3). Bromus
tectorum had more water throughout the
vadose zone above 3 m than the other
two species on similar soils and was most
similar to J. osteosperma, which had significant water below the hardpan at 1.1
m. The three communities on similar
soil type showed persistent dry (less than
−3 MPa) vadose zone layers at the deepest
depth sampled (3 m). However, the top
of this persistent dry layer varied from
approximately 1.8 m in Ar. tridentata
and Ag. desertorum to nearly 3 m in B.
tectorum (Fig. 4). The vadose zone under
J. osteosperma was much wetter at the
3-m depth, with substantial water found
below the hardpan (matric potentials
greater than −0.3 MPa).
All four communities displayed extraction of water in the near surface vadose
zone during the growing season, but
the depth and quantity varied (Fig. 3).
Artemisia tridentata displayed the greatest depth of seasonal extraction (to 1.5
m) reaching matric potentials of less
than −4 MPa, while B. tectorum had the
least depth of extraction (0.45 m) and
least drying (to approximately −1 MPa).
Water in the vadose zone of Ag. deserFig. 3. Interpolated soil matric potentials (MPa) for four plant communities at the study site from 0 to
3 m depth during portions of 1999 and 2000. Periods shown are when soil psychrometers were active.
torum was extracted to 1.2 m and in J.
Vertical dotted lines in the 2000 images indicate the period of data collection in 1999 for comparison.
osteosperma to at least 0.9 m. Patterns
The −0.5-MPa contour line is shown for Bromus tectorum for clarity. The hardpan at the 1.1-m depth
of extraction and percolation were simifor Juniperus osteosperma, is indicated with a horizontal bar. Data were interpolated using inverse distance squared (Grapher 8, Golden Software, Golden, CO) for daily average matric potentials measured
lar between years for each community,
with arrays of soil psychrometers at nine depths (0.3, 0.45, 0.6, 0.9, 1.2, 1.5, 1.8, 2.4, and 3.0 m).
but drying began later in 1999 for all
communities. Agropyron desertorum displayed increased vadose zone moisture in
decline in April in 2000 for all communities, reflecting the much
2000 0.9 to 2.0 m depth over patterns found in mid October 1999.
warmer spring in 2000. Bromus tectorum and J. osteosperma had
the highest maximum water in the vadose zone in spring. The B.
Maximum and minimum water content in the vadose zone at 0 to
tectorum community also had the highest minimum water content
3 m depth among the four communities during the growing season
during the growing season, least difference between wettest and
varied in both quantity (Fig. 4a,b) and time of minimum content.
driest periods, and the earliest dates of the minimum content (27
Water contents were high well into May in 1999, but began to
June 1999, 9 June 2000), after which water slowly accumulated
www.VadoseZoneJournal.org | 5
in soils from precipitation events.
Juniperus osteosperma had the next
wettest minimum water content,
but these both occurred at the end
of the water year (30 September).
The Ag. desertorum and Ar. tridentata communities had much lower
maximum and minimum water
contents and had the driest vadose
zone near the end of the water year
in 1999. Both were driest around
mid August in 2000, perhaps
reflecting precipitation inputs in
late August and September.
Vadose zone water distribution
by depth during the wettest and
driest periods varied among
communities (Fig. 4c–4f). The
Ar. tridentata community had
little change bet ween years,
except being dryer between 0.9
and 1.5 m in spring 2000. Both
grass communities exhibited
an increase in water below 0.9
m depth between years that
was evident in both spring and
summer/fall. For the J. osteosperma community, less water
was observed in the vadose zone
during the dry period in 2000,
especially at the 0.9- to 1.5-m
depths. This included a decrease
in water below the hardpan layer.
Loss of water below the hardpan layer was observed in 2000,
unlike in 1999.
Fig. 4. Equivalent depth of water for the period of greatest (dark bars) and least (light bars) water accumulation in the vadose zone (0–3 m depth) is shown for (a) 1999 and (b) 2000. Moisture profiles (0–3 m)
estimated from arrays of soil psychrometers at nine depths (0.3, 0.45, 0.6, 0.9, 1.2, 1.5, 1.8, 2.4, and 3.0 m)
for periods with (c,d) greatest (spring) and (e,f ) least (summer/fall) water accumulation in the vadose zone.
Vadose Zone Water Pool Dynamics
The four communities exhibited different dynamics in shallow
(0–0.3 m) and deep (0.3–3.0 m) vadose zone water pools. All
communities rapidly depleted water in the shallow pool (Fig.
5a,b) in spring of both 1999 and 2000 (up to 5 mm d−1), but
rapid depletion was initiated approximately a month earlier in
2000. In both years, the initiation of rapid decline in the shallow pool occurred about a month earlier in the Ar. tridentata
community than in the grass communities. The J. osteosperma
community paralleled Ar. tridentata in 2000, but data collection for J. osteosperma was not initiated early enough in 1999
to fully determine the timing of soil water loss. The B. tectorum community exhibited a period with the highest estimated
rate of water loss in the shallow pool, but extracted less total
water (Fig. 5a,b).
Shallow and deep water pools were not well linked in pattern of
depletion (Fig. 5a,b vs. c,d). For the B. tectorum community, the
deep pool was minimally accessed. For both Ag. desertorum and Ar.
tridentata, depletion of the deep water pool began about a month
or more after the initiation of rapid decline in the shallow pool.
For the J. osteosperma community, a similar pattern to Ag. desertorum and Ar. tridentata was found for 2000. But in 1999, little
depletion of the deep pool was found, suggesting the lateral flow
was sufficient for resupplying this pool below the hardpan, or that
minimal water was extracted in total from the deep pool.
The period of plant growth (Fig. 5e,f) was largely linked to water
availability in the shallow water pool (Fig. 5a,b) for three species
(growth in J. osteosperma was not monitored). All three species
exhibited cessation of growth 1 to 2 wk before near depletion of
shallow pool available water. Depletion of deep pools, however,
www.VadoseZoneJournal.org | 6
Fig. 5. Shallow (0.0–0.3 m) and deep (0.3–3.0 m) vadose zone water pools in four plant communities during (a,c) 1999 and (b,d) 2000. Note the scale
in the vertical axes in Parts a and b is half the vertical scale in axes in Parts c and d. Estimates were made from daily average matric potentials measured
with arrays of soil psychrometers at nine depths (0.3, 0.45, 0.6, 0.9, 1.2, 1.5, 1.8, 2.4, and 3.0 m). Periods of plant growth within three communities are
shown (growth for J. osteosperma was not monitored) for (e,f ) 1999 and 2000. Initiation of growth was not recorded, but began in mid to late March.
was not related to the cessation of growth, with significant depletion continuing well beyond the period of plant growth (Fig. 5).
This was especially evident for Ag. desertorum and Ar. tridentata,
where water use, largely from the deep pool continued throughout the summer. Artemisia tridentata consumed water in the
maintenance pool to similar levels in both years, while this pool
for the grass, Ag. desertorum, varied greatly in minimum quantity
between years (Fig. 3c,d).
maintained more than one-half of their active leaves throughout
the summer, with some leaves shed following cessation of growth.
The J. osteosperma trees maintained most active foliage throughout
the growing season. Estimated LAI for the grasses was less in 2000.
Bromus tectorum averaged 1.19 ± 0.13 (±SE) in 1999 and 0.76 ±
0.15 in 2000 (t test, t = 1.99, p = 0.075), while Ag. desertorum
averaged 1.20 ± 0.11 and 0.85 ± 0.03 in 1999 and 2000 (t test, t =
3.13, p = 0.040), respectively.
Plant phenology diverged following depletion of the shallow pool.
Bromus tectorum senesced shortly after cessation of growth and
seed set, and water depletion ceased in both pools. Agropyron
desertorum plants were largely dormant following cessation of
growth and seed set, but maintained a very low quantity of photosynthetically active material, primarily in stems, as evidenced
by a few green patches on most stems. Artemisia tridentata plants
Two types of pulses in vadose zone water pools were observed
in all communities and are exemplified by the Ar. tridentata
community in the 2000 water year (Fig. 1b). The largest pulse
corresponded to single pulse accumulation of water in the vadose
zone over the late October to mid-March period when air and
soil temperatures limit plant physiological function. Water from
precipitation events accumulated in the vadose zone during this
www.VadoseZoneJournal.org | 7
period and the pool increased in proportion to these
events. However, when temperatures became suitable
for plant transpiration, vadose zone water declined;
the decline was slow at first, but was greater as temperatures rose. The shallow pool declined to matric
potentials (approximately −1 MPa) where nutrient
diffusion and mass flow become very limited due to
increased tortuosity (Fig. 1b). Precipitation events
during the initial depletion phases of the soil water
pool slowed the decline of the vadose zone moisture, but were a small percentage of the whole pool.
However, as the water pool declined into summer,
precipitation events became sizable additions, particularly the shallow pool where these inputs more
than doubled the shallow soil pool size (Fig. 1b).
Hydraulic Redistribution
Evidence of hydraulic redistribution within the vadose
zone was found for all four plant communities (Fig. 6).
Following summer rain events, water was rapidly and
simultaneously distributed (within a few hours) to all
soil layers that had previously shown decline due to
plant water use (Fig. 6a). The estimated quantity of
redistributed water varied among communities for two
August rain events, with the most water hydraulically
redistributed by the woody species (~ 50%) and less
for the grass species (Fig. 6b). The maximum depth of
redistribution varied from 0.45 m in the B. tectorum
community to 1.2 m in the Ar. tridentata community. Movement was much more rapid than would be
expected via piston flow at the low matric potentials
observed and occurred simultaneously at multiple
soil layers in similar quantities, unlike a wetting front
(Ryel et al., 2002, 2003)
Fig. 6. (a) Matric potentials at multiple depths measured in four plant communities
before and after a 13.7-mm rainfall event on 20 Aug. 1999 (arrow). Additional, smaller
precipitation events (0.5–2.7 mm) are shown (black triangles). Matric potentials are
shown for all soil depths subject to recharge following the 20 Aug. 1999 rain event,
and the first measured layer below that did not exhibit recharge (deeper layers did not
exhibit recharge). (b) Estimated quantity of water moved below 0.30 m in the vadose
zone by hydraulic redistribution for two rain events, 20 Aug. 1999 and 30 Aug. 2000.
Value above bars indicates the maximum soil depth where redistribution was observed.
66Discussion
Patterns of Vadose Zone Water Use
Monotypic stands of Great Basin vegetation differed in patterns
of vadose zone water loss, including the timing and quantity of
water use, pattern of use of shallow and deeper water resource
pools, and in depth and quantity of rainwater hydraulically redistributed. This included use of both shallow and deep vadose zone
water pools. Use of shallow water was similar among species, with
rapid use in spring and early summer, but the woody species initiated significant soil moisture depletion earlier in the season than
the grasses. This was likely due in part to the transpiration of the
evergreen foliage of the shrub and tree that allowed for immediate transpiration once temperatures became suitable. Plant water
use associated with the shallow vadose zone was initially driven
by vapor pressure deficit, but when these soils became depleted,
water use became independent of this driver and likely was linked
to supply rates of water to the roots (Ryel et al., 2002).
An increase in vadose zone water between years was found for the
two grass communities. Anderson et al. (1987) found that evapotranspiration from monoculture stands of Ag. desertorum and Ar.
tridentata exceeded the average annual precipitation of their study
site in southern Idaho, while Trlica and Biondini (1990) found
that Ag. desertorum did not use all of the available deep (60 cm)
soil water at their study site in eastern Wyoming that received
nearly 100 mm more annual precipitation than Rush Valley. Cline
et al. (1977) found that soil water depletion in areas dominated
by B. tectorum was only about one-half that of the sagebrush
steppe because B. tectorum was not efficient at removing water
from depths below 0.5 m. Greater quantities of late summer/fall
vadose zone water was also found in B. tectorum (>15-cm depth)
and Ag. desertorum (>125-cm depth) communities, than in a mixed
sagebrush–bunchgrass community in south-central Washington
(Kremer and Running, 1996). Williamson et al. (2004) found conversion of chaparral to perennial grasslands resulted in differences
in vadose zone water distribution with depth during the dry season
and increased storage of water in the deeper vadose zone, especially
during years of average and above-average precipitation.
www.VadoseZoneJournal.org | 8
Recent studies in arid and semiarid ecosystems suggest that percolation of deep (>1 m) soil water has historically been rare and
that there is very little temporal variation in water potential at
these depths (Andraski, 1997; Scanlon et al., 1999; Seyfried et al.,
2005). However, our data indicate that there has been considerable
accumulation of soil moisture within the B. tectorum community,
as evidenced by the greater overall water content within the vadose
zone below 0.5 m compared to the other communities. A similar
accumulation was not found for Ag. desertorum, suggesting that
water accumulation in 1 yr may be used in subsequent years. Hence,
conversion of shrub steppe communities to exotic annuals such
as B. tectorum may result in increased vadose zone water content
at deeper depths by increasing the quantity of water subject to
downward displacement. If this deep percolation continues, there
may eventually be a reconnection of previously dry deep soils
with groundwater. In areas with poor groundwater quality, this
reconnection could lead to considerable soil degradation due to
increased soil salinity, as has been seen in some agricultural systems
in Australia and California (Dunne and Leopold, 1978; Schofield,
1992; Pierce et al., 1993; Bleby et al., 1997; Hobbs, 1998; Cramer
and Hobbs, 2002) and in afforestation of grasslands (Jobbagy and
Jackson, 2004).
Accumulation of water in B. tectorum and perhaps in Ag. desertorum communities may also change basic belowground processes
involved in nutrient cycling. Studies have shown that conversion
of shrub-lands to B. tectorum communities results in enhanced
decomposition and depletion of soil organic matter (Norton et al.,
2004) as well as increases in mineralization rates and NO3 concentrations (Norton et al., 2004; Saetre and Stark, 2005), and C and N
inputs to the soil (Hooker et al., 2008). Because cheatgrass senesces
early in the growing season, mineralization may extend beyond the
period of plant uptake. A possible consequence of this increase in
NO3 during periods of plant inactivity will be increased leaching
of NO3 resulting in accumulation of NO3 at depth (Walvoord et
al., 2003; Jackson et al., 2004; Hooker et al., 2008).
The rapid drying of the shallow vadose zone but lack of use of all
of available water at deeper depths by B. tectorum is consistent
with other studies of the effects of exotic annuals on the vadose
zone water balance. Holmes and Rice (1996) found lower lateseason matric potentials in perennial grass plots compared with
annual grass plots in a study of western California herbaceous
communities. Similarly, Borman et al. (1992) in a study of grasslands in southwest Oregon demonstrated that late, dry-season soil
matric potentials were lower among tussocks of the native perennial bunchgrass, Festuca idahoensis Elmer than among the exotic
annual forb, Cenchrus echinatus L.
The shallow vadose zone was rapidly dried by J. osteosperma at
the beginning of the season, and drying progressed downward
throughout the summer. However, J. osteosperma apparently did
not deplete water below our 1.2-m measurement depth during
either year, suggesting that lateral water movement below the petrocalcic layer may augment water available for transpiration. Leffler
et al. (2002) concluded that J. osteosperma vigorously utilizes water
in upper soils and remains tolerant of dry shallow soils as a strategy
to survive in harsh environments. This idea is consistent with rapid
responses of J. osteosperma to summer rainfall events that percolate
shallow soil layers (Leffler et al., 2002; Flanagan et al., 1992; Evans
and Ehleringer, 1994), although Donovan and Ehleringer (1994)
reported that adult J. osteosperma trees did not use measurable
quantities of summer precipitation at their northern Utah study
site. Use of summer rain by J. osteosperma may be related to interactions between root distribution and the timing and quantity of
rain events (Donovan and Ehleringer, 1994; Leffler et al., 2002).
Hydraulic Redistribution
Hydraulic redistribution of precipitation was found for all four
communities. Upward and downward redistribution of vadose
zone water occurs in several species common in the Great Basin,
including senesced B. tectorum (Leffler et al., 2005), J. osteosperma
(Leffler et al., 2002) Ar. tridentata (Richards and Caldwell, 1987,
Caldwell and Richards, 1989, Ryel et al., 2002, Ryel et al., 2003),
and Chrysothamnus nauseosus Pall. ex Pursh (Leffler et al., 2004).
Caldwell (1990) also presented evidence of upward redistribution
of soil water by Ag. desertorum. Our results indicate that while
hydraulic redistribution occurs in all of the species studied, the
amount of precipitation redistributed and depth of redistribution
varies among life forms, with woody species in this system exhibiting the greatest quantity of redistribution and the annual grass
the least. These redistribution patterns likely reflect differences
in root activity and phenology among these species. Bromus tectorum, Ag. desertorum, and Ar. tridentata have extensive shallow
root systems (Peek et al., 2005), as does J. osteosperma (Leffler et
al., 2002). However, the annual grass B. tectorum senesces locally
in early to mid summer (Leffler et al., 2005) and the perennial
grass, Ag. desertorum, senesces in mid to late summer, depending
on the availability of soil moisture (Trlica and Biondini, 1990).
Artemisia tridentata and J. osteosperma, on the other hand, are
both evergreen species that remain physiologically active during
the dry summer period. Hence, it is likely that the quantity of
precipitation redistributed by all of these species varies temporally
based not only on the quantity of precipitation and variation of
soil water potential with depth, but also with plant physiological
activity. It has been demonstrated, however, that B. tectorum roots
are capable of redistributing water even when the aboveground
portion of the plant is removed or fully senesced, as evidenced
by upward movement of water greater than would be expected
by other mechanisms (Leffler et al., 2005). Nevertheless, living
roots should have the potential to redistribute greater quantities
of water than nonliving roots in response to rainfall (Bauerle et
al., 2008) events because of increases in root length (Brady et al.,
1995) or through new root growth (Nobel and Sanderson, 1984;
Laurenroth et al., 1987???NOT IN REFERENCE LIST???; Peek
et al., 2005) in response to precipitation.
www.VadoseZoneJournal.org | 9
Comparison with Ecological Models
Rapid use of shallow vadose zone water in the spring by all species
does not support the “two-layer” model proposed by Walter (1971,
1973) for mixed species communities. Although the study communities were largely monocultures, the rapid use of the shallow
water resources was similar and indicates that species in mixed
communities would all heavily use this resource. Lack of niche partitioning for this limited resource is not surprising given that much
of the nutrient pool was likely associated with shallow vadose zone
water. Rapid use of this pool is consistent with the greatest biomass
production during the spring for all of our study species and little
temporal differentiation in maximum N uptake capacity in B. tectorum, Ag. desertorum, and Ar. tridentata (Bilbrough and Caldwell,
1997). Growth of these species is linked to the highly predictable
nature of the large spring pulse of water and nutrient recharge in
the Great Basin, and although each of the perennial species in this
study are capable of responding to summer pulses of water and
nutrients (e.g., Bilbrough and Caldwell, 1997;Leffler et al., 2002;
Ivans et al., 2003), long-term measurements on fitness-linked traits,
such as growth (Gebauer et al., 2002), suggest little long-term benefit from summer rain use.
Another important perspective for arid and semiarid soil water
dynamics is the “pulse reserve” model (Noy-Meir, 1973) where discrete pulses of precipitation are linked to plant processes, primarily
growth and reproduction (Goldberg and Novoplansky, 1997;
Schwinning and Ehleringer, 2001; Ivans et al., 2003; Schwinning
and Sala, 2004; Chesson et al., 2004). This model hypothesizes
that these individual events are important for plant performance
and may play a role in species composition (Chesson et al., 2004).
In our system, a single large spring pulse resulted from the accumulation of small events to result in a large single pulse that is
strongly linked to plant biomass production. Smaller events during
the late spring and summer add minimally to this pulse, and as
such seem to have limited effect on plant performance (Donovan
and Ehleringer, 1994). Reynolds et al. (2004) provided support
for this lack of correspondence with the pulse-reserve model by
indicating that individual precipitation events are not equal in
value and depend on antecedent soil water content and plant functional type and phenology. The “threshold-delay model” (Ogle and
Reynolds, 2004), developed in response to the limitations of the
pulse-reserve model, is more consistent with our system dynamics
since this model accounts for thresholds for precipitation events
that will elicit a rate response, physiological condition of plants,
and delay before a response to the pulse occurs.
The cessation of growth by the two grasses and Ar. tridentata as
the shallow vadose zone water pool became depleted supports the
function concept of “growth” pool as opposed to the deeper vadose
zone water resource pool, where use differed greatly among species
and where use was not linked to growth, but may serve as a “maintenance” pool for physiological activity during drought periods
(Ryel et al., 2008). The cessation of growth parallels the depletion
of water in the shallow vadose zone pool and strongly suggests that
this is linked to availability of nutrients, which are much greater
in concentration in shallow soils. Hooker et al. (2008) at nearby
sites found greater concentrations of various forms of nitrogen
were greater in shallow soils and that there was an accumulation
of NO3− in near-surface soils during dry periods in mid summer,
when accumulation by plants was likely minimal due to limited
diffusion. Jackson and Caldwell (1996) and Ryel and Caldwell
(1998) estimated for similar soils that diffusion of nutrients would
essentially stop below matric potentials of −1.5 MPa. Differences
in the time of depletion of the growth pool between years also corresponded to a difference in timing of cessation of growth, with
the later availability of this pool in 1999 linked to longer period of
plant growth in Ar. tridentata and both grasses. Use of the maintenance pool reflected phenology, with the greatest use in summer
by the two woody species. Artemisia tridentata reduced this pool
to similar levels in both years, while J. osteosperma depleted this
pool to a greater extent in the dryer year, perhaps due to demand
over a longer period or reduced lateral flow of vadose zone water in
this contrasting soil type. Numerous other studies have also documented that a majority of plant species rapidly deplete water from
shallow soils (e.g., Woods and O’Neal, 1965; Arya et al., 1975;
Herkelrath et al., 1977; Waring and Schlesinger, 1985; Yoder et
al., 1998) and then either become senescent or rely to varying
degrees on deeper soil water for maintenance (Comstock et al.,
1988; Manning and Groenveld, 1989).
66Synthesis
It has been suggested that the predictable nature of resource availability in the Great Basin has led to convergence on a general
pattern of water (Anderson et al., 1987) and nutrient (Bilbrough
and Caldwell, 1997) use among a majority of plant species. Our
research supports this idea, where we documented that dominant
species, ranging from an annual grass to trees, preferentially utilize
shallow vadose zone water during the spring, rapidly depleting this
resource. Because water is limiting for growth and reproduction
in arid environments, an obvious focal point of research regarding different water-use strategies has been to elucidate competitive
advantages based on the efficient acquisition of water in arid ecosystems (e.g., Eissenstat and Caldwell, 1988; Melgoza et al., 1990;
Yoder et al., 1998; Booth et al., 2003).
However, because nutrient availability, concentrated primarily
in shallow soils, is dependent on adequate moisture for diffusion
and bulk flow (Nye and Tinker, 1977), a major functional consequence of water depletion in the shallow vadose zone is that it
allows plants to influence nutrient availability. Hence, we postulate that cold-adapted plants in the Great Basin have converged
on a general pattern of rapidly utilizing soil moisture in shallow
depths, in part, to limit nutrient availability for other species. By
limiting nutrients, these plants also control the growth potential
www.VadoseZoneJournal.org | 10
of competitors, although disparity in the timing and rate of use
of this water pool among species will likely differentially affect
interactions with other species (Booth et al., 2003). Once shallow
soil water has been depleted, water in deeper soil layers is utilized
simply for maintenance for species that require water to maintain
physiological processes during drought periods, and as such does
not require such rapid use to affect the growth potential of competitors. This idea of rapid water use in shallow soils as a strategy
to influence nutrient availability is consistent with the important
role of nitrogen in determining plant productivity (Cline and
Rickard, 1973; Breman and de Wit, 1983; Fisher et al., 1988; Link
et al., 1995) and is also supported by root exclusion studies (e.g.,
Reichenberger and Pyke, 1990) and seedling establishment and
survival studies (Harris and Wilson,1970; Aguirre ,1991; Booth et
al., 2003; Humphrey and Schupp, 2004), which highlight the critical nature of water and nutrient availability in the shallow vadose
zone. Although this idea diverges considerably from current perspectives of plant water use strategies, there is evidence to support
it. For example, Link et al. (1995) observed that additional water
alone had no effect on growth of B. tectorum. Similarly, Uresk et
al. (1979) found only a weak relationship between soil water and
growth rates in B. tectorum. Cline and Rickard (1973) concluded
that 71% of B. tectorum growth was explained by nitrogen and
only 12% by soil water.
So, is vadose zone water really limiting? In the context of nutrient
resource availability through diffusion and mass flow in shallow
soils, water is likely a limiting resource for these processes and will
indirectly limit plant growth. But, what about deeper water or
water held at matric potentials insufficient for nutrient diffusion?
Although the grass species in our study senesced or went dormant
when shallow water became limiting for nutrient diffusion in
summer and did not use all available water at depth despite ample
access, the two woody species would still need sufficient water to
maintain transpiration and physiological function through this
period of drought. However, this resource could not become limiting for survival for these species, as stand-level mortality would
result when resources were depleted (Breshears et al., 2005, 2009b;
St. Clair et al., 2010); even the “fittest” individuals using the most
resources will suffer mortality when resources are depleted. Thus,
these maintenance pools must be sufficient to prevent subsequent
death to the woody plants, where dormancy or senescence is not
a life history option. Our work suggests that plant species have
evolved to limit use of deep water pools to only maintenance
functions (Ryel et al., 2008), even when abundant deep water is
available (McLendon et al., 2008), and plants may aid in the storage of this maintenance pool (Ryel et al., 2004). Predictability in
the delivery of sufficient water for plant water use from the maintenance pool becomes necessary for these long-lived woody species,
and some species may be able to shift to alternate resource pools of
different quantity and dynamic during drought periods (Nippert
et al., 2010). Circularly, the evolution of species that require the
dependability of these maintenance pools further suggests the
long-term persistence and lack of limitation of these pools.
These perspectives strongly suggest simple quantification of precipitation inputs is not sufficient to address plant water use and
availability, and that the perspective of total vadose zone water as
indicative of plant performance is not compelling. Additional elucidation of vadose zone water pool dynamics, as affected by plant
use and the processes involved in water delivery to plants, will be
essential for furthering our understanding of plant fitness, interactions among species for resources, species coexistence in arid and
semiarid ecosystems, and potential effects of climate change. Also,
it is likely that dynamics of vadose zone water pools will prove to be
important in less arid systems, where seasonal precipitation varies
and drought periods occur.
Acknowledgments
We thank Ann Mull for excellent field assistance, Charles Ashurst for superb instrumentation expertise, and Darrell Johnson for being very gracious to allow us to
establish our study site on his property in Rush Valley, Utah. This work was funded
by the National Science Foundation (DEB-9807097) and the Utah Agricultural
Experiment Station.
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