WETLANDS, Vol. 26, No. 1, March 2006, pp. 265–270
q 2006, The Society of Wetland Scientists
NOTE
EFFECTS OF SOIL WATER ON SEED PRODUCTION AND PHOTOSYNTHESIS
OF PINK SMARTWEED (POLYGONUM PENSYLVANICUM L.) IN
PLAYA WETLANDS
David A. Haukos1 and Loren M. Smith2
1
U.S. Fish and Wildlife Service
Department of Range, Wildlife, and Fisheries Management
Texas Tech University
Lubbock, Texas, USA 79409-2125
E-mail: david.haukos@ttu.edu
Wildlife and Fisheries Management Institute
Department of Range, Wildlife, and Fisheries Management
Texas Tech University
Lubbock, Texas, USA 79409-2125
2
Abstract: Soil moisture is frequently manipulated by wetland managers to achieve a desired composition
and production of plant species. Pink smartweed (Polygonum pensylvanicum) is a species valued by wetland
managers because of its potential seed production and food base for migrating and wintering waterbirds. To
improve ability to manage pink smartweed, we measured the influence of four growing-season soil moisture
regimes (flooded, field capacity, prescribed moist-soil management, and continuously dry) in playa wetlands
of the Southern High Plains of Texas. We used xylem water potential as an index of soil moisture available
to the plants and compared vegetation and seed production among the four moisture treatments. Daily net
photosynthesis was measured in each playa at 0900, 1200, and 1700 hours. The continuously-flooded wetland
had the lowest absolute xylem water potential value (20.294 MPa), followed by the playa at field capacity
(20.446 MPa), moist-soil managed playa (20.758 MPa), and dry playa (21.039 MPa). Vegetation biomass
(F3,40 5 97.7, P , 0.001) and seed production (F3,40 5 54.3, P , 0.001) of pink smartweed differed among
soil moisture treatments. Conducting moist-soil management (x̄ 5 6816 kg/ha) increased vegetation biomass
by 44% over the dry treatment (x̄ 5 4706 kg/ha). As soil moisture increased to field capacity (x̄ 5 9010
kg/ha) vegetation increased by 32% above the moist-soil treatment. Maintaining standing water in a playa
maximized vegetation production (x̄ 5 12497 kg/ha) of pink smartweed. Seed production was greatest when
soil moisture was maintained at field capacity (x̄ 5 695 kg/ha). There was no difference in seed production
between the moist-soil managed (x̄ 5 492 kg/ha) and flooded playas (x̄ 5 514 kg/ha), which was 28% lower
than the field capacity treatment. Seed production in the dry treatment (x̄ 5 377 kg/ha) was 46% lower than
the field capacity treatment and 25% below the flooded and moist-soil managed treatments. Net photosynthesis and xylem water potential decreased throughout the day in all treatments except continuously flooded,
which corresponded to the increased vegetation production in this treatment. Slight increases in water stress
relative to the continuously flooded treatment resulted in a redistribution of resources from vegetation to
seed production. Managers should consider maintaining wetland soils at field capacity to increase seed
production of pink smartweed.
Key Words: biomass, moist-soil management, photosynthesis, pink smartweed, playas, Polygonum pensylvanicum, seed production, Texas, water stress
INTRODUCTION
timum limit their rate of resource (i.e., nutrient) acquisition, growth, and reproduction (Grime 1989).
Available soil water influences production, availability
of soil nutrients, and nutrient transport by wetland
plants (Sculthorpe 1967, Kramer 1983, Pezeshki 1994,
2001, Pezeshki et al. 1999). Soil water is frequently
manipulated by wetland managers to promote certain
Many abiotic factors affect growth and reproduction
of plants including light, nutrients, temperature, and
soil moisture (Raven et al. 1986). Although most wetland plants can persist under a range of environmental
conditions, levels of these factors deviating from op265
266
plant communities for wildlife (Smith et al. 1989), but
seldom have managers considered the influence of soil
moisture on production within a species.
Water deficits primarily occur when availability of
soil water decreases to cause a decline in plant physiological activities such as photosynthesis within a species or genotype (Kramer 1983, Osmond et al. 1987,
Brown 1995). Extended water deficits, or drought,
generally result in stressed plants producing fewer, if
any, flowers and seeds (Brown 1995). Further, because
nutrient uptake is reduced under water-stress conditions (Haukos and Smith 1996), the nutritional value
of plants for wildlife can be diminished (Haukos and
Smith 1995). Extended flooding can also limit growth
and production of many wetland plants (McKee and
Mendelssohn 1989, Ernst 1990). Under flooded conditions, many species decrease photosynthetic rates
and production in response to reduced conditions (Pezeshki 1994, 2001). Death from anoxia or exposure to
high levels of ferrous iron or sulfide may also occur
(Levitt 1980, Otte 2001).
Moreover, as soil moisture varies, plants may shift
production to either vegetative or reproductive (i.e.,
seed) strategies (e.g., Raven et al. 1986, Sultan and
Bazzaz 1993, Otte 2001). From a wetland management
perspective, a primary goal is often to increase seed
production for wildlife, which has the foremost influence on carrying capacity of wetland-dependent birds
during migration and winter (Anderson and Smith
1998). Moist-soil management is a technique used by
managers to increase seed production of certain annual
plants in wetlands, including playas, to enhance wildlife habitat (Haukos and Smith 1993). Seeds are then
available for consumption by wetland-dependent birds
and other wildlife (Smith et al. 1989). The term
‘‘moist-soil’’ refers to the soil condition wetland managers attempt to create to promote germination,
growth, and seed production of these plants (Haukos
and Smith 1993). However, the influence of varying
levels of soil moisture, beyond that needed to establish
certain communities, on production are seldom examined, leading to erratic annual seed production (e.g.,
Haukos and Smith 1993, Anderson and Smith 1998,
Smith et al. 2004).
Pink smartweed (Polygonum pensylvanicum L.) is a
moist-soil species often targeted by wetland managers
because of its potential for high seed production and
superior relative nutritive quality (e.g., Haukos and
Smith 1993, 1995). Pink smartweed is one of the most
common and widespread species in playas and capable
of persisting under a wide range of soil moisture conditions (Wieland and Bazzaz 1975, Haukos and Smith
1997, 2004). However, optimal soil-moisture for establishment, growth, and reproduction appear to be at
or near field capacity (Haukos and Smith 2004). Man-
WETLANDS, Volume 26, No. 1, 2006
agers in the semi-arid region of the Great Plains and
western United States strive for optimum pink smartweed growth conditions to maximize cover and production of nutritious seeds with minimum water applications because of increasing water costs and declining water availability (Smith 2003). Our objective
was to examine the influence of growing season soil
moisture, as indexed by xylem water potential, on pink
smartweed production (vegetation and seeds) and daily
photosynthesis patterns to provide improved prescriptions for management of pink smartweed.
METHODS
Study Area
The study was conducted in four playa wetlands in
Lubbock (N 338 399, W 1018 499) and Floyd (N 348
009, W 1018 209) counties in the Southern High Plains
of Texas, USA. Playas are shallow, depressional recharge wetlands that naturally receive water only
through precipitation and runoff; they lose water
through recharge to the underlying Ogallala Aquifer,
evaporation, and transpiration (Smith 2003). There are
more than 19,000 playas in the Southern High Plains
of Texas (Guthery and Bryant 1982), comprising approximately 2% of the region (Haukos and Smith
1994), and they are underlain with the hydric soil
Randall clay (Allen et al. 1972). Precipitation averages
48 cm annually in the region, with most occurring
from thunderstorms from May to September (Bolen et
al. 1989).
Soil Moisture Conditions
We randomly selected four playas (x̄ 5 6.08 ha) that
had water-management capabilities allowing us to manipulate soil moisture. Based on sampling of extant
vegetation during the previous growing season, pink
smartweed was a dominant species in all playas (Smith
et al. 2004). Moist-soil conditions (i.e., saturated, exposed soil) were created either via wetland drawdown
or flooding using irrigation systems from late March
through early April to promote germination of pink
smartweed (Haukos and Smith 1993). Following establishment and initial growth of pink smartweed, one
of four growing-season water regimes was randomly
applied to each playa. In playa A, we maintained
flooded conditions throughout the remainder of the
growing season (1 June–31 August) by sustaining 3–
5 cm of water on the soil. Playa A represented saturated soil and reducing conditions as water filled air
spaces in the soil matrix. Soil of playa B was maintained at approximate field capacity by establishment
of satuated soil with no standing surface water each
Haukos & Smith, SOIL MOISTURE AND SMARTWEED PRODUCTION
week. Because playa soils have a negligible drainage
rate following wetting, approximate field capacity
should be established shortly after soil saturation (Cassel and Nielsen 1986). By sustaining approximate field
capacity through frequent irrigation events in playa B,
the maximum amount of water was held by the soil
throughout the growing season without inducing reducing conditions. Playa C was managed according to
recommendations of Haukos and Smith (1993) by creating moist-soil conditions in June and August and allowing fluctuation among flooded, moist, and dry environments the remainder of the growing season. Playa
D was kept dry (plants wilting during the day)
throughout the growing period.
Field and Laboratory Procedures
Beginning 1 June 1993, we determined xylem water
potentials (i.e., matric potential) every two weeks
through 31 August. Determination of xylem potential
provides a measure of total water potential (Waring
and Cleary 1967, Turner 1988). Xylem pressure potential is therefore an accurate measure of water available to a plant. We determined xylem water potentials
at dawn using a pressure chamber (Scholander et al.
1965, Turner 1988). Ten replicate plants were randomly selected for xylem potential measurements in each
playa for each biweekly sampling period. A leafy portion of the plant stem was placed in the pressure bomb
chamber. Pressurized nitrogen was introduced into the
chamber until water was forced from the xylem on the
exposed cut stem. At the pressure when water was
forced from the stem, water potential was measured as
2MPa. Water potential values were averaged first for
each sampling period, and these were averaged across
the growing season for each playa to provide relative
growing-season available soil water for pink smartweed among the four playas.
Following seed maturity in September, we clipped
12 randomly selected 0.25-m2 plots in homogenous
stands of pink smartweed in each playa. Plants were
clipped at the soil surface. In the field, we separated
seeds from vegetative matter. In the laboratory, we
dried each to a constant mass at 408C and weighed
each sample to the nearest 0.01 g. We converted seed
and vegetation biomass measurements to kg/ha prior
to analyses.
Over a four-day period in late August, we determined daily net photosynthesis patterns for 10 randomly selected smartweed plants in each playa. We
took photosynthesis readings in each playa at 0900,
1200, and 1700 hours using a LI-6200 portable photosynthesis system (Licor Inc., Lincoln, NE, USA).
This technique used a closed canopy system to provide
a measure of CO2 flux. The difference in CO2 levels
267
of air entering the chamber and leaving the chamber
provided an index of photosynthetic rate. The assimilation rate of CO2 is presented as mmhos CO2/L/hr,
where L represents the volume of air pumped through
the chamber and characterizes an average net photosynthesis rate. We also determined xylem pressure potential at each of these times in each playa for relative
comparison.
Statistical Analyses
Average growing-season xylem water potential defined the differences among the four treatments for the
independent variable of soil moisture. We compared
seed and vegetative biomass among these four treatments using a completely randomized design analysis
of variance (ANOVA; SAS 1999). Following a significant F-test (P , 0.05), we used a least-significant
difference test to separate treatment means. Statistically, inference of our results is limited to the sampled
playas. However, because of the homogeneity of playa
structure and response to moist-soil management
(Haukos and Smith 1993, 1994), results should be repeatable to other similarly managed playa wetlands.
The effects of soil moisture levels on daily trends in
net photosynthesis were determined by using ANOVA
comparing playas within each time period.
RESULTS
The continuously-flooded playa had the lowest absolute xylem water potential value (20.294 MPa), followed by the playa at field capacity (20.446 MPa),
moist-soil managed (20.758 MPa), and continuously
dry playas (21.039 MPa). Vegetation biomass differed
(F3,40 5 97.7, P , 0.001) among soil moisture treatments. As soil moisture increased, vegetation biomass
increased in each treatment (Figure 1). Conducting
moist-soil management increased vegetation biomass
by 44% over the dry treatment. Maintaining soil moisture at field capacity increased vegetation by 32%
above the moist-soil treatment. Further, maintaining
standing water during the growing season in a playa
maximized vegetation production of pink smartweed
(Figure 1).
Seed production of pink smartweed differed (F3,40 5
54.3, P , 0.001) among soil moisture treatments but
had a different pattern than vegetation biomass (Figure
1). Seed production was greatest when soil moisture
was maintained at approximate field capacity. There
was no difference in seed production between the
moist-soil and flooded treatments, which was 28%
lower than the field capacity treatment. Seed production in the dry treatment was 46% lower than the field
268
WETLANDS, Volume 26, No. 1, 2006
capacity treatment and 25% below the flooded and
moist-soil managed treatments (Figure 1).
Daily photosynthesis patterns were similar to changes in xylem water potential throughout the day (Figure
2). The flooded treatment had consistent net photosynthesis and xylem water potentials from 0900 through
1700 hours. The remaining three treatments showed
decreasing net photosynthesis and xylem water potential as the day progressed. At 0900, all treatments differed with the field capacity treatment at the greatest
net photosynthesis (Figure 2). However, by 1200, the
flooded and field capacity treatments had similar net
photosynthesis, as did the moist-soil and dry treatments. This pattern was evident at 1700 as well (Figure 2). Xylem water potential never surpassed 22.0
MPa by 1700 hours in any treatment.
DISCUSSION
The goal of many wetland managers is to provide
soil moisture conditions that maximize seed production and biomass of certain plant species. Pink smartweed can persist in a wide range of environmental
conditions within playas (Haukos and Smith 2004) and
is a target species of management (Haukos and Smith
1993, 1995). Soil moisture during the growing season
greatly affected vegetation biomass and seed production of pink smartweed, but soil moisture influenced
vegetation and seed production in a different manner.
Pink smartweed vegetative production increased
with increasing soil moisture. The greatest values of
biomass were attained when a water depth of a few
cm was maintained on the soil surface. However, seed
production was greatest when soils were maintained at
field capacity throughout the growing season. Soils
subjected to flooded conditions or prescribed periods
of moist-soil management had similar seed production,
both of which were greater than that for the dry soils.
Sultan and Bazzaz (1993) also found variation in vegetative production and seed production of an ecologically similar smartweed species (Polygonum persicaria L.) as soil moisture varied but found that optimal
plant-growth conditions occurred when the soil was at
field capacity.
As net photosynthesis decreased throughout the day
in soils that were not inundated, absolute values of
xylem water potential increased. Net photosynthesis,
however, did not change during the day in the flooded
soils, indicating that pink smartweed can translocate
water rapidly even during high water-demand periods
provided water was available. Although the absolute
values likely vary throughout the growing season, we
believe that the relative photosynthetic patterns remain
consistent, contributing to the difference in production
among the treatments. In the Southern High Plains,
Figure 1. Average vegetation and seed biomass (6 1 SE)
of pink smartweed grown in playa wetlands subjected to
continuously flooded, field capacity, fluctuating wet/dry, and
dry soil conditions in the Southern High Plains of Texas
during August 1993. Means associated with the same uppercase letter do not differ (P . 0.05) between treatments.
temperatures commonly exceed 308C and are at their
maximum daily highs at 1700 hours (Bolen et al.
1989). Wieland and Bazzaz (1975) also found that
pink smartweed growing in water did not have an increase in xylem water potential during peak demand
periods.
Pink smartweed plants in the soils where soil moisture was at field capacity had the highest net photosynthetic activity during the early time period, but it
decreased during the day to levels measured in the
flooded treatment by 1200. At 1700 hours, the rate of
plants in those soils remained similar despite the widening discrepancy in xylem water potential by late afternoon (20.32 MPa vs. 20.78 MPa). This suggests
that pink smartweed has adaptive plasticity in resource
allocation in response to levels of soil moisture. Under
inundated conditions, pink smartweed does not reduce
daily photosynthetic rate and, as a result, allocates re-
Haukos & Smith, SOIL MOISTURE AND SMARTWEED PRODUCTION
Figure 2. Daily mean net photosynthesis and xylem water
potential of pink smartweed in four playa wetlands with different soil moisture regimes (continuously flooded, field capacity, fluctuating wet/dry, and dry soil conditions) measured at 0900, 1200, and 1700 in the Southern High Plains
of Texas during August 1993. Means associated with the
same uppercase letter do not differ (P . 0.05) between treatments within each time period.
sources to maximize biomass. Under other soil-moisture conditions, pink smartweed alters resource allocation to maximize seed production at the expense of
vegetative production likely in response to daily increasing water stress, which may be creating a similar
response within these plants (e.g., closing of stomata
guard cells). Sultan and Bazzaz (1993) reported that
the ecologically-related Polygonum persicaria showed
phenotypic plasticity by responding to declining soil
moisture with a reduction in whole plant photosynthesis, increasing root biomass to expand area of potentially available water and increasing proportional resource allocation to fruit. We found similar responses
by pink smartweed in playas.
Wetland plants possess various characteristics that
enable them to survive periodic soil saturation and accompanying changes in soil chemistry (Pezeshki
2001). Plants capable of persisting in playas must be
269
able to survive and reproduce under a wide range of
unpredictable, dynamic environmental conditions.
Common species in playas, such as pink smartweed,
adapt to changes in soil moisture by altering allocation
of resources to ensure seed production and persistence
within playas. Warwick and Brock (2003) stated that
in less predictable, drier climates, depth, duration, and
season (month) of flooding (i.e., soil moisture) influence completion the wetland plant life cycle through
selection for sexual reproduction.
Although we did not examine relative nutritional
value of seeds among the soil-moisture treatments,
there exists a potential for changing nutritional quality
of seeds under varying soil moisture levels (Haukos
and Smith 1994, Pezeshki 1994). Despite similar production of seeds under inundated and moist-soil management treatments, it is possible that nutritional quality differed between the treatments. Reduced soil conditions under flooding may affect nutrient uptake and
availability (Pezeshki 1994, Haukos and Smith 1995,
1996), which may alter nutritional quality of seeds.
For managers with the objective of increasing wetland carrying capacity for wetland dependent birds,
these results have important implications. First, maximum seed production will be achieved by maintaining
soils at field capacity conditions. If areas are inundated
even a few cm for extended periods, vegetative biomass will increase and seed biomass will decrease, reducing carrying capacity for seed-eating wetland wildlife. If greater vegetative biomass is necessary more
from a cover or secondary production standpoint (i.e.,
invertebrates), managers will need to maintain shallow
inundated conditions for a few weeks following germination. If water is kept deeper than a few cm, it will
reduce survival and vegetative biomass of the remaining plants (Haukos and Smith 2004). Managers can
increase vegetative and seed production of smartweed
stands in relatively dry soil conditions up until fruiting
times by simply providing additional water following
germination.
Under average conditions, we expected to produce
532 kg/ha of seed in moist-soil managed playas (Haukos and Smith 1993). This value is similar to that produced in the flooded and moist-soil playas of this study
but less than that produced when soil moisture is maintained at field capacity throughout the growing season.
The potential increase in seed production by maintaining a playa at field capacity is approximately 150 kg/
ha. This improvement in seed production increases options available for management of playa wetlands. If
only a few playas are available to a manager to conduct moist-soil management, then perhaps the most effective use of water would be to maintain soil moisture
at field capacity to maximize seed production and potential wildlife food availability. If a number of playas
270
WETLANDS, Volume 26, No. 1, 2006
are available for management and funds for water application are limited, then managers may want to follow previously recommended prescriptions (Haukos
and Smith 1993) and produce slightly less seed in all
managed playas. This trade off between water cost and
seed production should be assessed by each playa
manager.
These results reflect the findings for one species,
pink smartweed. Moist-soil communities typically are
more diverse and additional studies are needed on other common species. Moreover, moist-soil studies are
needed on the response of the entire community to
lower water application prescriptions that would maximize food production for wetland birds while using
the least amount of water.
ACKNOWLEDGMENTS
Funding was provided by the U.S. Fish and Wildlife
Service through the efforts of J. W. Haskins. R. Prather
and T. Monasmith assisted in the field and laboratory.
L. M. Smith was funded by the Caesar Kleberg Foundation for Wildlife Conservation. This is manuscript
T-9-1083 of the College of Agricultural Sciences and
Natural Resources, Texas Tech University.
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