WETLANDS, Vol. 21, No. 2, June 2001, pp. 274–280
q 2001, The Society of Wetland Scientists
TEMPORAL EMERGENCE PATTERNS OF SEEDLINGS FROM
PLAYA WETLANDS
David A. Haukos1 and Loren M. Smith
Department of Range, Wildlife, and Fisheries Management
Texas Tech University
Lubbock, Texas, USA 79409
1
Present Address: U.S. Fish and Wildlife Service
Department of Range, Wildlife, and Fisheries Management
Texas Tech University
Lubbock, Texas, USA 79409
E-mail: davidphaukos@fws.gov
Abstract: Playas undergo dynamic environmental changes throughout the growing season, resulting in the
need for a persistent seed bank for plants to respond to these changes. Therefore, we investigated seasonal
germination patterns of species found in seed banks of playa wetlands. We used the seedling-emergence
technique to determine recruitment patterns from seed banks of eight playas. In the greenhouse, seed-bank
samples were subjected to two treatments, drawdown or flooded, over a 210-day duration divided into seven
30-day time periods. In both treatments, seedling emergence differed among time periods and species but
was similar among playas. Approximately 52% of drawdown seedlings and 44% of seedlings occurring in
the flooded treatment germinated in the first 30 days. Plants occurring in playa seed banks had variable
germination strategies. Three patterns for common (.5% occurrence) species were identified in the drawdown treatment: (1) early germinators (those species that germinated rapidly after exposure to treatments
with low germination during the remainder of time periods), (2) late germinators (those that germinate after
specific environmental conditions have existed for some time), and (3) continuous germinators (those with
even germination rates throughout submersion). Two patterns were found for common species in the flooded
treatment: (1) early germinators and (2) continuous germinators. Germination throughout the period of suitable environmental conditions was the dominant strategy for persistence in the unpredictable playa environment. With only a few exceptions, species persisting in seed banks of playas do not show germination for
all available seeds upon creation of suitable environmental conditions but rather use viable dormant seeds
as a hedge against the unpredictable environment.
Key Words:
playas, seed bank, seedling emergence, Southern High Plains, wetlands
INTRODUCTION
Natural disturbance created by fluctuating water levels serves to maintain a species rich community in wetlands (Smith and Kadlec 1985). Typically shallow,
temporary wetlands, playas can be considered an
‘‘edge’’ community (Brock and Casanova 1997) and
are rich in plant species. Following germination, most
common wetland ‘‘edge’’ species either tolerate moderate changes in the environment or respond to changing conditions with changes in physiology or life form
(Brock and Casanova 1997). Plant communities in playas are diverse and represented by species capable of
persisting in the heterogeneous playa environment
(Haukos and Smith 1997). The majority of plant species persisting in playas are represented by ecotypes
capable of relatively rapid germination, growth, and
reproduction in the typically harsh playa environment.
The feature of closed watersheds combined with fluc-
Playas are small, depressional wetlands most commonly occurring in the U.S. in the High Plains Region
of the Southern Great Plains (Haukos and Smith
1994). Numbering approximately 25,000, the majority
of playas are characterized by (1) a dynamic, unpredictable hydroperiod, (2) being positioned as the terminus of a closed watershed, and (3) a changing,
adaptable flora and fauna responding to the frequently
fluctuating wetland conditions (Haukos and Smith
1994, 1997). The lack of a ground-water connection
limits the flood duration in playas to periods following
runoff events from precipitation, which usually occurs
from May through September. Recent changes in irrigation practices have limited irrigation runoff that
once extended flood duration and flood frequency in
playas.
274
Haukos & Smith, SEEDLING EMERGENCE IN PLAYAS
tuating germination conditions limits establishment of
wetlands plant communities through proximate colonization by propagules from other wetlands. These
characteristics result in the presence of extant playa
flora being virtually dependent on the seed bank (Haukos and Smith 1993). There is usually limited time for
vegetative reproduction following germination.
Species and their relative abundance in the existing
plant community of each playa are dependent on three
factors, which are dictated by patterns of natural disturbance (Haukos and Smith 1993). First, past environmental conditions in each playa dictate which species are potentially able to germinate and reproduce,
thus adding to the seed bank. Second, the rate of sediment accumulation in a playa influences species response to the changing environment by burying potentially viable propagules (Luo et al. 1997, 1999). Finally, the persistence of existing environmental conditions restricts the number of species capable of
germinating in the wetland (Pederson and Smith
1988).
Typically, plant species persisting in an unpredictable environment such as playas are characterized by
ungerminated viable seeds upon creation of suitable
conditions as a hedge against the unknown future
(Freas and Kemp 1983, Rosenzweig 1996:171). We
predict that this strategy would be shown in germination patterns of common plant species persisting in
playa seed banks. Moreover, knowledge of seed-bank
response to the period of existence of certain environmental conditions (e.g., drawdown, flood) is essential
for evaluation of future wetland management techniques. A number of playas are, for various reasons,
being subjected to attempts to stabilize their hydroperiod over time either through continuous flooding or
water diversions to maintain perpetual dry conditions.
These types of management usually limit the productivity of wetlands (Smith 1990). Efforts to remove natural disturbance from playas require an assessment of
the seed bank response to these unnatural conditions.
We investigated temporal germination patterns of
plant species occurring in seed banks of playa wetlands under continuous drawdown and flooded management conditions (Pederson and Smith 1988). Germination strategies should be reflected in seedlingemergence patterns from existing seed banks (Poiani
and Johnson 1988). Our specific objectives were to test
predictions that following creation of flooded and
drawdown wetland environmental conditions, (1) time
since the start of the germination trial does not affect
species recruitment from seed banks in playas, (2) recruitment patterns across time are similar for species
germinating in flooded and drawdown conditions, and
(3) recruitment patterns are similar among playas.
275
METHODS
Seed-Bank Assessment
We assessed seed banks of eight playa wetlands
within a three-county area (Floyd, Hale, and Crosby)
of the Southern High Plains (SHP) of Texas, USA for
temporal emergence patterns. Study playas averaged
13 ha in area. Field vegetation at the time of seedbank collection was dominated by Polygonum spp. and
Echinochloa crusgalli. Nomenclature follows Haukos
and Smith (1997).
Seed-bank assessment followed the seedling-emergence technique (Smith and Kadlec 1983, Pederson
and Smith 1988). Poiani and Johnson (1988) found the
method to be accurate in evaluating composition of
wetland seed banks. Twenty soil samples (20 X 20 cm)
were collected to a depth of 4 cm in each playa. Samples were collected 10 m apart along a 200-m transect
from the southwest corner to the center of the playa.
This stratified sampling scheme followed the prevailing wind direction and incorporated the entire playa
elevation gradient. Sample collection was made during
late March, when all playas were dry, thus allowing
for any required dormancy break to occur as a result
of overwinter chilling (Baskin and Baskin 1985).
In a greenhouse, we mixed, divided, and placed
each soil sample into two, 4 X 10 X 20 cm plastic
trays, lined with 2 cm of sterilized potting soil (i.e.,
40 trays per playa representing the 20 sample points).
One tray of each sample pair was placed in drawdown
treatment, with the sample watered daily with distilled
water to maintain drawdown conditions without standing water. The remaining tray was subjected to constantly flooded conditions (4 cm of distilled water
maintained over the tray). All of the trays in the flooded treatment were placed in a single, long container.
We monitored the samples from 1 April through 31
October 1989, which corresponds to the growing season in Floyd County (Neitsch and Blackstock 1978).
Samples were subjected to the same photoperiod and
similar daily temperatures as found in the field.
We identified and counted emerged seedlings weekly. Identified seedlings were carefully removed to prevent soil disturbance. Unidentified seedlings were
transplanted to their own containers and grown until
each could be positively identified. Seedlings were
identified following Correll and Johnston (1979) and
Godfrey and Wooten (1981), with verification by
voucher specimens.
Statistical Analyses
Seedling emergence data were assembled into
groups representing 30-day intervals. The percent of
emerging seedlings in each time period was calculated.
276
WETLANDS, Volume 21, No. 2, 2001
Figure 1. Percent total seedlings emerging during seven
30-day periods in drawdown and flooded treatments from
the seed banks of 8 playa wetlands on the Southern High
Plains of Texas. Percents of germination for the time periods
within a treatment with the same letter are not different
based on multivariate separation of periods.
A repeated-measures multivariate analysis of variance
(MANOVA) was used to test the following hypotheses: (1) time since inception of either drawdown or
flooded conditions had no effect on seedling emergence; (2) the effect of time on seedling emergence
was the same for all playas; and (3) the effect of time
on seedling emergence was the same for all species.
The variable time consisted of seven 30-day periods.
Chi-square analysis was used to test frequency of occurrence of all seedlings across time periods within
treatments.
A randomized block design MANOVA was used to
compare seedling densities of common species (.5%
of total seedlings) across 30-day periods. Estimates of
seedling density (seedlings/m2) were log-transformed
prior to analyses to satisfy parametric statistical assumptions. Playas served as blocks in each analysis.
Following a significant (P , 0.05) MANOVA, species
were separated based on multivariate techniques (Harris 1975:104) for allocation into similar germination
groups. Repeated-measures analysis of variance was
used to examine effects of independent variables on
each dependent variable following a significant MANOVA.
RESULTS
Germination began within 3 days of being exposed
to treatments. A total of 4,549 and 6,456 seedlings
germinated in the drawdown (34 species) and flooded
(11 species) treatments, respectively. Approximately
half of the seedlings germinated within the first 30
days in both treatments, 43.6% and 52.1% in the drawdown and flooded treatments, respectively (Figure 1).
Following the first 30 days, total germination in the
drawdown treatment remained relatively constant
across the time periods, whereas in the flooded treat-
ment, germination, gradually decreased throughout the
study (Figure 1).
In the drawdown treatment, (1) time since establishment of drawdown conditions affected seedling emergence (Wilks’ lambda 5 0.13, P , 0.001); (2) the
effect of time on seedling emergence was similar for
all playas (Wilks’ lambda 5 0.41, P 50.77); and (3)
the effect of time on seedling emergence differed
among species (Wilks’ lambda 5 0.008, P , 0.001).
In the flooded treatment, (1) time since flooding affected seedling emergence (Wilks’ lambda 5 0.04, P
, 0.001); (2) the effect of time on seedling emergence
was similar among playas (Wilks’ lambda 5 0.14, P
5 0.14); and (3) the effect of time on seedling emergence differed among species (Wilks’ lambda 5 0.08,
P , 0.001).
Seedling densities of common (.5% of total) wetland species (8 drawdown, 5 flooded) differed among
the 30-day periods (drawdown, Wilks’ lambda 5 0.01,
P , 0.001; flooded, Wilks’ lambda 5 0.07, P , 0.001)
(Table 1). In both treatments, seedling densities of
each common species differed (P , 0.04) across periods (Table 1, Figures 2, 3). Multivariate separation
of common species across time periods indicated three
of strategies of germination in the drawdown treatment
and two in the flooded treatment (Table 2).
DISCUSSION
Similar to plants in vernal pools (Bliss and Zedler
1998), germination in playas was initiated within a few
days of being exposed to the treatments. Roughly 50%
of seedlings in our treatments germinated within the
first 30 days of being exposed to treatments, with variable rates thereafter. Specifically, in the drawdown
treatment, 43.6, 49.7, and 63.0% of seedlings germinated within 0–30, 0–60, and 0–90 days, respectively;
whereas, in the flooded treatment, 52.1, 62.2, and
77.0% of seedlings germinated within 0–30, 0–60, and
0–90 days, respectively. These are smaller percentages
than reported from seed-bank studies of other, more
predictable, wetland environments. From a Canadian
freshwater marsh, Pederson (1983) found that approximately 90% of seedlings were recruited within the
first three months of being exposed to drawdown and
flooded treatments. Welling et al. (1988) also reported
that nearly all recruitment of seed banks from prairie
pothole wetlands in the drawdown treatment occurred
in the first two months of exposure. Haag (1983) reported that most seedlings from seed banks of a Canadian lake emerged within 30 days after being exposed to treatments. Our data support the hypothesis
that plants in playas are capable of rapid response to
changes in the playa environment but can also hedge
against the uncertain future if not all seed capable of
abcd
75
17
81
57
59
26
12
71
24
49
(27)bc
(41)a
(86)a
(386)a
(73)a
47
878
2277
414
197
(31)a
(10)b
(34)b
(51)ab
(29)b
36 (31)a
27 (17)b
9 (6)bc
0b
3 (2)b
39 (18)a
28 (18)a
47 (43)bc
9 (6)bc
7 (3)bc
0e
0b
4 (2)b
12 (8)ab
0b
13 (12)cd
(207)a
(46)ab
(12)ab
(2)a
(54)a
0c
0b
0d
307
87
38
9
102
(15)c
(7)b
(40)bc
(20)ab
(19)bc
61–90
x̄ (SE)
31–60
x̄ (SE)
0–30
x̄ (SE)
Means with the same superscript do not differ within species across months (P . 0.05).
Drawdown
Echinochloa crusgalli (L.) Beauv
Rumex crispus L.
Polygonum lapathifolium L.
Polygonum pensylvanicum L.
Chenopodium leptophyllum Wats.
Rorippa sinuata (Nutt.) Hitchc.
Ammannia auriculata Willd.
Coreopsis tinctoria Nutt.
Submerged
Sagittaria longiloba Engelm.
Echinochloa crusgalli
Eleocharis macrostachya Britt.
Heteranthera limosa (Sn.) Willd.
Ammannia auriculata
Treatment
Species
41
2
21
9
70
(15)ab
(1)bc
(9)cd
(8)bc
(30)b
9 (8)c
6 (5)c
26 (16)ab
0b
3 (2)b
12 (8)ab
5 (3)ab
15 (12)bc
91–120
x̄ (SE)
33
1
12
7
72
(17)bc
(0.7)c
(5)cd
(6)bc
(40)bc
21 (17)a
21 (9)b
30 (17)a
0b
1 (1)b
21 (11)ab
9 (6)ab
52 (35)b
121–150
x̄ (SE)
Days Following Initiation of Treatments
30 (21)bc
0.4 (0.4)c
6 (3)e
0c
35 (24)cd
11 (9)bc
5 (4)c
5 (3)cd
0b
0b
2 (2)b
28 (21)a
213 (87)a
151–180
x̄ (SE)
25 (9)b
0c
5 (2)e
0c
35 (25)cd
14 (13)bc
6 (4)c
3 (3)de
0b
0b
2 (1)bc
5 (2)ab
0d
181–210
x̄ (SE)
3.11
13.28
30.18
3.68
9.82
16.18
7.29
11.32
9.45
6.10
3.70
2.43
12.92
F
0.013
0.0001
0.0001
0.005
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.005
0.042
0.0001
P
Univariate
Across Months
Table 1. Germination means (SE) (seedlings/m2, averaged from 8 playas and 20 samples/playa) for common (.5% of total seedlings) drawdown and submerged treatment plant
species from the Southern High Plains of Texas. Drawdown treatment was watered daily with no standing water, and the submerged treatment was under 4 cm of water.
Haukos & Smith, SEEDLING EMERGENCE IN PLAYAS
277
Figure 2. Percent of total seedlings emerging during seven
30-day periods for common species representing the three
identified germination strategies under drawdown conditions
from the seed banks of 8 playa wetlands on the Southern
High Plains of Texas. Groups 1, 2, and 3 represent early,
late, and continuous germinators, respectively.
germinating in a specific environmental condition germinates immediately.
Unlike most seed-bank studies of freshwater wetlands (e.g., van der Valk and Davis 1978, Smith and
Kadlec 1985), more seedlings germinated in the flooded treatment compared to the drawdown treatment.
This is likely the result of different species being represented in the drawdown-adapted seed bank. The
278
WETLANDS, Volume 21, No. 2, 2001
Table 2. Independent variable separation from multivariate analysis of variance for common plant species in drawdown and flooded treatments for 8 playas on the Southern High Plains of Texas.
Drawdown
Flooded
Chenopodium leptophyllum
Polygonum pensylvanicuma
Rumex crispusab
Echinochloa crusgalliab
Polygonum lapathifoliumb
Coreposis tinctoriac
Rorippa sinuatac
Ammannia auriculatac
a
Heteranthera limosaa
Echinochloa crusgallia
Eleocharis macrostachyaa
Ammannia auriculatab
Sagittaria longilobab
a,b,c
Species with the same superscript form a germination strategy group
based on multivariate separation.
Figure 3. Percent of total seedlings emerging during seven
30-day periods for common species representing the two
identified germination strategies under flooded conditions
from the seed banks of 8 playa wetlands on the Southern
High Plains of Texas. Groups 1 and 2 represent early and
continuous germinators, respectively.
drawdown treatment of other studies is frequently
dominated by perennial emergents such as Typha spp.
and Scirpus spp., which have much greater seed production than those drawdown species found in playa
wetlands.
Multivariate analyses indicate potential strategies of
emergence in playa wetlands. In the drawdown treatment, three strategies were identified based on their
temporal germination patterns. Early germinators are
those species (Chenopodium leptophyllum, Polygonum
pensylvanicum) for which most of the viable seeds germinate as soon as favorable conditions are present.
Late germinators are those species (Coreopsis tinctoria, Rorippa sinuata, Ammannia auriculata) that germinate after specific environmental conditions have
existed for some time. The continuous germinators are
those species that germinate approximately 50% of
seed within the first 30 days of being exposed to the
treatment but, unlike the early germinators, continue
to produce seedlings at relatively the same rate
throughout the growing season under drawdown conditions. They include species such as Polygonum lapathifolium, Rumex crispus, and Echinochloa crusgal-
li. Haukos and Smith (1997) found that early germinators were among the more frequently occurring species in playas (occurred in an average of 53.6%
playas), followed by continuous germinators (occurred
in an average of 36.2% playas), and late germinators
(occurred in an average of 22.6% playas).
Based on temporal emergence patterns, two strategies were identified in the flooded treatment. Early
germinators (Heteranthera limosa, Echinochloa crusgalli, and Eleocharis macrostachya) showed most seed
germination (average of 89.9%) within 30 days of being exposed to the treatment and had a low percent
germination during the remainder of the study. Whereas, continuous germinators (Ammannia auriculata,
Sagittaria longiloba) had relatively similar emergence
throughout the growing season. Haukos and Smith
(1997) found that early germinators were among the
more frequently occurring species in playas (occurred
in an average of 43.6% playas), followed by group 2
(occurred in an average of 9.3% playas).
The two species common to both treatments, Ammannia auriculata and Echinochloa crusgalli, had different germination patterns depending on treatment
and relative to the other species specific to each treatment. In both treatments, E. crusgalli germinated the
majority of seeds within 30 days of being exposed to
a treatment, with the only difference between treatments being the length of time that germination continued following exposure. Echinochloa crusgalli is
unable to survive prolonged submergence (Haukos and
Smith 1993). Therefore, by germinating throughout
the growing season in shallowly flooded conditions,
the species would be established if drawdown conditions developed, which could happen at any time during the growing season as a flooded playa dries. Ammannia auriculata is more adapted to flooded than
drawdown conditions (Correll and Johnston 1979) and,
therefore, takes advantage of flooded conditions with
relatively continuous germination. The lack of a dom-
Haukos & Smith, SEEDLING EMERGENCE IN PLAYAS
inant temporal germination pattern for persistence of
wetland species was also found for some species in
tidal freshwater marshes, which have a considerably
more predictable environment than playas (Leck and
Simpson 1995). This lack of a consistent germination
strategy for both unstable and predictable wetland environments signifies the importance of the contributions that diverse seed banks provide in the development of the structure and function of wetlands.
In a freshwater wetland with a predictable environment, Leck and Simpson (1987) identified three germination strategies: (1) transient winter seed banks,
which consist of species that germinate all their seeds
in spring and required annual renewal, (2) species with
seeds that persist throughout the year and germinate
during or later in the growing season, and (3) species
with persistent seed banks that change little with season and that are large relative to annual seed input. In
playas, only species representing groups 2 and 3 are
possible. The unpredictable environment would not allow for the persistence of group 1 in playas because
of the uncertainty for annual renewal.
Examination of seedling emergence from seed
banks throughout a growing season indicates that the
floristic community structure in playa wetlands would
be dynamic in the short term even if the environment
is stabilized. However, stabilized conditions will eventually result in a decrease in species diversity (Haukos
and Smith 1994, 1997) as the seed banks of species
requiring differing environmental conditions are unable to replenish themselves. Stabilizing the hydroperiod would not allow the playa to develop the diverse
plant communities and associated seed banks needed
to respond to changes in the environment if, after several years of stabilized conditions, natural fluctuations
once again were allowed to exist. In other wetlands, a
stabilized environment has resulted in the replacement
of native species by exotic species (Brock and Casanova 1997). Furthermore, we anticipate that the continued loss of natural disturbance would result in diminished plant communities consisting of only a few
species, potentially causing deleterious impacts to the
structure and function of playas (Smith 1990).
Through the influence of natural disturbance, common
species in seed banks of playa wetlands have evolved
mixed strategies of differential temporal emergence in
response to the unpredictable playa environment. Further research is needed to determine specific germination requirements (e.g., temperature, moisture, light,
oxygen) and cues for dormancy break of species in
playas to fully understand persistence mechanisms.
ACKNOWLEDGMENTS
This research was funded by the Texas Advanced
Technology Program. L. M. Smith was supported by
279
the Kleberg Foundation for Wildlife Conservation. R.
E. Sosebee, F. C. Bryant, J. C. Zak, and M. R. Willig
reviewed early drafts of the manuscript. D. R. Lucia
provided field and greenhouse assistance. D.B. Wester
provided statistical assistance. This is paper T-9–886,
College of Agricultural Sciences and Natural Resources, Texas Tech University.
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Manuscript received 6 November 2000; revisions received 5 February 2001; accepted 12 March 2001.