WETLANDS, Vol. 25, No. 3, September 2005, pp. 520–530
q 2005, The Society of Wetland Scientists

A COMPARATIVE ASSESSMENT OF SEEDLING SURVIVAL AND BIOMASS
ACCUMULATION FOR FOURTEEN WETLAND PLANT SPECIES GROWN
UNDER MINOR WATER-DEPTH DIFFERENCES
Lauchlan H. Fraser1,2 and Jason P. Karnezis1
1
Department of Biology
University of Akron
Akron, Ohio, USA 44325-3908
Present address:
Department of Natural Resource Science
Thompson Rivers University
Kamloops, British Columbia, Canada V2C 5N3
E-mail: lfraser@tru.ca
2

Abstract: The purpose of this experiment was to investigate the survival and biomass accumulation of
wetland plant species under different water depths in controlled microcosms. In the greenhouse, two-weekold seedlings were randomly assigned to one of seven water-depth treatments (26, 24, 22, 0, 12, 14, and
16 cm relative to the soil surface) and allowed to grow for six months. Species included five perennial
sedges, four perennial and one annual grasses, and two perennial and two annual forbs. Twelve of the species
had their lowest biomass and lowest survivorship at water depths greater than 0 cm. The root:shoot ratio,
however, did not change across water-depth treatments. Biomass accumulation differed by plant form (sedges.forbs.grasses). Annuals had the greatest biomass values across the widest range of water depths compared to perennials. Of the fourteen plants tested, Lythrum salicaria (purple loosestrife), one of the two
invasive, non-native species tested, had the greatest biomass at water depths from 26 to 12, whereas Phalaris
arundinacea (reed canarygrass), the other invasive, had comparatively small mean biomass values. Ranking
of biomass between species was highly concordant between non-flooded treatments but not significantly
concordant between flooded treatments indicating that plant species have distinct responses to flooding. This
research suggests that newly established plant seedlings in wetland restorations should not be submerged,
or if submergence is unavoidable, annuals and sedges may be more tolerant of prolonged flooding.
Key Words:

wetland restoration, flooding, hydrology, fundamental niche, hydrophyte

INTRODUCTION

Changes in land use have resulted in destruction and
fragmentation of wetlands in North America over the
past century, which has affected water quality, water
recharge, and wildlife habitat. As a result, wetland restoration has recently become a common and widespread activity. Wetland restoration research has focused primarily on seed viability, seed germination,
and seed-bank dynamics, as they relate to hydrology,
nutrients, temperature, and time (van der Valk 1981,
Keddy and Ellis 1985, Schneider and Sharitz 1986,
Gerritsen and Greening 1989, Budelsky and Galatowitsch 1999, van der Valk et al. 1999, Leck 2003,
Jensen 2004). Little information is available regarding
how species perform, once germinated, over a range
of hydrologic treatments (Weiher et al. 1996, Budelsky
and Galatowitch 2000). Experiments have determined
that an increase in water level generally reduces the
growth and affects morphological responses of wetland plants (Coops et al. 1996, Newman et al. 1996,

Much of the literature in wetland research is focused
on how hydrologic regimes affect the structure and
function of a given wetland (Spence 1982, Gerritsen
and Greening 1989, van der Valk et al. 1994, Barendregt et al. 1995, Owen 1995, Busch et al. 1998,
Moore et al 1999, Keddy and Fraser 2000, Baldwin et
al. 2001, Johnson et al. 2004). Laboratory, field, and
modeling techniques have all been employed to gain
insight into the processes that determine how abiotic
factors contribute to wetland plant community structure (van der Valk et al. 1994, Ellison and Bedford
1995, Weiher and Keddy 1995, Casanova and Brock
2000, Cole and Brooks 2000, Baldwin et al. 2001, De
Steven and Toner 2004, Johnson et al. 2004). Of all
potential abiotic factors, hydrologic regimes have generally been shown to correlate most strongly with vegetation type (Keddy 2000, De Steven and Toner 2004).
520

Fraser & Karnezis, WATER DEPTH EFFECTS ON PLANT BIOMASS

521

Table 1. Fourteen wetland plant species that were used in the experiment and grown at seven water depths.
Scientific Name and Authoritya

Code

Common Name

Life Form

Life Cycle

Calamagrostis canadensis (Michx.) Beauv.
Carex lurida Wahlenb.
1
Carex stipata Hudson
2
Carex tribuloides Wahlenb.
1
Carex vulpinoidea Michx.
2
Elymus virginicus L.
2
Glyceria canadensis (Michx.) Trin.
1
Lythrum salicaria L.
1
Mimulus ringens L.
1
Phalaris arundinacea L.
1
Rumex orbiculatus A. Gray
1
Scirpus cyperinus (L.) Kunth.
2
Verbesina alternifolia (L.) Britt.
2
Zizania aquatica L.

Cc
Cl
Cs
Ct
Cv
Ev
Gc
Ls
Mr
Pa
Ro
Sc
Va
Za

Blue-joint grass
Hop sedge
Sawbeak sedge
Bristlebract sedge
Fox sedge
Virginia wildrye
Rattlesnake mannagrass
Purple loosestrife
Monkey flower
Reed canarygrass
Great water dock
Woolgrass
Yellow ironweed
Annual wildrice

Grass
Sedge
Sedge
Sedge
Sedge
Grass
Grass
Forb
Forb
Grass
Forb
Sedge
Forb
Grass

Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Perennial
Facultative annual
Facultative annual
Perennial
Perennial
Perennial
Perennial
Annual

1
1

from Crow and Hellquist (2000)
indicates field-collected seed.
2
indicates seed that was purchased from Ernst Conservation Seeds.
a

1

Lentz and Dunson 1998, Lenssen et al. 1999, Miller
and Zedler 2003); however, the number of species tested is generally less than five for each study, the waterdepth treatments tend to differ by at least 5 cm, and
the plants tested are generally not seedlings but vegetative ramets of mature plants (but see Kercher and
Zedler 2004).
Wetland restorations generally involve the new establishment of a plant community, either by natural
seed dispersal, introduced seeds, or planted seedlings.
The establishment phase of the plant community is
critical to colonization and stand development. Controlled mesocosm experiments have shown that control
of water depth during the first year can significantly
influence the potential for growth, mortality, and stand
development in later years (Weiher and Keddy 1995,
Budelsky and Galatowitsch 2000).
In natural systems, isolating an individual environmental factor’s effect upon that system is nearly impossible. Using laboratory techniques, we can more
clearly delineate the role of hydrology on a number of
species (Fraser and Keddy 1997). Isolating the effect
of water depth on a range of wetland species can reveal fundamental niche boundaries, information that
can be applied to predictive models of plant establishment and wetland restoration.
The purpose of this experiment was to investigate
the growth responses (measured as seedling survival
and accumulated biomass) under different water
depths at small (2 cm) increments in controlled microcosms. Although static water levels are generally not
normal or realistic conditions found in natural marshes
or wet meadows, we were interested in identifying the
constraints of absolute water depth to initial establishment and performance of seedlings over a relatively

short time period. Fourteen wetland species, representing a range of growth forms, were grown at seven
static water depths, from 26 cm to 16 cm in 2 cm
increments relative to the soil surface. Two-week-old
seedlings were used because it is already well-established that flooding is a constraint to germination (van
der Valk 1981, Keddy and Ellis 1985, Casanova and
Brock 2000, Jensen 2004), but the gap in the literature
regards the survivorship and growth of seedlings to
water depth. The following questions were posited. (1)
Does water depth affect biomass and root:shoot ratios
of these wetland plants? (2) Is there a difference in
seedling survivorship between water depths and between species? (3) Does biomass at different water
depths differ between life forms (grass, forb, and
sedge) and life cycles (perennial and annual)? (4) Does
the hierarchy of growth performance remain consistent
across water-depth treatments? (5) Do non-native invasive species (Lythrum salicaria and Phalaris arundinacea) produce more biomass across the seven water
depths compared to the twelve native species tested?
MATERIALS AND METHODS
Twelve species were selected to represent a broad
range of plant life forms typically found in temperate
eastern North American marsh and wet meadow wetlands (Crow and Hellquist 2000) that are commonly
used in wetland restoration projects (Hammer 1997,
Cronk and Fennessy 2001) (Table 1). In addition, we
included two invasives, Lythrum salicaria (purple
loosestrife, a facultative annual forb) and Phalaris
arundinacea (reed canary grass, a perennial grass),
commonly found throughout North America (Apfelbaum and Sams 1987, Thompson et al. 1987). Species

522
represent three life forms (forbs, sedges, and grasses)
and two life cycles (facultative or obligate annuals and
perennials). Seeds were either purchased through Ernst
Seed Company (Meadville, PA) or collected in the
field. The seed source from Ernst Seed Company is a
combination of field-collected and cultivated plants
that are bulk processed so that there is genetic variability. Field-collected seed were taken from multiple
individuals and from at least three sites, which were
then combined by species.
Seven water-depth treatments were created to test
the individual effect of hydrologic regimes on the fourteen species. The hydrologic treatments were 26, 24,
22, 0, 12, 14, and 16 cm relative to the substrate.
Species were tested individually over the seven hydrologic treatments, resulting in a 14 x 7 factorial design
with five replicates of each water depth, yielding 490
individual microcosms. The microcosm consisted of
an outer 700-mL cup with holes drilled at the appropriate water depth (26, 24, 22, 0, 12, 14, and 16
cm) and an inner 300-mL cup filled with a 3:1 peat
and sand mixture in which the plant was grown. The
300-mL cup was 10 cm in height; therefore, the distance of the water level to the bottom of the cup were
4, 6, 8, 10, 12, 14, and 16 cm from lowest to highest
water depth. Holes were drilled at the bottom of the
inner 300-mL cup for water uptake.
A set of fourteen microcosms was placed on a single
tray, representing the fourteen plant species at one of
the seven water depths. Since each water-depth treatment had five replicates, there were a total of 35 trays.
The arrangement of trays and microcosm position on
each tray were randomized at the beginning of the experiment.
Germination
The microcosms were seeded on March 22 and 23,
2001 with approximately ten seeds per microcosm.
Seeds of each species were also placed in petri dishes
filled with peat to act as a reserve for microcosms with
no germination. The seeds in all microcosms were kept
moist and partially covered with plastic to encourage
germination. Following germination within microcosms, only one healthy seedling was kept, and the
remaining seedlings and seeds were removed. If no
germination was noted in a microcosm, germinated
seeds from the petri dishes were transplanted into the
appropriate microcosm. The timing of germination
was relatively uniform within species so that we could
randomly select a cohort of 2-week-old individuals,
separated by no more than 24 hours at germination.
Water-depth treatments were started two weeks after
germination. There was a large discrepancy between

WETLANDS, Volume 25, No. 3, 2005
species relating to germination date, but no more than
two months separated the first and last species.
Standardized Conditions
In order to isolate the effects of the water-depth
treatment on the fourteen species better, other abiotic
factors were standardized. A daily, 14-hour photoperiod was provided by four 1000 watt, High Pressure
Sodium bulbs providing an average photosynthetically
active radiation (measured with a LiCor LI-250 light
meter) of 150.5 mmol s21 m22 (6 8.2 SD) on the experimental plants. Windows in the room were covered
to prevent incident solar radiation. Temperature ranged
between 20 and 248C and was maintained by air-conditioner units. Humidity ranged between 40 and 50%
and was self-maintained due to the evaporation of the
water from all the microcosms. After the initial two
weeks of germination, plants were watered once per
day. A watering wand was used to apply water to the
outer sleeve of each individual microcosm. A bi-weekly nutrient regime of 20 ml of full-strength Rorison’s
solution (Hendry and Grime 1993), containing 1.12
mg of nitrogen and 0.62 mg of phosphorous, was administered into each microcosm for the duration of the
experiment. Algae grew in some of the flooded microcosms but were physically removed immediately so
that algae were never present longer than 24 hours.
Harvesting, Drying, and Weighing
Plants were harvested after six months of growth.
Since the last species cohort germinated two months
later than the first species cohort, the whole experiment
ran for approximately eight months. Each plant’s biomass was divided into aboveground and belowground
biomass, placed in a drying oven for 48 hours at 808C,
and weighed to the nearest ten-thousandth gram. Dead
plants were scored as having zero biomass.
Statistical Analyses
A two-way, fixed-effects ANOVA was conducted
for the combined wetland species by water-depth-treatment effect on total biomass. Tukey’s LSD test was
run to determine statistical significance between
means. Biomass data were square-root transformed to
improve homoscedasticity and normality of residuals.
A general, non-linear modeling procedure was used to
test for the combined effects of wetland species and
water depth on root:shoot ratios, with dead plants excluded from the analysis. A logarithmic transformation
was applied to the ratio data. A one-way, fixed-effects
ANOVA was used to test the effect of water depth on
survivorship, with Tukey’s LSD applied post-hoc to

Fraser & Karnezis, WATER DEPTH EFFECTS ON PLANT BIOMASS

523

Table 2. Results of 2-way ANOVA examining the effects of
plant species (Species) and the water-level treatments (Water
depth) on the mean plant biomass.
Source of variation
Species
Water depth
Species 3 Water depth
Error

SS

Df

F-ratio

711.335 13 18.287
6 42.940
770.893
734.994 78 3.149
1172.923 392

P
,0.001
,0.001
,0.001

determine statistical significance between means. Survivorship of each species across the seven water
depths, and all species at each water depth, was calculated with the equation: Ni/Nt; where Ni is equal to
the number of individuals collected (maximum 5) divided by Nt the total number of individuals planted per
treatment (5). Survivorship data were arcsine-transformed to improve homoscedasticity and normality of
the data. Two-way, fixed-effects ANOVAs were conducted to test effects of (1) combined life form (grass,
forb, and sedge) and water depth, and (2) life cycle
(perennial and annual) and water depth on biomass.
Response variables were square-root transformed to
improve homoscedasticity and normality of residuals.
Kendall’s test of concordance W (Siegel 1956) was
used to test for the degree of association in the rank
order of species biomass response in the different water depths. W ranges between 0 and 1, with 1 representing total concordance. Significance of W was calculated with a null hypothesis of no correlation of
rankings between water-depth treatments. The general
linear model, all ANOVAs, and the post-hoc analyses
were conducted using Systat 8 (SPSS 1998).
RESULTS
The effects of species and water depth on plant biomass were statistically significant (Table 2). An analysis of the combined biomass of all species showed
that the greatest biomass occurred at water depths between 24 and 0 cm, with a significant decrease in
biomass at lower (26 cm) and higher water depths
(12, 14, 16 cm) (Figure 1). Of the fourteen species
tested, only two species showed no significant differences in means across water depths, Elymus virginicus
and Verbesina alternifolia, with mean biomass less
than 1.0 g at each water depth, and were therefore not
presented in the histograms in Figure 2. For the other
twelve species, the lowest biomass was recorded at the
highest water depths (i.e., under flooded conditions)
(Figure 2). The mean biomass of plants at each individual water depth was below 4.0 g for nine out of 14
spp. However, Lythrum salicaria (one of the two ‘‘invasives’’ tested) had a mean biomass of greater than

Figure 1. Effects of water depth on the mean biomass of
fourteen wetland plants (see Table 1). N 5 490. Error bars
represent 11 SE. Bars sharing the same letter are not significantly different using Tukey’s LSD.

4.0 g at 6 cm and greater than 6.0 g from 24 to 12
cm (Figure 2g). Phalaris arundinacea, the other ‘‘invasive,’’ had comparatively lower growth across water
depths, with a mean biomass greater than 2.0 g (2.375)
at only 26 cm (Figure 2i). Other species with a mean
biomass greater than 4.0 g included Carex lurida (Figure 2b), C. tribuloides (Figure 2d), C. vulpinoidea
(Figure 2e), and Zizania aquatica (Figure 2l). Although Glyceria canadensis (Figure 2f) and Mimulus
ringens (Figure 2h) had significant differences in mean
biomass between water-depth treatments, the differences were comparatively small, and mean biomass
was never greater than 2.0 g across all water depths.
Mimulus ringens and Lythrum salicaria were the only
species to flower, and M. ringens flowered at all seven
water-depth treatments.
There was a large variation in root:shoot ratio between species. The general linear model analyzing the
effects of species and water depth on root:shoot ratio
was statistically significant (F2,305 5 9.553, p , 0.001),
but only for species (t 5 24.350, p , 0.001) (Figure
3) not for water depth (t 5 1.017, p 5 0.310). Carex
stipata, C. tribuloides, Lythrum salicaria, Rumex orbiculatus, and Zizania palustris had mean values close
to four, but the majority had mean values below two
(Figure 3). Since root:shoot ratio did not differ by water depth, no further analyses were conducted using
root:shoot ratios.
Water depth affected survivorship (F6,91 5 17.083,
p , 0.001). The highest mean percent survivorship by
water depth of all species combined was found at the
lowest water depths: 26, 24, 22, and 0 cm (Table 3).
By species, survivorship ranged from 94 percent (Zizania aquatica) to 26 percent (Verbesina alternifolia),

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WETLANDS, Volume 25, No. 3, 2005

Figure 2. Effects of water depth on the mean biomass of (a) Calamagrostis canadensis, (b) Carex lurida, (c) Carex stipata,
(d) Carex tribuloides, (e) Carex vulpinoidea, (f) Glyceria canadensis, (g) Lythrum salicaria, (h) Mimulus ringens, (i) Phalaris
arundinacea, (j) Rumex orbiculatus, (k) Scirpus cyperinus, and (l) Zizania aquatica. Elmyus virgicus and Verbesina alternifolia
are not included because the data were not significant. For each figure, N 5 35. Error bars represent 11 SE. Bars sharing the
same letter are not significantly different using Tukey’s LSD.

but no significant difference between species was detected. Only four species (Zizania aquatica, Elymus
virginicus, Mimulus ringens, and Phalaris arundinacea) had at least one survivor across all water depths.
We did not measure timing of death, but the majority

of mortality in the flooded treatments occurred within
the first month of the water-depth treatment.
A two-way ANOVA to determine the combined effect of life form and water depth on biomass was significant (Table 4). Sedges had the highest mean bio-

Fraser & Karnezis, WATER DEPTH EFFECTS ON PLANT BIOMASS

Figure 2.

mass, statistically greater than forbs and statistically
greater than the grasses (Figure 4). By water depth,
the mean biomass of sedges was greatest at 26, 24,
22, and 0 cm water depth; however, the mean biomass
of forbs was greatest at 12 cm.
A two-way ANOVA to test the combined effect of
plant life cycle and water depth on biomass showed
statistically significant results (Table 5). At each water
depth, annuals consistently had greater mean biomass
than perennials (Figure 5).

525

Continued.

The rankings of mean biomass of the 14 species at
the seven water depths had a concordance, W, of 0.354
(F14 5 32.238; P ,0.01) (Table 6). A W of 0.354 is
unexpectedly low considering the results were statistically significant and therefore not independent of
each other. When calculations of concordance were
separated according to whether plants are grown above
or below 0 cm, the rankings below the substrate (26,
24, and 22 cm) had a concordance, W, of 0.861 (F14
5 33.578; P ,0.01); Lythrum salicaria and Carex vul-

526

WETLANDS, Volume 25, No. 3, 2005
Table 4. Results of 2-way ANOVA examining the effects of
plant life form (Life form) and the water-level treatments (Water
depth) on the plant biomass.
Source of variation

SS

Df

F-ratio

2 15.747
149.938
6 25.832
737.903
236.420 12 4.138
2232.893 469

Life form
Water depth
Life form 3 Water depth
Error

P
,0.001
,0.001
,0.001

DISCUSSION
We asked if water depth affects survivorship and
accumulated biomass of 2-week-old wetland seedlings
exposed to static water-depth treatments for a period
of six months. Wetland plants can tolerate periodic
flooding; however, as this study demonstrates, wetland
plant seedlings do not seem to be able to tolerate extended flooding. It is clear from our results that mean
plant biomass and survivorship in general is strongly
affected by growth at different water depths. Figure 1
could represent a typical plant physiological response
curve for any number of abiotic variables of importance, where there is a maximal performance at some
intermediate abiotic value (Lambers et al. 1998). Despite the relatively small standard error bars, when individual species were analyzed, there was a large difference in species’ biomass response across the seven
water depths. For example, two species (Elymus virgincus and Verbesina alternifolia) showed no statistical difference in biomass between water depths. How-

Figure 3. The mean root:shoot ratios of fourteen wetland
plants (see Table 1 for species’ codes). N 5 308. Error bars
represent 11 SE. Bars sharing the same letter are not significantly different using Tukey’s LSD.

pinoidea were consistently in the top three ranking,
while Elymus virginicus, Verbesina alternifolia, and
Glyceria canadensis were always in the bottom three
ranking. Conversely, rankings of mean biomass at the
three water depths above the substrate (12, 14, and
16 cm) had a concordance, W, of 0.495 (F14 5 19.303;
P .0.05); thus, the rankings were independent of each
other.

Table 3. Percent survivorship of each species at each water depth (n 5 5). Far right column is the mean percent survivorship of species
across all water depths. Bottom row is the mean percent survivorship of all species by water depths. Numbers sharing the same letter are
not significantly different using Tukey’s HSD. Values in parentheses are standard error 61.

Species

26

24

22

0

2

4

6

Mean
Survivorship
by Species

C. canadensis
C. lurida
C. stipata
C. tribuloides
C. vulpinoidea
E. virginicus
G. canadensis
L. salicaria
M. ringens
P. arundinacea
R. orbiculatus
S. cyperinus
V. alternifolia
Z. aquatica
Mean Survivorship
by Water Depth

60
60
100
100
80
100
100
80
100
80
100
80
60
100

60
80
100
100
100
100
100
80
80
80
100
100
80
100

100
60
80
100
100
100
80
100
60
80
80
100
20
100

80
40
80
80
80
100
100
80
100
80
60
100
0
100

0
0
0
20
40
80
20
100
60
60
20
100
20
100

20
0
20
0
0
60
0
0
80
60
0
40
0
100

0
20
40
0
0
80
0
0
40
20
0
0
0
60

46 (15)
37 (12)
60 (15)
57 (18)
57 (17)
89 (6)
57 (18)
63 (17)
74 (8)
66 (8)
51 (17)
74 (15)
26 (12)
94 (6)

86b (4)

90b (4)

83b (6)

77b (8)

44a (10)

27a (9)

19a (7)

Water Depth (cm)

Fraser & Karnezis, WATER DEPTH EFFECTS ON PLANT BIOMASS

527

Table 5. Results of 2-way ANOVA examining the effects of
plant life cycle (Life cycle) and the water-level treatments (Water
depth) on the plant biomass.
Source of variation
Life cycle
Water depth
Life cycle 3 Water depth
Error

Figure 4. Effects of water depth and plant life form on the
mean biomass of fourteen wetland plants (see Table 1). Error
bars represent 11 SE. Bars with slanted lines are grasses,
open bars are forbs, and bars with horizontal lines are sedges.

ever, the flooding treatment (.0 cm) generally killed
plants, especially at 14 and 16 cm. Lythrum salicaria,
Carex tribuloides, C. vulpinoidea, Leersia oryzoides,
and Scirpus cyperinus all had mean biomass values
above 4 g for at least one of the water-depth treatments
between 26 and 0 cm, but at 14 and 16 cm, biomass
was minimal and survivorship was very low.
Unlike other studies, which have shown a decrease
in root:shoot ratios with increasing water depth (Waters and Shay 1992, van den Brink et al. 1995, Coops
et al. 1996, Edwards et al. 2003), we report no significant effect of water depth on root:shoot ratio. A possible explanation may be the small scale of our experiment in combination with the small differences in
our water levels. The size of our microcosms probably
limited the overall growth of the plants. Furthermore,
species’ response to microcosm size may not be in the
same (linear) fashion, such that there may be differences in fine structure roots versus storage tissues belowground. The experiments noted above were in larger containers than ours, with much larger differences
in water depths, and used ramets not seedlings. For
example, Edwards et al. (2003) found a decrease in
root:shoot ratios with an increase in water depth in
Eleocharis cellulosa, but they were comparing growth
response of field-collected ramets in fixed water depths
of between 17 and 154 cm in 3.8 L containers. Although we found that root:shoot ratios differed by species, there was no apparent pattern in species’ response.
Not only was biomass generally reduced with an
increase in water depth, but survivorship was also significantly reduced. An analysis of survivorship offers

SS

Df

F-ratio

1 20.502
105.488
6 19.343
597.159
6 2.093
64.615
2449.149 476

P
,0.001
,0.001
0.053

interesting comparisons between species. Biomass responses of Elymus virginicus and Verbesina alternifolia at first glance appeared very similar. However,
survivorship scores for each species were in stark contrast to each other, with E. virginicus scoring 89% and
V. alternifolia scoring 26% across the seven water
depths. This apparent discrepancy may be attributable
to their differences in growth form. Elymus virginicus
is a grass with slender blades no more than 2 cm wide
and stems 1.5 m long, and V. alternifolia is a forb with
an erect stem and leaves up to 6 cm wide and 12 cm
long.
Mimulus ringens, an annual forb that can have stems
up to a meter in height and leaves as large as 2 cm
wide and 10 cm long, was one of only two species
that produced flowers (the other was Lythrum salicaria) and the only species to produce seeds over the
course of the experiment. This species was able to survive and reproduce across all seven water-depth treatments, even under flooded conditions.
The most striking aspect of all species responses
across the water depths is that a 2-cm difference in
water level was sometimes the difference between

Figure 5. Effects of water depth and plant life cycle on the
mean biomass of fourteen wetland plants (see Table 1). Error
bars represent 11 SE. Filled bars are annuals, open bars are
perennials.

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WETLANDS, Volume 25, No. 3, 2005

Table 6. Rank orders of decreasing biomass in fourteen species at seven different water-depth treatments. See Table 1 for species’ codes.
1

2

3

4

5

6

7

8

9

10

11

12

13

14

Water depth (cm)
Ls
26
Ct
24
Ls
22
Ls
0
Ls
2
Za
4
Za
6

Cs
Cv
Cv
Ct
Za
Va
Ev

Cv
Ls
Ct
Cv
Sc
Ro
Mr

Pa
Cl
Za
Sc
Cv
Sc
Sc

Za
Sc
Cl
Cc
Va
Cc
Cs

Sc
Za
Cs
Cs
Mr
Ev
Ct

Cl
Cs
Sc
Za
Ev
Mr
Pa

Mr
Ro
Ro
Ro
Pa
Pa
Cv

Ro
Cc
Cc
Pa
Ct
Cv
Ro

Ct
Pa
Mr
Mr
Gc
Cl
Cl

Cc
Mr
Gc
Cl
Cc
Cs
Va

Ev
Gc
Ev
Ev
Ro
Gc
Gc

Va
Ev
Pa
Gc
Cl
Ct
Ls

Gc
Va
Va
Va
Cs
Ls
Cc

growth and senescence. Once plants had been given
two weeks to establish, the water-depth treatments
were initiated, leaving all plants in the flooded treatments (.0 cm above soil level) partially or fully submerged. Although we initiated our treatments on twoweek-old seedlings, after six months of growth, the
plants can be considered juvenile or, in the case of the
flowering Lythrum salicaria and Mimulus ringens, mature. However, informal observations of all of the species in flooded conditions revealed that unless species
could stay or grow above the water level, species could
only maintain current growth, and most died within
one month. Flooded conditions produce anaerobic
soils and soils with lowered redox potential, which
generates an even greater demand for oxygen within
the soil (Armstrong et al. 1994, Pezeshki 2001). It
seems that flooding a seedling, even though it is a
wetland plant, can be considered a stress (Otte 2001).
Other studies have shown that some of the same species we tested had greater biomass accumulation and
survivorship in flooded treatments (e.g., Lempe et al.
2001, Miller and Zedler 2003), but these studies used
ramets, not seedlings. Kercher and Zedler (2004) tested the effects of water depth on the biomass production of seventeen wetland plants at approximately three
months old grown from seed, two of which we tested
(P. arundinacea and C. canadensis). The two invasives tested (P. arundinacea and Typha latifolia) consistently had greater biomass accumulation than the
other species (Kercher and Zedler 2004).
Plant Form and Plant Life Cycle
We separated our species by life form and life cycle
in order to determine general patterns of biomass response to growth at different water depths. When
grouped by life form, the sedges performed similarly
in their response to water depth and generally had the
largest biomass values across all water levels. The only
exception was that biomass of Lythrum salicaria was
greatest at 12 cm. Grasses, on the other hand, had the
lowest biomass values across all water depths, even

though survivorship was relatively high compared to
the other life forms. If biomass is used as an indicator
of establishment, then it seems that the sedges as a
group could establish across the widest range of water
levels, particularly at greater water depths. These results are in accordance with Coops et al. (1996), who
compared the growth response of two grasses (Phalaris arundinacea and Phragmites australis) and two
sedges (Scirpus maritimus and S. lacustris) at five different water depths (280, 255, 230, 25, and 120
cm). Kercher and Zedler (2004) also found that sedges
tolerated flooding better than the broadleaf forbs. The
responses reflect the zonation of life forms along the
water-depth gradient: sedges in relatively deep water,
forbs and grasses in shallow to very shallow water
(Spence 1982, Keddy and Ellis 1985).
Our investigation of life cycle showed that annuals
had much greater biomass under flooded conditions
and generally had high survivorship values compared
to perennials. Perhaps this is because annuals may be
better able to respond to the disturbance conditions
caused by flooding (Grime 1977). In fact, Mimulus
ringens and L. salicaria flowered during the experiment, which therefore limited their allocation to nonreproductive shoot and root growth. Nonetheless, we
only tested three annual species, compared to eleven
perennials for our comparative analysis, and it is evident that L. salicaria strongly influenced the large biomass result for annuals. Lempe et al. (2001) also found
that L. salicaria (as well as four other species from
the family Lythraceae) responded to flooding by increasing growth as total plant height.
Species Rank
In order to answer whether species performed differently in relation to one another across the waterlevel treatments, a ranking of all species across all water-level treatments was created. Kendall’s test of concordance showed that hierarchies remained statistically
consistent across all water depths; however, the concordance value of 0.354 (where 1.0 is the highest) was

Fraser & Karnezis, WATER DEPTH EFFECTS ON PLANT BIOMASS
quite low. It is clear, based on the two additional tests
of concordance for the flooded (12, 14, 16 cm) and
non-flooded (26, 24, 22 cm) water-level treatments
that flooding disrupts the ranking of species response.
Under non-flooded conditions, species rank was predictable, and Lythrum salicaria, a non-native invasive,
was consistently ranked among the highest, by biomass, of all species. When the ranking of species’ biomass was compared between the flooded conditions,
there was no concordance. This reflects the distributional zonation patterns typically found on river banks
and lake shores (Spence 1982). Zonation patterns develop and persist because different species have varying properties that enable them to establish, survive,
and colonize at particular water depths (Keddy 1984,
Grace 1987, van der Valk and Welling 1988, Keddy
and Fraser 2000), and a mere 2-cm increment has the
effect of disrupting the hierarchical order in the flooded treatments.

529

can tolerate flooded conditions. However, the plants in
those studies were not seedlings less than one month
old at the time of flooding. More work is needed to
understand the relationship between the timing of
flooding and the life stage of the plant.
Constructing performance indices, such as survivorship and biomass at different water depths, as in this
experiment, are necessary for a better understanding of
natural species distribution and abundance (Keddy
2000). Our study helps to define the fundamental niche
of fourteen wetland plants with respect to water depth,
which allows a predictive tool for wetland plant community establishment. However, this is a step-wise
process, and other interacting abiotic and biotic variables such as fluctuating water levels (Budelsky and
Galatowitsch 2000), sediment type (Lenssen et al.
1999), competition (Grace 1987), and herbivory (Sheldon 1987) need to be considered as well.
ACKNOWLEDGMENTS

Restoration Implications
The fact that species’ growth performance, measured as biomass and survival, was generally very poor
in the flooded treatment (. 0 cm above the soil surface) has implications for restoration. Hammer (1997)
stated that the watchful maintenance of hydrology in
a newly restored or created wetland over the first two
to three years will largely determine the level of restoration success for any given wetland. If seedlings are
to be planted at a site, the result of flooding has been
clearly demonstrated with this experiment. If new
shoot growth is submerged, survival is reduced and
growth will generally not occur. Timing of death was
not measured, but we observed that the majority of
mortality in the flooded treatments was within the first
month. The sedge life form and the annual life cycle
in our study were two general classification groups
best able to tolerate prolonged flooding. Care must be
taken in extrapolating these results to other species.
More work is needed across a larger number of species
to make general statements on the performance of life
form and life cycle to growth under flooded conditions.
Our results suggest the potential use of slightly different water-level manipulations to control the growth
and survivorship of the invasives Phalaris arundinacea and Lythrum salicaria. Flooding (above 0 cm relative to soil level) prevented the growth of the invasive
P. arundinacea, but flooding of greater than 2 cm
seems to be needed to prevent growth of L. salicaria.
The life stage or age of the plant when it is flooded is
probably critical because other studies have shown that
both P. arundinacea (Miller and Zedler 2003, Kercher
and Zedler 2004) and L. salicaria (Lempe et al. 2001)

We thank Adam Landaw for his help in maintaining
the experiment. Randy Mitchell and Steve Weeks offered helpful comments on early versions of the manuscript. The research was supported by a U.S. Geological Survey grant through the Ohio State University
Water Resources Research Institute Program to L.
Fraser. Finally, this manuscript benefited greatly from
the suggestions of two anonymous reviewers and Dr.
Rachel Budelsky.
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