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Ecological Studies at the Woodworth Study Area

Effects of Water Level Changes on Prairie Pothole Vegetation Structure and Diversity in the Woodworth Study Area, North Dakota

Debra L. Taylor* and Naomi E. Detenbeck
U.S. Environmental Protection Agency
Mid-Continent Ecology Division
6201 Congdon Boulevard
Duluth, MN 55804

Introduction and Methods

Vegetation diversity and community structure were monitored in 1993 and 1994 in 20 seasonal wetlands near Woodworth, ND. These wetlands were the site of a three-year study examining the effects of agricultural practices and sedimentation on seasonal prairie wetlands. The wetlands were chosen for similar area (mean 0.18 ha, range 0.09 to 0.47 ha) similar watershed size (range 0.49 to 2.18 ha), and none had surface water inlets or outlets at the beginning of the study. Radial transects were established in each wetland in April 1993 when shallow marsh and wet meadow zones were delineated. In May 1993, five wetland watersheds were tilled to the wet meadow/upland boundary. In addition, another five watersheds were tilled but 25 ft (7.62 m) buffer zones were left around these wetlands. Five wetlands were native prairie, and the final five had been managed for Conservation Reserve Program vegetation for at least ten years. During the 1993 and 1994 growing seasons, vegetation density and diversity in each wetland was measured using the point-centered quarter method (1). Densities were measured separately for species within each of five growth form categories: shrubs, forbs, graminoids, giant burreed (Sparganium eurycarpum), and cattails (Typha spp.). The graminoid growth form included grass, sedge, and rush species. These vegetation surveys were conducted in June and September 1993 and in May, June, and July 1994.

Water levels at the deepest point in each wetland ranged from 1 cm to 62 cm (mean 26 cm) in June 1993. Soon after the June 1993 survey, unusually high rainfall began in the Woodworth area. This increased precipitation continued through the autumn; precipitation in winter and spring, 1994 was also higher than normal. By June 1994, the 20 study wetlands had water depths of 29 cm to 130 cm (mean 80 cm). Water levels in most of the wetlands did not drop significantly in mid-summer 1994. The mean July depth was 78 cm, with a range of 20 to 131 cm.

Vegetation of the five native prairie (NP) and the five Conservation Reserve Program (CRP) sites will be described here. June 1993 data will be compared with both the June 1994 and July 1994 observations. High water levels and the cool spring temperatures in 1994 retarded plant growth. Thus, the July 1994 observations may better represent peak summer biomass for 1994.


Vegetation density decreased overall and mean plant height increased between June 1993 and June/July 1994 in response to the increased water levels. Community structure in both the shallow marsh and wet meadow zones changed between summer 1993 and summer 1994. Species generally associated with semi-permanent wetlands, such as cattail and giant burreed, became established in the centers of several wetlands by July 1994, although none of these species became dominant in any wetland. No submersed vegetation was found in any wetland in June 1993. However, a submersed species, bladderwort, (Utricularia vulgaris) was found in five of the ten NP/CRP wetlands during the summer of 1994.

In June 1993, the average water depth at the deepest point in the NP sites was 31 cm (Table 1). This had increased to 106 cm in June 1994, and was measured at 103 cm in July 1994. The CRP wetlands tended to be shallower than the NP sites throughout the study, with a mean water depth of 15 cm in June 1993, 49 cm in June 1994 and 46 cm in July 1994.

In the NP sites, percent open water in the shallow marsh zones increased from 61% in June 1993 to 81% in July 1994 (Table 1). Virtually no open water existed in the wet meadow zones of the NP wetlands in June 1993; open water covered 69% of the area in June 1994 and 46% a month later in July. The percent open water in the shallow marsh zones of CRP sites remained fairly constant during the 1993 and 1994 growing seasons. The CRP wet meadow zones showed an increase in open water area, from 8% in June 1993 to 25% in June and 20% in July 1994.

Overall vegetation density, defined as shoots/m2, decreased in both the NP and the CRP wetlands (Table 1). In the NP shallow marsh zones, graminoids showed little decrease in density between June 1993 and June 1994. However, by July 1994, the graminoid density in the shallow emergent zones of NP wetlands had decreased significantly. Forbs showed large decreases in density after flooding. In the wet meadow zones of the NP sites, graminoids dropped to a third of their June 1993 density by June/July 1994. Forbs also decreased in density, thinning from 272 to 28 shoots/m2.

Graminoid density decreased in both vegetation zones in the CRP wetlands. Shallow marsh zone forbs showed a small drop, from 123 to 88 shoots/m2 (Table 1). The density of forbs actually increased in the CRP wet meadow zones, from an average of 126 shoots/m2 to 186 shoots/m2 in July, 1994, largely due to increased densities of water smartweed (Polygonum amphibium).

In both the NP and the CRP sites, nearly all the area that was not open water was covered by live, erect vegetation. Bare earth, filamentous algae, and/or floating-leaf vegetation constituted no more than 5.5% of the cover in any of these sites during any of the three sampling periods.

Vegetation heights increased in both the shallow marsh and wet meadow zones in the NP and in the CRP sites (Table 1). In the NP sites, average vegetation heights in both vegetation zones increased about 300% after flooding. For the CRP sites, with shallower water depths, the vegetation height increases were not as great, averaging 200 to 250%.

The average number of species with ten or more shoots/m2 decreased in both the shallow marsh and wet meadow zones of the NP sites during the study period. For the shallow marsh zone, the decrease was from an average of 5.4 species in June 1993 to 1.2 species in June and July 1994. In the NP wet meadow zones, the decrease was from 7.0 species in June, 1993 to 5.0 species in June 1994, and a further decrease to 3.6 species by July 1994.

The decrease in the average number of species with at least ten shoots/m2 in the CRP wetlands was not nearly as great, from 5.6 to 3.8 species in the shallow marsh zones and 7.8 to 6.6 species in the wet meadow zones.

In the shallow marsh zones of the NP sites, slough sedge (Carex atherodes) and whitetop (Scolochloa festucacea) replaced quack grass (Agropyron repens) and a number of forbs following the water level rise (Table 2). Water smartweed was the only forb whose densities exceeded 10 shoots/m2 during the summer of 1994.

In the shallow marsh zones of the CRP sites, whitetop and river bulrush (Scirpus fluviatilis) replaced brome grass (Bromus inermis), reed canary grass (Phalaris arundinacea), spikerush (Eleocharis palustris), and alkali-grass (Puccinellia nuttalliana) (Table 2). Sedge species (including C. atherodes) and quack grass were found in the CRP shallow marsh zones during both study years. Dominant forbs shifted to wetter-zone species (2).

In the wet meadow zones of the NP sites, brome grass and fowl bluegrass (Poa palustris) were succeeded by slough sedge, reed canary grass, whitetop, and prairie cordgrass (Spartina pectinata) (Table 2). Forbs changed from a mixture of Canadian thistle (Cirsium arvense), goldenrods (Solidago spp.), asters (Aster spp.), anemone (Anemone canadensis), woundwort (Stachys palustris), and dandelion (Taraxacum officinale) to water smartweed and sea blite (Suaeda depressa). At least five graminoids and sow thistles (Sonchus arvensis) were found in the NP wet meadow zones during both years. The most common shrub, wolfberry (Symphoricarpos occidententalis), died out as the water levels rose.

Four species of grass and a number of small sedges were present in the CRP wet meadow zones in 1993 and again in 1994 (Table 2). Spikerush and alkali-grass were found only in 1993. After the water levels rose, these graminoids were replaced by slough sedge, true rush (Juncus balticus, J. interior), prairie cordgrass, bulrush (Scirpus spp.) and a small, stunted, unidentifiable grass. As in the NP sites, wet meadow forbs also moved toward more hydrophytic species, but the effect was not as pronounced in the CRP sites.


When emergent wetland vegetation is subjected to an increase in water level, either the emergent vegetation is largely eliminated, or vegetation zones migrate up slope toward more optimal water depths (3). van der Valk (3) suggests that the die-out model is more likely to apply to large and/or abrupt water level increases, while the migration theory is more likely to prove true when water level increases are small or occur gradually over a number of years. Shallow emergent prairie pothole vegetation (sedges, water smartweed, whitetop, spikerush) is adapted to seasonal wetlands, which have an annual wet/dry cycle. Millar (4) observed that shallow marsh vegetation will experience at least a partial die-off after one winter of standing water conditions.

In this study, water levels increased substantially in one year. We saw more evidence of drier end species dying out than evidence of emergent vegetation migrating up slope (vegetation densities decreased in both the shallow emergent and wet meadow zones, per cent open water increased in the NP wetlands). Although the dominance of several shallow marsh species (slough sedge, whitetop, water smartweed) increased with flooding, their overall densities decreased by July of 1994.

The combined decrease in density and increase in vegetation height is consistent with the conclusions of Squires and van der Valk (5). They stated that for emergent species to survive in increased water depths, the plants must be able to maintain sufficient area above the water surface for gas exchange. To accomplish this, emergent plant species tend to produce fewer and longer shoots, which results in increased height and decreased density.

An examination of Table 2 shows an overall shift from drier end grasses and forbs to slough sedges, whitetop, water plantain (Alisma plantago-aquatica), bulrush, and burreed in the shallow emergent zones. Smaller and less water-tolerant forbs had lower chances for survival than forbs adapted for standing water conditions (water plantain, water smartweed). The rising water table killed shrubs in the wet meadow zones of the NP sites.

When compared to the CRP sites, the NP sites were deeper overall and had a greater percent of open water in their wet meadow zones. Predictably, the wet meadow zones of the NP wetlands showed a more dramatic change to wetter-zone, shallow marsh-type species than did the species mix in the CRP wet meadow zones (2).

Also, the decrease in vegetation diversity was more obvious in the NP sites. This, too, may be due to the greater post-flooding water depths in the NP sites. It is possible that more species were able to survive the shallower inundation depths found in the CRP wetlands (5).

The presence of bladderwort, after only one year of continuous flooding, is consistent with the findings of Millar (4) who concluded that bladderwort was one of the first submerged species to become established under conditions of year-round inundation.

1.  Barbour, M. G., Burk, J. H. and Pitts, W. D. (1987) Terrestrial 
         Plant Ecology, 2nd ed.  The Benjamin/Cummings Publishing 
         Company, Menlo Park, CA.

2.  Great Plains Flora Association, McGregor, R. L., Coordinator 
         (1991) Flora of the Great Plains.  University Press of 
         Kansas, Lawrence.

3.  van der Valk, A. G. (1994) Effects of prolonged flooding on 
         the distribution and biomass of emergent species along a
         freshwater wetland coenocline.  Vegetatio 110,185-196.

4.  Millar, J. B. (1972) Vegetation changes in shallow marsh 
         wetlands under improving moisture regime. Can. J.Bot.

5.  Squires, L. and van der Valk, A. G. (1992) Water-depth tolerances of 
         the dominant emergent macrophytes of the Delta Marsh, Manitoba.
         Can. J. Bot. 70,1860-1867.

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