Northern Prairie Wildlife Research Center
George A. Swanson¹
U.S. Fish and Wildlife Service
Northern Prairie Wildlife Research Center
Jamestown, ND
A prairie wetland complex located in the eastern edge of the Missouri Coteau in south-central North Dakota was investigated from 1967 to 1992. The study was designed to define the hydrology of a prairie wetland complex, the role of hydrology in influencing wetland chemistry, and the combined influence of hydrology and chemistry on plant and animal communities. Wetland communities were determined by the position that each wetland basin occupied within the landscape with respect to elevation and the associated groundwater gradients that controlled dissolved salt concentrations. Wetland communities occupying basins located along hydrologic gradients were further modified by annual fluctuations in water level and dissolved salts, which established and maintained vegetative zones, and by long-term trends in climatic conditions, which cycled semipermanent wetlands between extremes of flooding and drawdown. Aquatic communities dominating the individual wetland basins reflected hydrologic and chemical gradients from landscape features. Knowledge of the physical and biological factors that determine the functions and values of natural wetland complexes is prerequisite to establishing a comprehensive management plan for manipulating wetland communities.ABSTRACT
The semiarid climate that prevails in the prairie pothole region interacts with the glaciated landscape to produce highly dynamic wetland complexes that cycle between extremes in wet and dry conditions over extended periods of time. Knowledge of the physical and biological factors that determine functions and values of natural wetlands is prerequisite to establishing a comprehensive wetland management plan. Unless wetland management guidelines are based on information derived from studies of the dynamics of natural wetland complexes, the process becomes one of trial and error with low probability of duplicating the functions and values of natural wetlands.INTRODUCTION
An interdisciplinary, cooperative study was initiated in the prairie pothole region of North Dakota by U.S. Fish and Wildlife Service biologists and U.S. Geological Survey hydrologists to define the hydrology of a prairie wetland complex, the role that hydrology plays in influencing wetland chemistry, and the combined influence of hydrology and chemistry on wetland plant and animal communities (Winter and Carr 1980, LaBaugh et al. 1987, Swanson et al. 1988). By combining the information derived from this study with a previous description of hydrology in the Cottonwood Lake Waterfowl Production Area (Eisenlohr 1972) it is possible to describe hydrologic trends and subsequent vegetative changes that have occurred over a 32-year period (1961-92). This paper describes the response of emergent vegetation, with emphasis on cattails (Typha spp.), to hydrologic trends observed in The Cottonwood Lake study area (CLSA).
The CLSA is located in Stutsman County near the eastern edge of the Missouri Coteau in south-central North Dakota (Fig. 1). This area was purchased by the U.S. Fish and Wildlife Service on August 6, 1963 to be managed as a waterfowl production area. The site at the time of purchase contained 281 acres of native grasses, 94 acres of wetlands, and 93 acres of cultivated land that was seeded to perennial grasses and legumes. The CLSA is bordered on the east and west by pasture, on the north by hayed land, and on the south by cultivated land. Most of the cultivated land has recently been enrolled in the U.S. Department of Agriculture Conservation Reserve Program.THE STUDY AREA
The study area contains 10 semipermanently flooded basins (Stewart and Kantrud 1971), designated by the prefix "P" on Fig.2, and 8 seasonally flooded basins. All wetlands with the prefix "T" (Fig. 2) are seasonally flooded, with the exception of T2, which is semipermanently flooded. Seasonally flooded basins overflow into adjacent semipermanent basins during periods of high water. Wetlands T2 and P8 are integrated, semipermanent wetlands that routinely overflow into wetland P9. Wetlands P1, P6 and P7 are isolated semipermanent wetlands that are nonintegrated. Wetlands P2, P4 and P5 are semipermanent wetlands that integrate during periods of high water but, as a unit, are closed to surface outflow. Wetland P11, a semipermanent wetland located two miles to the west in a topographic low, functions as a hydrologic sump. Salts accumulate because dominant water loss is to the atmosphere. Semipermanent wetlands (with the exception of P11) are dominated by hardstem bulrush (Scirpus acutus) and cattails (Typha angustifolia, T. latifolia, and their hybrid T. X glauca). Wetland P11 is dominated by salt-resistant plants such as alkali bulrush (Scirpus maritimus) and sago pondweed (Potamogeton pectinatus). Seasonal wetlands that function as groundwater recharge areas are dominated by marsh smartweed (Polygonum coccineum) and slough sedge (Carex atherodes). Seasonal wetlands that receive groundwater contain higher concentrations of dissolved salts and are dominated by whitetop (Scolochloa festucacea). The wetland complex has not been grazed since 1966.
This study was initiated in the spring of 1967. Wetland water conditions and the response of emergent vegetation to hydrologic trends were documented with staff and constant-recording water-level gages, ground and aerial photographs, and 0.25-m2 plots of emergent vegetation located along randomly selected transects. Annual trends in wetland zonation were documented with vertical aerial low-altitude 35-mm color photographs. Plant species identifications within zones were confirmed by ground-truthing. Methods used to investigate wetland hydrology and chemistry on the study area are described by Winter and Carr (1980) and LaBaugh et al. (1987).METHODS
RESULTS AND DISCUSSION
Landscape features in concert with climatic trends dictated the biotic structure of the wetland complex by controlling basin hydrologic functions and their influence on water quality. Wetland basins located on topographic highs functioned as groundwater recharge areas, accumulated atmospheric water low in dissolved salts (dominated by calcium bicarbonate), and supported plant and invertebrate communities unique to a freshwater seasonal water regime (Stewart and Kantrud 1971). Cattails that invaded seasonal wetlands during periods of high water were eliminated during drought.
Basins located at intermediate levels in the landscape received groundwater that contained dissolved salts and lost water through evapotranspiration, groundwater recharge, and, on occasion, surface outflow. Basins that received groundwater tended to be higher in dissolved salts dominated by calcium and magnesium sulfates and bicarbonates, and supported plants and invertebrates typical of a semipermanent water regime (Stewart and Kantrud 1971). Cattail flourished under this water regime and achieved dominance following drawdown. Extended periods of drought increased the salt content of the water and, consequently, salt-tolerant species germinated during drawdown. As the water table dropped in response to extended drought, flow-through systems began to function as recharge areas. Rainfall flushed surface salts lower in the basin soil profile. Under these conditions salt concentrations were reduced at the surface allowing cattail seeds to germinate on sites that were previously too high in salt to support cattails.
Wetland basins located in topographic lows functioned as groundwater discharge areas, lost water through evapotranspiration, and concentrated salts dominated by sodium sulfate. These basins functioned as hydrologic sumps and supported plants and invertebrates that tolerated high salt concentrations. Cattails were restricted to groundwater discharge sites that were lower in dissolved salts than the open water. During drawdown, exposed salt crystals were removed by wind action and deposited in the upland. The influence of wind action on wetland salt concentration is being assessed.
Aquatic communities that dominated basins within the wetland complex reflected the hydrologic and chemical gradients that tracked landscape features. Semipermanent wetlands cycled between extremes in water level over time, from highs that eliminated emergent vegetation, to drawdowns that exposed mud flats, germinated seeds, and reestablished emergent vegetation. Basins with surface outflow maintained stable water low in dissolved salts and supported floating mats of cattail. Basins closed to surface outflow tended to be highly dynamic in both water level and salt concentration as they responded to climatic trends. Seeds of salt-tolerant species germinated during drawdown, when specific conductance of the water approached 8,000 µS/cm. Cattails on the CLSA were controlled by elevated salt concentrations, flooding, and drawdown.
During this investigation (1967-91), three periods of drought (1973-74,1976-77 and 1988-91) initiated a trend in the wetland complex toward lower pond and ground-water levels. This trend, which periodically exposed wetland substrates and supported seed germination on exposed mud flats, caused all of the semipermanent wetlands in the complex to undergo dominance conversion from open water to dense, monotypic stands of cattail. The drought of 1973-74 converted wetland P4 from a basin dominated by open water to a monotypic cattail stand. The drought of 1976-77 converted four additional semipermanent wetlands on the site (P2, P3, P6, and P7) to monotypic cattail stands. The drought of 1988-91 caused the central open water zone of the remaining semipermanent wetlands to draw down and germinate cattails. Once the central zone of a wetland basin of the complex was dominated by cattail, the wetland never reverted to the open-water conditions observed during the early years of the investigation. Management techniques that can manipulate flooding, drawdown, and salt concentrations potentially will control cattail monotypes. Cattail-dominated wetlands are a major problem for managers in the prairie pothole region because they concentrate migrating blackbirds and lack the open water emergent cover ratios that are attractive to breeding waterfowl.
SUMMARY
Integrated (flow-through) wetland basins that receive and discharge surface water have their maximum depth controlled by the elevation of the outlet. Suspended solids entering the basin are released and deposited in the basin substrate as water velocity decreases. This deposition process reduces the volume of water that wetlands can store by reducing mean depth. Hybrid cattails can tolerate deeper water than other cattail species and expand as the basins fill in and water depth decreases. The resulting wetlands are dominated by monotypic stands of cattail that cannot be managed by flooding unless the elevation of the outlets are increased.
A combination of wetland drainage, which accelerates integration of the remaining wetland basins, and soil erosion, related to runoff from cultivated fields, has converted previously nonintegrated wetland basins to silt traps that tend to be dominated by cattail as filling progresses. Once the natural cycle of flooding and drawdown that manipulates wetland emergent vegetation has been eliminated by siltation, and cattails become dominant, management options are limited to physical or chemical control. Installing water-control structures at the outlet of basins that have been silted in, or excavating basin sediments to increase water depth for cattail control, is usually too costly especially when incoming waters carry heavy silt loads from cultivated fields. Unless wind and water erosion is controlled in the watershed, increasing water depth will only serve as a temporary solution to cattail dominance.
lPresent address : 122 18th Av. NE, Jamestown North Dakota 58401
Appreciation is extended to H. K. Kantrud, N. H. Euliss, Jr., and D. P. Fellows for critical review of this manuscript.
Eisenlohr, W. S., Jr. 1972. Hydrologic investigations of prairie potholes
in North Dakota, 1959-68. U.S. Geol. Surv. Prof. Paper 585. 102pp.
Grue, C. E., L. R. DeWeese, P. Mineau, G. A. Swanson, J. R. Foster, P. M.
Arnold, J. N. Huckins, P. J. Sheeham, W. K. Marshall, and A. P. Ludden.
1986. Potential impacts of agricultural chemicals in waterfowl and
other wildlife inhabiting prairie wetlands: an evaluation of research
needs and approaches. Trans. N. Amer. Wildl. Nat. Resour. Conf. 51:357-383.
LaBaugh, J. W., T. C. Winter, V. A. Adomaitis, and G. A. Swanson. 1987.
Hydrology and chemistry of selected prairie wetlands in the Cottonwood
Lake area, Stutsman County, North Dakota 1979-82. U.S. Geol. Surv.
Prof. Pap. 1431. 26pp.
Stewart, R. E., and H. A. Kantrud. 1971. Classification of natural ponds
and lakes in the glaciated prairie region. U.S. Fish Wildl. Serv.
Resour. Publ. 92. 57pp.
Swanson, G. A. 1987. An introduction to the Cottonwood Lake area. Proc.
N. D. Acad. Sci. 41:25.
Swanson, G. A., T. C. Winter, V. A. Adomaitis, and J. W. LaBaugh 1988.
Chemical characteristics of prairie lakes in south-central North
Dakota--their potential for influencing use by fish and wildlife.
U.S. Fish Wildl. Serv. Tech. Rep. 18. 44pp.
Winter, T. C., and M. R. Carr. 1980. Hydrologic setting of wetlands in the
Cottonwood Lake area, Stutsman County, North Dakota. U.S. Geol. Surv.
Water Resour. Invest. 88-99. 42pp
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://www.npwrc.usgs.gov/resource/plants/cattail/swanson.htm
Page Contact Information: Webmaster
Page Last Modified: Saturday, 02-Feb-2013 06:05:31 EST
Menlo Park, CA [caww54]