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Wetland plant species prove sensitive to hydrologic conditions

The nationís wetlands represent remnants of increasingly endangered plant communities. Biodiversity of native plant species is threatened in many wetland ecosystems by loss of acreage to encroaching agricultural and urban development and by degradation related to the use of surrounding land and the influx of exotic species. To develop management strategies that can sustain native plant communities in wetlands located in disturbed landscapes and to design restorations that can support native vegetation, an understanding of hydrologic requirements of native and introduced species is critical.

Western Ecology Division scientists have taken an initial step in identifying hydrologic conditions that may support common native species in wetlands in disturbed settings while limiting introduced species. They assessed the relationship between water depth and water level variation and the distribution of 31 species commonly found in the Portland, OR, metropolitan area. Both plant species and plant communities could be identified relative to their optimal hydrologic conditions. Often those optima were narrowly separated, however, and the conditions favoring desirable plants often were bracketed by conditions that favored undesirable species. For example, the assemblage richest in native species occurred under intermediate hydrologic conditions while drier conditions and less variability in water depth favored plant assemblages dominated by pasture grass, and higher water with greater variability favored plant assemblages dominated by invasive reed canary grass. The findings suggest that minor changes in water levels (approximately 10 centimeters) and variability, (plus or minus 2 cm) could promote a shift from assemblages of native plants to those dominated by invasive or alien varieties. (Contact M.E. Kentula, 541-754-4478; kentula.mary@epa.gov)

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