The impacts of trace metals on grass communities along the floodplains of Soda Butte Creek, Montana and Wyoming

Stoughton, J.A.

in: Bozeman, MT: Montana State University, pp. 110


Existing research documents that high levels of copper and other, trace metals can be toxic to plants, but little has been written about the spatial extent of ecosystem disturbance due to trace metal deposition. Outside of Yellowstone National Park, in Montana and Wyoming, tailings from an old mine have been deposited downstream by large flood events. Elevated levels of trace metals from the displaced tailings have been identified 25 km downstream (Meyer, 1993). This study evaluates whether grass species diversity, density, and biomass: (1) decrease as trace metal concentrations increase; and (2) decrease as soil pH decreases. In order to assess the role of environmental variables unrelated to mine tailings, the study also evaluated whether grass species diversity, density, and biomass: (1) vary with soil clay content and salinity; (2) vary as a function of distance from the stream channel or elevation above the. channel; or (3) change in the downstream direction. Trace metal levels frequently exceed Maximum Acceptable Concentrations for agricultural soils (Kabata-Pendias and Pendias, 1992) at points within all four meadows sampled along Soda Butte Creek. pH levels in the four sites mark the presence of strongly acid and moderately acid soils, with values as low as pH 3.4. Results from four meadow sites along Soda Butte Creek indicate that vegetation diversity, density, and biomass decrease at threshold levels of trace metals and soil pH. CuSum plots of diversity in relation to trace metal levels show a decrease in mean diversity at 3I5ppm copper, 22ppm arsenic, 4.2% iron, 65ppm lead, and 17Oppm zinc. Densities of Phleum pratense and Poa pratensis were significantly lower (p<=0.001) on plots with more than 25Oppm copper. Above-ground biomass of Phleum pratense was also significantly lower on plots with copper levels above 250ppm. Decreased mean grass density was found on plots with pH<6.4, but the only statistically significant difference was for Juncus balticus. which had increased density on plots with pH<6.4. In contrast to the clear impacts of trace metals and pH on vegetation, factors such as soil salinity, soil clay content, site elevation above stream and distance from stream did not alter vegetation patterns.

Patrick Cross1995