Persistent Impacts of Trace Metals from Mining on Floodplain Grass Communities Along Soda Butte Creek, Yellowstone National Park.

/ In Yellowstone National Park, tailings and associated trace metals from past mining have been deposited along 28 km of Soda Butte Creek by large flood events. This study documents grass species diversity, density, and biomass; trace metal concentrations in soils; and soil pH, salinity, and clay content in four selected floodplain meadows contaminated by these tailings. Trace metal levels frequently exceed acceptable concentrations for agricultural soils at sampling points within the meadows. pH levels within flood-deposited tailings are strongly to moderately acid, while pH levels outside of tailings deposits are neutral. The data analysis: (1) shows that metals and acidity associated with tailings affect plant biomass, density, and diversity; (2) documents that the vegetation/metal and vegetation/pH associations are more of a threshold than a linear relationship; and (3) suggests that other factors may be involved in structuring the community. Vegetation diversity, density, and biomass decrease at threshold levels of trace metal concentrations and soil pH in all four meadows. CuSum plots of diversity in relation to trace metal levels show a decrease in mean diversity at 315 ppm copper, 22 ppm arsenic, 4.2% iron, 65 ppm lead, and 170 ppm zinc. Densities of Phleum pratense and Poa pratensis were significantly lower (P </= 0.001) on plots with more than 250 ppm copper. Above-ground biomass of Phleum pratense was also significantly lower on plots with copper levels above 250 ppm. 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, other site characteristics did not alter measured vegetation characteristics.