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 pas t 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 cont ent in four selected floodplain meadows contaminated by these tailings. Tra ce metal levels frequently exceed acceptable concentrations for agricultura l soils at sampling points within the meadows. pH levels within flood-depos ited tailings are strongly to moderately acid, while pH levels outside of t ailings deposits are neutral. The data analysis: (1) shows that metals and acidity associated with tailings affect plant biomass, density, and diversi ty; (2) documents that the vegetation/metal and vegetation/pH associations are more of a threshold than a linear relationship; and (3) suggests that o ther factors may be involved in structuring the community. Vegetation diver sity, density, and biomass decrease at threshold levels of trace metal conc entrations and soil pH in all four meadows. CuSum plots of diversity in rel ation to trace metal levels show a decrease in mean diversity at 315 ppm co pper, 22 ppm arsenic, 4.2% iron, 65 ppm lead, and 170 ppm zinc. Densities o f Phleum pratense and Poa pratensis were significantly lower(P less than or equal to 0.001) on plots with more than 250 ppm copper. Above-ground bioma ss of Phleum pratense was also significantly lower on plots with copper lev els above 250 ppm. Decreased mean grass density was found on plots with pH < 6.4, but the only statistically significant difference was for Juncus bal ticus, which had increased density on plots with pH < 6.4. In contrast to t he clear impacts of trace metals and pl-l on vegetation, other site charact eristics did not alter measured vegetation characteristics.