COS 69-3
Manipulating soil harshness to reduce plant invasion at a mine restoration site
Harsh sites, defined as those having high levels of abiotic stress, are thought to be less invaded by non-native plant species. Serpentine grasslands contain a number of abiotic stressors including low Ca:Mg, low levels of nutrients, high levels of heavy metals, and shallow topsoil. These communities have been historically less invaded than non-serpentine grasslands, and experimental evidence has demonstrated that amelioration of abiotic stress in serpentine soils increases invasion. We examined whether abiotic harshness influenced the distribution of native and non-native grassland species at a serpentine restoration site, and whether harshness could be manipulated to decrease dominance of non-native plants and increase dominance of natives. Within eight plots at a former mercury mine site, mine waste material with high levels of mercury and low levels of nutrients was overlaid with stockpiled topsoil in depths ranging from 0 to 50cm to create a gradient of decreasing soil harshness. Native grass species were seeded into the plots, while non-native grasses and forbs recruited from the surrounding area. Plant density and aboveground biomass of native and non-native plants were measured for two years, and shoot nutrients for a selected native species were assessed in the first year. Density data were used to calculate genus-level Shannon-Wiener diversity. Topsoil depth was measured at each location where plant data were collected.
Results/Conclusions
Topsoil depth significantly influenced the plant community in both years of the study. In Year 1, diversity, native grass biomass, and shoot N and P of the native annual Festuca microstachys were positively correlated with topsoil depth. In contrast, native grass density was negatively correlated with topsoil depth. Non-native biomass and density were not significantly affected by depth in the first year. In Year 2, native and non-native density and biomass were all positively correlated with topsoil depth. Although density of natives was initially higher in the harsher end of the gradient, results suggest that the less harsh, more fertile deep topsoil may have enabled both native and non-native plants to produce more viable offspring, leading to higher density and biomass of both in deeper topsoil in the second year. Findings indicate that although increasing soil harshness may reduce non-native plant growth, native growth (and possibly fecundity) was reduced by soil harshness as well.