Management Regime and Field Age Affect Species Richness and Cover of Native Forbs and Exotic Species in Virginia Grasslands

Abstract

The majority of grasslands in the eastern United States are maintained through agricultural use (livestock grazing and hay production), intermittent management as fallow fields, or active management for ecological or recreational purposes. Management following agricultural use can follow a variety of practices from benign neglect to active planting of native grasses and forbs. We surveyed 64 grasslands in a 15-county region of northwestern Virginia to assess their plant species composition, with emphasis on the response of exotic species and native forb species richness to time since agricultural use. With regard to agricultural use, we found that livestock grazing resulted in low levels of native species richness and increased exotic species prevalence, while hay production increased native forb richness. In these fields, eutrophication (as measured by phosphorus levels) was a strong positive predictor of exotic species. Post-agricultural fields, maintained through mowing (fallow), retained native species but also experienced sharp increases in exotic species. When post-agricultural management included the establishment of native grasses and forbs, a higher initial richness of native species resulted. However, fields disked during establishment lost native species and gained exotic species with increasing field age; an outcome not observed when field establishment did not involve disking. The management practices applied to post-agricultural fields significantly impact their ability to support biodiversity, their propensity to harbor exotic species, and their ability to maintain native diversity and resist invasions of exotic species with increasing age since abandonment.