Vegetation structure of field margins and adjacent forests in agricultural landscapes of the North Carolina Piedmont

Irregularly disturbed forests surrounding crop fields in agricultural landscapes often serve as ecological buffers that separate undesirable agricultural elements such as agrichemicals and weedy species from adjacent ecosystems. However, the nature of this interface between fields and forests remains little studied, particularly within the context of how species composition changes with field distance and how far exotic species penetrate into forest interiors. In three agricultural landscapes in the North Carolina Piedmont, we surveyed plant communities in multi-scale, nested quadrats arrayed along transects perpendicular to field boundaries and penetrating >200 m into adjacent forests. Rates of species turnover, patterns of species richness, and the distribution of exotic species were assessed for 18 transects. Plant communities exhibited high rates of compositional turnover within 50 m of field boundaries, but species turnover was considerably reduced beyond this threshold. Vegetation composition within borders immediately adjacent to cornfields was dominated by a relatively predictable set of weedy forb and graminoid species, of which a substantial proportion was exotic. Composition of surrounding forest communities more reflected local environmental conditions and contained few exotic species overall. Species richness was not influenced by field proximity. We suggest that agricultural influences on landscape-scale vegetation patterns are most apparent in plant communities located very close (<50 m) to continuous agricultural operations. Although weed communities associated with agricultural management are represented by a large pool of exotic species, relatively few of these species are able to penetrate forest boundaries.