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. INTRODUCTION The ubiquity of agricultural ecosystems and their significant role as a major source of nutrients (Rew et al. 1992, Weathers et al. 2001), pesticides (Marshall 1987, de Snoo et al. 1999), species (crops, weeds), and even new genes (Marvier 2001), makes the documentation and understanding of ecological communities surrounding intensively cropped agricultural fields of great importance in planning and management of both agricultural landscapes and nearby natural communities (Le Coeur et al. 1997, 2002; Marshall and Moonen 2002). In agricultural landscapes the vegetation surrounding crop fields can serve as an ecological buffer, potentially isolating undesirable agricultural elements such as agrichemicals and weedy species from adjacent non-agricultural ecosystems (Moonen and Marshall 2001). One concern is whether weedy borders of crop fields operate as source populations of invasive exotic species that may spread to surrounding communities (Fraver 1994, Honnay et al. 2002, Merriam 2003). Unfortunately, there are relatively few studies of the structure and composition of plant communities surrounding agricultural fields in relation to their distance from those fields (but see Fraver 1994, Boutin and Jobin 1998, Honnay et al. 2002, Harper et al. 2005, Piessens et al. 2006). Of particular interest is the distribution of exotic species with respect to distance from field edges, whether particular species traits are associated with distance from field, and whether there are structural patterns to the *email address: [email protected] Present address: Department of Biology, Syracuse University, 107 College Place, Syracuse, New York 13244. Received November 24, 2008; Accepted April 24, 2009. CASTANEA 74(4): 327–339. DECEMBER 2009