Habitat destruction results in fragmentation of previously continuous environments. These fragments have greatly increased edge to interior habitat ratios. Such edge-proximate areas experience a wide variety of potentially deleterious abiotic and biotic changes. These 'edge effects' can be further modified by edge aspect. Edge effects are becoming of greater concern in the Appalachian region due to suburbanization. However, there have been no investigations into how edge effects impact the pollination and reproduction of American ginseng (Panax quinquefolius L.), an ecologically and culturally important plant species in the region due to its annual harvest and subsequent export. Five out of 30 monitored natural populations of P. quinquefolius are within 50m of anthropogenic edge. To investigate responses of this forest interior species, a total of 567 flowering-size P. quinquefolius were planted at four distances in linear transects parallel to the edge at two edges (northeast and southwest aspects) at each of three forested sites. An additional control transect was planted in interior forest (>80 m from edge) at each of the three sites. Three flower manipulation treatments were imposed to distinguish the sources of pollen at each distance in this selfing species: (1) no treatment (open pollinated), (2) bagged, and (3) emasculated.
Results/Conclusions
Among all aspects, there was no observed effect of distance from edge on seed-set among plants that produced seeds. Possible explanations of this result include compensatory pollination by disturbed area-dwelling pollinators like the European honeybee (Apis mellifera) or the natural Halictid bee pollinators being unaffected by proximity to edge. While P. quinquefolius did not have a significant overall reproductive response to distance from edge, individuals at southwest aspects had significantly greater seed-set than individuals at northeast aspects, thus suggesting abiotic edge effects of southwest aspects actually increase P. quinquefolius pollinator activity, embryo survival, and subsequent successful seed maturation relative to individuals at northeast aspects. As hypothesized, plants at southwest aspects displayed a stronger response than those at northeast aspects, however the response of flower manipulation treatment as a function of distance from edge was complex. Seed-set of control, bagged, and emasculated plants at northeast aspects remained constant constant across all distances from edge. However, seed-set of emasculated plants increased, seed-set of bagged plants remained constant, and seed-set of control plants decreased as distance from edge increased at southwest aspects. These complex results could potentially be explained by changes in pollinator guild or activity coupled with edge-dependent changes in geitonogamy rate.
