Forest edges and tree growth rates in the North Carolina Piedmont

Forest fragmentation is a common process in forests worldwide, with implications for tree species composition and abundance. In particular, the effects of forest-non-forest edges on microclimate are often profound, usually resulting in increased light available to plants along the forest-non-forest edge. Using dendrochronological techniques, we assessed the effect of edges on the growth rates of four overstory species common in the North Carolina Piedmont: Acer rubrum, Liriodendron tulipifera, Liquidambar styraciflua, and Pinus taeda. Transects from the edge into the forest interior were established on 62 edges of varying aspect and ages in the Duke Forest (Durham, North Carolina, USA). Within a transect, all stems >10 cm in diameter were cored, and their spatial positions relative to the edge were recorded. Along each transect, a set of environmental and edaphic variables was measured including soil texture, soil concentrations of plant nutrients, and percentage of canopy closure. All transects were geo-referenced, and land cover data classified from a time-series of Thematic Mapper images were used to assess the age of each edge. Two species, P. taeda and L. tulipifera, had significant increases in growth rates within 5 in of an edge relative to the forest interior. However, edges explain a substantially smaller portion of the variance in growth rate than soil texture, soil nutrients, and topographically derived variables. Possible, interactions between soil texture and effects of edges on tree growth rate for P. taeda were found, although the interaction was opposite from our hypothesis that increased water availability would increase the positive effect of edges on growth rates. Although the edge effect on tree growth rate is small, as forest fragmentation becomes more prevalent worldwide, even small responses to edge effects by individual trees could have profound effects on regional patterns in carbon fixation and species composition over time.