MICROENVIRONMENT VARIATION WITHIN AND AMONG FOREST EDGE SITES IN THE EASTERN UNITED-STATES

Width of the edge zone is critical to the existence of interior habitat in forest fragments. To determine how edge zone width varies among stands, microenvironmental gradients were described at ten edge sites in southeastern Pennsylvania and northern Delaware, USA. Sites were selected to allow comparisons of edge orientation and successional development. Significant edge effects were detected in light, temperature, litter moisture, vapor pressure deficit, humidity, and shrub cover, in some cases affecting the forest microenvironment up to 50 m from the edge. Variables which were dependent on direct beam radiation (vapor pressure deficit, temperature, and litter moisture) showed strong edge-oriented gradients at edges facing east, west, and south, but none at north-facing sites. Exclusion of light by closure of the side canopy curtailed edge effects in these variables. By contrast, edge-oriented gradients of shrub cover and humidity occurred independently of edge aspect and beyond the limits of direct beam radiation. Edge zone microenvironment remained distinguishable from that of continuous forest even after closure of the side canopy. These findings suggest that a large proportion of forest in small- and medium-sized fragments is climatically altered by edge proximity. Such effects recede over successional time, but are not entirely eliminated.