Linking forest edge structure to edge function: An experimental and synthetic approach

Habitat fragmentation is occurring worldwide, greatly increasing the structural heterogeneity of landscapes. Consequently, habitat patches are smaller, and a greater proportion of the habitat core is close to a boundary so that the probability of interacting with contrasting habitat types is high. The fragmentation of forests has created forest edges as boundaries between forest and non-forest habitats. Edges may function to mediate the flux of organisms, material, and energy between these two habitats. Though edges represent a small fraction of the ground, their potential as a regulator of external influences on internal forest dynamics makes edges prominent features of modern landscapes.; Research on forest edges has been primarily static and descriptive. Here we take a functional approach and consider how edges interact with fluxes in the landscape. We propose that edge function is linked to edge structure and present a conceptual model to organize our research on deciduous forest edges. We identify and define edges by quantifying abiotic and vegetative gradients across them. Our results suggest that edges are distinct forest zones and that the location of the edge and the extent of edge effects differ for each variable measured. The vegetation gradients indicated that generalized responses of vegetation structure on edges overwhelm site-specific effects on vegetation patterns. In addition, we provided evidence that, below the canopy, edges receive greater nutrient inputs than forest interiors.; Our hypothesis that edges function to mediate landscape fluxes was tested experimentally by altering the structure of the edge vegetation. All vegetation lower than 1/2 canopy height was removed, including lateral branches of the canopy trees, small trees, and shrubs. Differences in fluxes across the altered and the intact edge determined how the structure of the vegetation influenced the interaction of the edge with the flux, and, therefore, the function of the edge. It was established that edges function as barriers to seed flux, as significantly more seeds traversed the altered edge than the intact edge. The structure of the vegetation on the edge also influenced the movement of herbivores, as measured by damage to seedlings. More seedlings were damaged on the edge than in the interior and more seedlings were damaged on the intact edge than the cut edge. In addition, the structure of the edge influenced the identity of the dominant herbivore. These results indicate that the density and composition of the seedling community on edges may be influenced by the structure of the edge. Finally, to guide future research, the complexity and richness of interactions between edges and fluxes in the landscape have been incorporated into a conceptual model linking edge structure to forest regeneration through nutrient fluxes and plant consumers.