Edge Effects of Linear Canopy Openings on Tropical Rain Forest Understory Microclimate

We investigated microclimatic edge gradients associated with grassy powerlines, paved highways and perennial creeks in wet tropical forest in northeastern Australia during wet and dry seasons. Photosynthetically active radiation, air temperature and vapor pressure deficit, soil temperature, canopy temperature, soil moisture, and air speed in the rain forest understory were measured during traverses perpendicular to the forest edge. Light intensity was elevated near the edges of powerlines, highways, and creeks, but this effect was strongest for creek edges. Air temperature and vapor pressure deficit were elevated near powerline edges in the dry season and highway edges in both wet and dry seasons but were not elevated near creek edges in either season. In contrast, soil moisture was lowered near creek edges but not near either powerline or highway edges. No edge gradients were detected for air speed. Canopy temperature was elevated near highway edges and lowered near powerline edges in the wet season but no edge gradients in canopy temperature were detected near creek edges in either the wet or the dry season. We suggest that these different edge gradients may be largely the result of differences in the fluxes of latent and sensible heat within each type of linear canopy opening, with periodic flood disturbance assisting by maintaining a more open canopy near creek edges. Our data indicate that the nature of the linear canopy opening is at least as important as the width in determining the nature and severity of microclimatic edge effects, analogous to the "matrix effect" of traditional fragmentation studies.