A MODEL OF CONDUCTIVE HEAT-FLOW IN FOREST EDGES AND FRAGMENTED LANDSCAPES

Although the creation of edges during forest fragmentation can have im portant abiotic and biotic impacts, especially under conditions of fut ure climate change, mechanistic models of edge effects have not been f orthcoming. A simple numerical model of two-dimensional heat flow is d eveloped and applied to a vertical forest/clearcut edge profile and to simulated fragmented landscapes. Height-specific thermal diffusivity and conductivity in the forest were assumed to vary in proportion to f oliage densities measured in the central Amazon. In the edge profile, the clearcut that abutted the edge served as a heat source and its tem perature was maintained at a constant value higher than in the initial ly cooler forest. In the fragmented landscapes, simulated treefall gap s were heat sources whose temperature varied with sun movements during the day. Gap frequency was varied so as to approximate the gap covera ge observed in selectively logged forests. In one set of simulations, temperature in the openings was systematically varied; in another, the rmal diffusivity of the forest was varied. Along the edge profile, hig h temperatures in the clearcut were rapidly transmitted into the upper canopy due to additive edge effects. Temperatures in the forest under story were also very sensitive to clearcut temperatures due to relativ ely sparse understory foliage. An overall increase in forest diffusivi ty led to markedly higher temperatures close to the edge and a more ev en temperature distribution among height strata. In fragmented landsca pes, total gap coverage and additivity from neighboring gaps strongly influenced forest temperatures. At low conductivities, heat flowed onl y into the forest close to the gaps and hence forest temperature incre ased almost linearly with gap area. However, at high conductivities, h eat flowed far into the forest and forest temperature varied as a func tion of gap density in the surrounding neighborhood. Because of these additive effects, slight increases in total gap area led to disproport ionate changes in the thermal profile of the landscape. These results have important implications for the conservation of forest ecosystems.