A model of conductive heat flow in forest edges and fragmented landscapes

Although the creation of edges during forest fragmentation can have important abiotic and biotic impacts, especially under conditions of future climate change, mechanistic models of edge effects have not been forthcoming. A simple numerical model of two-dimensional heat flow is developed 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 foliage densities measured in the central Amazon. In the edge profile, the clearcut that abutted the edge served as a heat source and its temperature was maintained at a constant value higher than in the initially cooler forest. In the fragmented landscapes, simulated treefall gaps were heat sources whose temperature varied with sun movements during the day. Gap frequency was varied so as to approximate the gap coverage observed in selectively logged forests. In one set of simulations, temperature in the openings was systematically varied; in another, thermal diffusivity of the forest was varied. Along the edge profile, high temperatures in the clearcut were rapidly transmitted into the upper canopy due to additive edge effects. Temperatures in the forest understory were also very sensitive to clearcut temperatures due to relatively sparse understory foliage. An overall increase in forest diffusivity led to markedly higher temperatures close to the edge and a more even temperature distribution among height strata. In fragmented landscapes, total gap coverage and additivity from neighboring gaps strongly influenced forest temperatures. At low conductivities, heat flowed only into the forest close to the gaps and hence forest temperature increased almost linearly with gap area. However, at high conductivities, heat flowed far into the forest and forest temperature varied as a function of gap density in the surrounding neighborhood. Because of these additive effects, slight increases in total gap area led to disproportionate changes in the thermal profile of the landscape. These results have important implications for the conservation of forest ecosystems.