Abstract
It is suggested that local heterogeneity in soil properties and surface microenvironment, caused by the influence of physical factors within gaps in the forest canopy, may underlie part of the geographical differences in tree species diversity. The hypothesis is based on latitudinal trends in the angle and intensity of solar radiation, precipitation, temperature and the distribution of nutrients between soil and vegetation. It is argued that the temporary environmental gradients in such openings are much broader in the tropics than in the temperate regions.