ity. Because the dynamics of these changes in land use as well as the species richness of the forests also change according to country, region, and economic conditions, it behooves scientists to be extremely careful when projecting local experiences to global scales.
The Holdridge Life Zone Classification System identifies some 120 ecological life zones in the world, 68 of which are tropical or subtropical (Holdridge, 1967). Thirty-two of the tropical and subtropical life zones are capable of supporting forests. About 19 million square kilometers of mature forests exist in the tropics and are distributed as follows: 42% in the dry forest life zones, 25% in the wet and rain forest life zones, and 33% in the moist forest life zones (Brown and Lugo, 1982). Statistically significant relationships suggest that life zone conditions relate to characteristic numbers of tree species (Holdridge et al., 1971), biomass and rate of primary productivity (Brown and Lugo, 1982), and capacity to resist and recover from disturbance (Ewel, 1977). These relationships are based on climatic data.