Looking at the vegetation map of the world we are struck by the fact that much of the land surface is unfavorable for plant growth. Desert is the largest of the biomes. It is only in the tropical rainforest that water and temperature are continuously favorable for plant growth. This makes it the most diverse and productive biome. There can be as many as 10,000 species of plant per hectare.
Microbial activity is very rapid and little organic matter accumulates in the soil.
Fertility declines after clearance and the soils quickly become useless for agriculture. The loss of diversity, productivity and fertility make clearance a threefold tragedy. When temperature and water supply are temporarily favorable for plant growth we tend to get mixed deciduous forests. These conditions exist mostly in the Northern hemisphere. In the summer these systems approach tropical rain forest in terms of productivity and species diversity can be up to 2,000 per hectare.