ABSTRACT
The worldwide evidence that high hills and mountains usually have both more rainfall and more natural forest than do the adjacent lowlands has, historically, led to confusion of cause and effect. Forests contribute to the stability of watersheds by protecting the soil surface from the direct impact of intensive tropical rainstorms. Only two tropical forests, covering the watersheds of the Congo and the Amazon, are large enough to have some effect on the moistening of major airflows. Early quantitative evidence on the effects of forests on streamflow came from the southern United States, where a severely eroded watershed of 90,000 square kilometers was restored, beginning in the 1930s. By providing a physical obstruction that slows overland flow, by protecting the topsoil, and by strengthening riverbanks, forests minimize the transport of soil and thus moderate the main mechanism of watershed degradation. Protection of steep upland watersheds must therefore depend on government policy and resources for land management.
