SAVANNA

In areas of reduced annual precipitation, or prolonged annual dry seasons, open tropical and subtropical deciduous forests give way to a kind of open grassland with scat-lered shrubs and trees, called savannas. The savanna biome, on a global scale, is in a sense transitional between tropical rain forest, which is evergreen, and desert. Generally 90 to 150 centimeters of rain fall each year in savannas, which generally grow on nutrient-poor soils that are often rich in aluminum, a substance that is toxic to most plants. Soil conditions apparently exert the major influence on the formation of savannas, at least in Latin America, but water relationships are Important also. There is a wider fluctuation in temperature here during the year than in the tropical rain forests, and there is seasonal drought. These factors have led to the evolution of an open landscape, often with widely spaced trees, in which large grazing mammals are sometimes characteristic, as in Africa. Periodic fires also are an important factor in the maintenance of savannas.
Some tens of thousands of years ago, toward the close of the Pleistocene Epochs vegetation similar in appearance to that of today's African savannas was widespread in North America. It disappeared as the climate became more and more like it is now-with even greater extremes of temperature and longer periods of seasonal drought, especially in the West. In many areas, humans seem to have slaughtered the remaining large animals that made up the extraordinary Pleistocene herds, and the vegetation evolved into the modern communities that we know today.
In savannas, the trees are usually deciduous and lose their leaves in the dry season. Savannas often gradually give way on their drier borders to thorn forest, plan' communities dominated by thorny trees that are seasonally deciduous. In Southeast Asia, a similar plant community, called monsoon forest, occurs in such dry regions, Under and among the scattered trees of these communities, perennial grasses and other plants with food stored in roots or underground stems are common.