Rainfall is highest at the equator, where sun's evaporative power causes high evapotranspiration, and rising air cools to shed its moisture. Precipitation tapers off as one moves away from the equator, and dry belts are found at 25-30 latitude. Local variation is also great due to trade winds, ocean currents, land masses and mountain ranges. Africa Evergreen forests are replaced by deciduous forests as precipitation becomes seasonal. Wherever dry periods are of several months or longer duration, leaves are shed as the dry season takes hold, providing a winter-like visual appearance.
Leaves re-appear in anticipation of the onset of the rains.
The growing season is shortened, and so forest productivity is less than in the evergreen forests of the more humid tropics. Boundaries are gradual, and sensitive to changing conditions. A prolonged period of wetness or drought, or human intervention, can cause dramatic changes because of the transitional nature of environmental conditions. Humid tropical forests appear superficially similar everywhere, but in fact differ widely in species composition and ecosystem attributes. In the coterminous 48 United States, botanists recognize 135 natural plant formations based on dominant species.