Most of sub-Saharan Africa lies within the tropics and has a tropical climate determined mainly by rainfall, ocean currents, prevailing winds, elevation, and latitude. The region contains rainforests near the equator with very high rainfall, canopy trees over 150 feet tall, and cash crops that can be grown. However, the rainforests are threatened by deforestation from farmers and loggers. Tropical savannas with seasonal rainfall and scattered trees cover almost half the region, including the famous Serengeti grasslands that can reach 15 feet tall. The Sahel region receives declining rainfall and is expanding southward due to overgrazing and population growth. Much of southern, northern, and eastern Africa is desert