Several factors seem to play important roles in savanna formation. A. Savannas can form in areas with a five to eight month wet season, but they more commonly have a longer wet season. B. Soil stress caused by drought, extreme moisture, or low nutrient levels favors the formation of savannas. C. Studies conducted in various regions indicate that an upper layer of white, sandy, soil is present in most permanent savannas. D. Drought stress affects trees and shrubs in savannas far less than it affects savanna grasses. E. Frequent fire is a major factor contributing to the formation and maintenance of savannas. F. In some areas, human cutting and burning is associated with savanna formation, and increase in pastureland had led to savanna expansion.