In the northern Sahel, dunes are covered with scrub grasses and spiny acacia trees. Farther south, greater rainfall permits denser vegetation. Sands begin to give way to clay. Large date palm plantations are found on the Tagant Plateau, and savanna grasses, brushwood, balsam, and spurge cover fixed dunes. Occasional baobab trees dot the flat savanna grasslands of the southern Sahel. Forest areas contain palm trees and baobabs. Vast forests of gum-bearing acacia grow in Trarza and Brakna regions. Farther south, particularly in Assaba and the northern portion of Guidimaka regions, rainfall is high enough to support forms of sedentary agriculture.