Approximately the lower two thirds of the region is covered in rain forest, which grows progressively thicker as one travels south. The forests are composed of hardwood evergreens of species such as dibetu, ebony, iroko, mahogany, obeche, and sapelli, some of which grow to heights of 70 metres or more. There are also numerous ferns and orchids. This forest is stratified into several levels. The first is composed of straight, thick-rooted trees averaging about 40-metres tall. After this grow shorter, thinner trees with highly placed clusters of leaves. Sparse grasses and other plants make up the ground layer. In areas closer to settlements and roads, human groups have harvested the larger trees, thus exposing the forest floor to more direct sunlight and allowing thick vegetation to thrive. The upper third of the province (beginning at about the latitude of Bertoua and Batouri) is dominated by humid, wooded savanna. Trees here are sparser, but still may grow to be as tall as 20 metres.