Changes in irradiance and soil properties explain why typical non-arboreal savanna species disappear under tree encroachment

Abstract

Savanna vegetation maintains its openness and its diverse plant composition because of frequent fire events; however, when these are suppressed, encroachment is caused by increases in the tree density. In the neotropical savanna (cerrado of Brazil), typical forest trees are invading savanna areas, altering abiotic conditions and affecting the persistence of their exclusive species. Here we studied changes in abiotic conditions, species richness and diversity of a non-arboreal community (herbs, vines, grasses, subshrubs and shrubs) in a gradient of encroachment at a site where fire has been suppressed for more than 50 years in south-eastern Brazil. Encroached communities were more shaded and possessed a wetter and richer soil (higher contents of organic matter and P) compared with the typical savanna. These abiotic changes were related to a less rich and less diverse plant community in encroached savanna compared with typical savanna. The most important abiotic variables driving such changes were photosynthetically active radiation (PAR) reaching the understorey and soil P content: communities with lower species richness and diversity had lower PAR incidence and higher soil P content. Our results suggest that non-arboreal savanna species may be under serious threat of extinction given the expected ecological changes caused by the widespread expansion of forest on the savannas in the absence of fire.