Grass species richness decreases along a woody plant encroachment gradient in a semi-arid savanna grassland, South Africa

Context Savanna grasslands are undergoing rapid land cover transformation as a consequence of woody plant encroachment. It still remains unclear how increasing woody plant density and cover in savannas influences tree-grass-soil interactions. Objectives This study determined the effect of increasing tree density on grass species richness, evenness, diversity and assessed how nutrients change along an encroachment gradient of a shallow plinthic savanna soil. Methods Vegetation was quantified and soil samples analysed from eighteen randomly distributed 10 m x 10 m plots along a tree density gradient spanning from open, transitioning into intermediate and intensively encroached savanna grassland in northern South Africa. Results We found that an increase in tree density along the encroachment gradient decreased grass richness (GR) by 53% in the intensively woody-encroached grassland. Higher tree density and canopy cover led to a 90% dominance of Panicum maximum grass species. Conversely, higher tree density increased concentrations of carbon, nitrogen, phosphorus and exchangeable calcium and magnesium in the topsoil layer. Conclusions This study highlights that woody plant encroachment decreases grass species richness and composition, but increases nutrients in the uppermost layer of a plinthic savanna soil. Such information is critical to improve understanding of the mechanisms underlying tree-grass interactions in savannas.