Variable effects of termite mounds on African savanna grass communities across a rainfall gradient

Abstract

Questions: Termite mounds of the genus Macrotermes are prominent features inAfrican savannas, forming nutrient hotspots that support greater plant diversity, which is of higher nutritional value than the surrounding savannamatrix. However, little is known about grass communities on and around mounds or how the functional importance ofmounds varies across sites. Asmean annual rainfall increases, savannas in southern Africa become increasingly dystrophic through increased denitrification (including pyrodenitrification) and the leaching of soil nutrients. The functional importance of mounds is concomitantly expected to increase as the difference in foliar nutrient levels between mounds and thesavannamatrix increases.We tested this prediction on grass communities acrossa rainfall gradient to: (i) determine the degree to which grass assemblages differbetween termite mounds and the savanna matrix; (ii) determine the spatial extent to which mounds influence grass communities; and (iii) investigate whether these patterns differ across savanna types.Location: Kruger National Park, South Africa.Methods: Grass communities were surveyed at three savanna sites differing in mean annual rainfall (550–750 mmyr 1). Grass diversity and tissue nitrogen concentrations were measured on and off termite mounds and along transects away from mounds in order to calculate the spatial influence of termite mounds on savanna grass communities. Using termite mound densities estimated fromairborne LiDAR, we up-scaled field-based results to determine the percentage of the landscape influenced by Macrotermes termite activity.Results: Although species richness of grasses was lower on mounds than in thesavanna matrix, the assemblage composition varied significantly, with higher nutrient concentrations in grasses located on mounds. This pattern became more distinct with increasing rainfall. The spatial extent of these nutrient-rich grasses also differed across the rainfall gradient, with a larger sphere of influence around mounds in wetter areas. Mounds distinctly altered grass communities over ca. 2%of the entire landscape.Conclusions: Our results show that Macrotermes mounds are important componentsof savanna heterogeneity, and reveal that the functional importance of mounds increases with increasing rainfall.