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 in African savannas, forming nutrient hotspots that support greater plant diversity, which is of higher nutritional value than the surrounding savanna matrix. However, little is known about grass communities on and around mounds or how the functional importance of mounds varies across sites. As mean 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 the savanna matrix increases. We tested this prediction on grass communities across a rainfall gradient i) to determine the degree to which grass assemblages differ between termite mounds and the savanna matrix, ii) to determine the spatial extent to which mounds influence grass communities, and iii) to 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 mm.yr-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 from airborne LiDAR, we upscaled 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 the savanna 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 ~2% of the entire landscape. CONCLUSIONS : Our results show that Macrotermes mounds are important components of savanna heterogeneity, and reveal that the functional importance of mounds increases with increasing rainfall.