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.
