Why is Abundance of Herbaceous Legumes Low in African Savanna? A Test with Two Model Species

Although fire is frequent in African savanna ecosystems and may cause considerable loss of nitrogen (N), N 2 ‐fixing herbaceous legumes—which could be expected to benefit from low N conditions—are usually not abundant. To investigate possible reasons for this scarcity, we conducted a pot experiment using two common plants of humid African savannas as model species, the legume Cassia mimosoides and the C 4 grass Hyperthelia dissoluta . These species were grown at different levels of water, N and phosphorus (P), both in monoculture and in competition with each other. In the monocultures, yields were significantly increased by the combined addition of N and P in pots receiving high water supply. In pots with interspecific competition, the legume grew poorly unless P was added. Foliar δ 15 N values of legume plants grown in mixtures were considerably lower than those in monocultures, suggesting that rates of symbiotic N‐fixation were higher in the presence of the grass. Grass δ 15 N values, however, were also lower in mixtures, while N concentrations were higher, indicating a rapid transfer of N from the legume to the grass. We conclude that the main reason for the low abundance of C. mimosoides is not low P availability as such, but a greater ability of H. dissoluta to compete for soil N and P, and a much higher N‐use efficiency. If other C 4 grasses have a similar competitive advantage, it could explain why herbaceous legumes are generally sparse in African savannas. We encourage others to test these findings using species from other types of savanna vegetation.