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

Abstract

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 d 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 d 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.

Vegetation state changes in the course of shrub encroachment in an African savanna since about 1850 CE and their potential drivers

“…This perennial grass spreads across disturbed fields forming dense tall swards, which inhibit the growth of other grasses (Cech, Edwards, & Olde Venterink, 2010;Jordaan, 2017). This perennial grass spreads across disturbed fields forming dense tall swards, which inhibit the growth of other grasses (Cech, Edwards, & Olde Venterink, 2010;Jordaan, 2017).…”

A dominance shift in arid savanna: An herbaceous legume outcompetes local C4 grasses

“…Thereby, it is generally assumed that the living grasses use their superior competitive abilities (Cech, Edwards, & Olde Venterink, 2010) to maintain their dominance by suppressing the germination, reducing the growth, and thus regulating the establishment of trees and other competing species (Bond, 2008; D’Odorico, Okin, & Bestelmeyer, 2012; Sankaran, Ratnam, & Hanan, 2004). However, some studies contradict these findings and found, for example, seedlings of Acacia mellifera to grow and establish even when growing within grass tussocks (Joubert, Smit, & Hoffman, 2012; Rothauge, 2011).…”

Biological nitrogen fixation: rates, patterns and ecological controls in terrestrial ecosystems

“…The last mechanism also could be influenced by biotic interactions; for example, Cech et al [73] demonstrated that a dominant C 4 grass could outcompete an herbaceous legume for otherwise-available P (and N) in a tall-grass Tanzanian savannah ecosystem, thereby suppressing legume growth and N fixation.…”