Figure 1.
Conceptual framework of African savanna where N2‐fixing trees and grasses, as well as ungulate browsers and grazers, occupy opposite positions in an interconnected cycle of processes (a). Forage (b) and dung samples (c) were collected in a semiarid savanna at the Mpala Research Centre (36°52′E, 0°17′N) in Kenya. Leaves of six individual trees of the N2‐fixing species Acacia (Senegalia) brevispica, A. mellifera, Acacia (Vachellia) drepanolobium, and A. gerrardii were collected, together with leaves of up to 20 individuals of the dominant C4‐grass species Brachiaria lachnantha, Pennisetum mezianum, P. stramineum, and Themeda triandra in January 2016. Additionally, fresh dung of the browser species Guenther's dikdik (n = 6), eland (n = 3), giraffe (n = 5), greater kudu (n = 3) and Thomson's gazelle (n = 6), and the grazer species African buffalo (n = 5), Burchell's zebra (n = 6), Grevy's zebra (n = 2), and common warthog (n = 3) were collected. Forage and dung samples were dried, ground, and analyzed for total N and P concentrations. Classification of a herbivore species as a browser or grazer was based on dung δ13C values, with browsers ranging from −28 to −25 and grazers from −16 to −13, according to (Codron & Codron, 2009; Codron et al., 2007). Comparisons between N2‐fixing trees and C4‐grasses, and between browser and grazer dung, were performed with balanced ANOVA and unbalanced ANOVA, respectively. Panel (d) shows hypothesized effects of dung N:P ratio and dung quantity on the competitive strength of N2‐fixing tree seedlings; that is, biomass when growing in a mixture with C4‐grasses relative to biomass in the absence of the competing grasses (values >1 indicate that tree seedlings are superior competitors and values <1 that grasses are superior)