Growth of N 2 ‐fixing African savanna Acacia species is constrained by below‐ground competition with grass

Summary 1.  The coexistence of leguminous trees and C 4 grasses in many African savannas forces competition between these two distinct plant life‐forms. The nature of tree–grass coexistence and the consequent competition for resources remains poorly understood. 2.  We hypothesized that grass suppression of tree seedling growth was due to competition for N in plants not capable of fixing N 2 and due to the costs of N 2 fixation in those capable of nodulation. 3.  Growth and N 2 fixation of nodulating ( Acacia karroo , Acacia nilotica , Acacia tortilis and Acacia nigrescens ) and non‐nodulating ( Acacia ataxacantha Acacia brevispica and Acacia schweinfurthii ) Acacia spp. in response to soil characteristics and fertilizer variations in irrigated field experiments were compared. The extent of N 2 fixation was determined from δ 15 N values. 4.  Grass suppressed the growth of all species, but particularly of the non‐nodulating species. The N 2 ‐fixing tree seedlings had high tissue N and high water‐use efficiencies (WUE, based on δ 13 C) relative to non‐nodulating legumes, making the N 2 ‐fixing species strong competitors with C 4 grasses, which have low tissue N contents and high WUE. The suppression of seedling growth by grass was smaller for plants grown in a soil with 66 mg kg −1 than in a soil with 9 mg kg −1 of Bray II P. The tissue N : P ratios (15.1) indicated that P‐limitation constrained growth of seedlings in soils with low Bray II P when grass was present, rather than the energetic costs of N 2 fixation. 5.Synthesis . In Acacia savannas and in the absence of water limitation, competition for N is the main constraint imposed by grass on growth of Acacia seedlings and that the ability to fix N 2 overcomes this limitation. However, the growth of N 2 fixing Acacias may be additionally limited by grass competition for P.