Inducible defences in Acacia sieberiana in response to giraffe browsing

Abstract

Acacia trees comprise a principal component of the diet of many African browsers. The resultant browsing pressure has led to the evolution of both physical and chemical responses in Acacia trees. In an observational study, we investigated the physical and chemical defenses in Acacia sieberiana var. woodii in response to different intensities of giraffe herbivory. Trees with high browsing intensity had significantly longer spines, smaller leaves and higher total cyanide (prussic acid) concentrations than trees with low browsing intensity. No significant differences were present in the density of spines and leaves between high and low browsing intensity sites. Similarly, no significant differences were found in the concentrations of condensed tannins and total leaf nitrogen. However, comparisons with other Acacia species revealed that the sampled A. sieberiana trees had substantially higher condensed tannin concentrations than many other species. It therefore appears that condensed tannins may form a high constitutive, or fixed, defence in A. sieberiana in this population, while prussic acid and spine length (with a decrease in leaf size) form important inducible defences.