Community-level interactions between ungulate browsers and woody plants in an African savanna dominated by palatable-spinescent Acacia trees

Abstract

We studied the composition of a savanna woody plant community across a natural herbivory gradient maintained by both browsing and grazing ungulates in an arid part of the Kruger National Park, South Africa. We focused on (1) short-term browsing effects on reproductive and morphological traits of a dominant-palatable woody species, Acacia nigrescens, Miller, (2) the relationship between browsing–grazing intensity and soil parameters, (3) the effects of herbivore–soil interactions on woody species richness and composition, and (4) browser-induced effects on the representation of woody plant functional traits.

We show that the number of pods carried by A. nigrescens trees as well as the internode length of external tree branches both decreased significantly at high browsing intensity. Moreover, we found that total soil nitrogen (N) and soil cation (Ca, Na, Mg, and K) concentrations varied significantly according to grazing rather than browsing intensity with soil nutrients decreasing at heavily grazed sites. Although herbivory and soil properties together explained 79% of the total variability in woody species composition, neither herbivory intensity nor soil properties taken separately could explain variation in woody species richness across sites. Browsing intensity could also not account for variation in the mix of evergreen, spinescent and preferred browse plants across sites. Ordination analysis showed that the palatable but spinescent A. nigrescens had the lowest fit on the first two ordination axes and was distributed across study sites irrespective of herbivory intensity and soil properties. We suggest that community-level interactions between browsers and woody plants are different in arid eutrophic African savannas where the dominance by spinescent trees might prevent shifts in species composition under high browsing intensity, which is contrary to what has been observed in woodlands at higher latitudes.