Ungulate browsing as an ecosystem process: browser-plant-soil interactions in a Southern African Savanna

Abstract

Ungulate browsing and its ecological effects at plant, population, community, and ecosystem levels were addressed in a eutrophic southern African savanna. This was to test predictions of prevailing hypotheses, which are based on research in boreal and temperate forests. Changes in plant morpho-functional traits and population structure of a staple palatable species, Acacia nigrescens Miller were addressed over a two-year survey among vegetation stands with very different histories of attack from herbivores. Moreover browser-induced effects on functional composition of a woody plant community were addressed along a strong browsing gradient. Nutrient cycling was investigated through measurements of leaf litter decomposition rates, as well as soil and leaf chemistry analyses. Finally, a modelling approach was used to make predictions on plant productivity and changes in soil nutrient availability under ungulate browsing according to opposite plant defensive traits (i.e. tolerance vs resistance). I found evidence that long-term selective browsing may negatively affect soil nutrient pool, at least in the vicinity of palatable woody plants. I proposed this might be due to the drastic reduction of leaf-twig litter mass returned to the soil, which likely decreased decomposer activity and negatively affected N mineralization rates. Hence, the processes responsible for a loss of nutrients in the soil were different from those proposed for northern hemisphere scenarios, which were instead due to decreased litter quality. However, I found evidence of high plant resilience in heavily browsed sites where Acacia stands shown (1) higher leaf N during the main growing season, (2) higher N release from leaf litter, (3) high concentrations of nitrate (NO3) and ammonium (NH4), and (4) similar, or even faster, litter decomposition rates than in lightly browsed sites. Firstly, this suggests that tree pruning triggers and maintains a fast nutrient cycling within the plant-browser system. The accelerating effect is supported by high mass compensatory growth abilities from highly palatable, fast-growing Acacia trees that produce highly decomposable litter. Secondly, browsing may have a long-term decelerating effect on N cycling through quantitative changes in litter production rather than through qualitative changes in litter chemistry. Further studies should better address processes related to soil nutrient cycling to confirm such hypothesis. I discuss how the interactive effects of browsing-grazing-soil fertility may influence nutrient cycling trough different ecological processes.