Kruger National Park (KNP), a biodiversity hotspot in South Africa, recently experienced one of its worst droughts on record, with a moderate drought in 2015 followed by an exceptional drought in 2016. This drought resulted in significant declines in net primary production, forage availability, and surface water as well as dramatic declines in large herbivore populations, with the remaining animals left to consume low quality and unpalatable forage. For almost a decade, we have been assessing how this savanna grassland responds to alterations in fire frequency and herbivory, key determinants of plant community structure and ecosystem function, providing us with baseline understanding of how fire regimes and grazing pressures interact under standard climatic conditions. We build on this dataset to address how fire and grazing regimes interact with extreme drought conditions to impact herbaceous plant communities and ecosystem functioning. We hypothesize that the exceptional 2015-2016 drought will induce a rapid shift in herbaceous community composition from dominance by unpalatable grasses to dominance by palatable grasses, while simultaneously increasing plant species diversity.
Results/Conclusions
For 10 years prior to the 2015-2016 drought, plots with annual and intermediate burn frequencies were dominated by a single unpalatable grass species, Bothriochloa radicans, while plots that remained unburned were co-dominated by palatable grass species and displayed low to moderate abundance of B. radicans. Even after ten years of herbivore removal, grazing had little effect on dominance prior to drought and no effect on plant richness. Conversely, in this first year of post-drought recovery, the community shifted from dominance by unpalatable to palatable grasses and forbs, while simultaneously increasing plant species diversity across all treatments, although this effect is stronger in annual and intermediate burn frequencies. Surprisingly, this shift was seen in both grazed and ungrazed plots, suggesting that B. radicans may be grazing resistant but lacks drought tolerance. Thus, an exceptional drought with severe negative short-term impacts has caused a transformation of the savanna grassland, potentially increasing available large herbivore forage and biodiversity in the long run.