Grass community responses to drought in an African savanna

Drought is often discussed as a major perturbation in savanna ecosystems, associated with lasting shifts in savanna structure and functioning. However, historically, droughts have occurred frequently in southern African savannas, suggesting that plants might benefit from being resilient to drought. Here, we evaluate whether the 2014-2016 drought was an extreme event in terms of effects on the herbaceous vegetation and specifically on grasses. We monitored herbaceous responses, pre-, during and post-drought in the Kruger National Park and compared responses with a previous extreme drought (1991-1992). Grass biomass decreased during both droughts, but recovered quickly once rains returned, regardless of whether areas burned in the years preceding drought. However, species functional composition was slower to recover, with shifts in dominance that closely matched grass photosynthetic machinery. Species drought tolerances were best predicted by their bundle sheath morphology: species characterised by XyMS- bundle sheaths (often associated with NADP-ME photosythesis) declined in abundance consistently during drought, whereas those characterised by XyMS+ bundle sheaths (associated with PEP-ck and NAD-ME photosynthesis) either increased or were unaffected by drought. The response of grass to drought, at least in terms of productivity and functional composition, is relatively predictable.