Droughts are expected to increase in frequency and severity around the world, with potentially major consequences for vegetation. In tropical and subtropical savannas, little is known about the susceptibility of the tree layer to drought-induced mortality, nor the potential long-term consequences of drought for woody composition and function. Of particular interest is how drought may intersect with other ongoing patterns of vegetation change, such as woody encroachment (a widely observed phenomenon of increasing woody biomass and cover in grassy ecosystems); depending on the drought sensitivity of woody encroachers, drought could either mitigate or accelerate encroachment. A severe drought in South Africa in 2014-2016 provided a rare opportunity to observe savanna vegetation responses over multiple years and answer key questions about variation among tree species in drought sensitivity. Here, we combine results from four years of post-drought tree monitoring in Kruger National Park, South Africa with an analysis of key functional traits distinguishing the most drought-sensitive and drought-resistant species. We also examine how these traits relate to woody encroachment propensity, and use vegetation survey data and historical rainfall maps to study how the frequency of past droughts has influenced the distribution of a dominant woody-encroaching species (Dichrostachys cinerea).
Results/Conclusions
Following the 2014-2016 severe drought in Kruger National Park, we observed high rates of tree mortality and overall declines in tree densities, followed by multiple years of slow recovery. Species varied widely in their responses, with some species much more sensitive than others. The most drought-sensitive species were characterized overall by lower investments in non-structural carbohydrates and polyphenols, higher rates of root-suckering, and higher wood density. The same suite of traits also distinguished woody-encroaching species, which were over-represented among drought-sensitive species, suggesting a potential trade-off between drought resistance and rapid proliferation. The most dramatic example of this was in Dichrostachys cinerea, a widespread woody encroacher that suffered high rates of drought-induced mortality. We also found that D. cinerea distributions throughout Kruger were limited both by mean annual rainfall and by the frequency of past droughts, suggesting that slow rates of post-drought population recovery can have long-lasting impacts on the extent and abundance of this otherwise highly successful species. As droughts increase in frequency and severity, they will increasingly shape the composition of savanna woody communities, and could potentially make woody encroachment by drought-sensitive species easier to control.