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Droughts are expected to become more frequent and severe, due to climate change, with uncertain consequences for savanna vegetation. Drought-driven tree mortality has been observed in some savannas, but little is known about how tolerant savanna trees are to drought, or what determines differences among species in drought vulnerability. Here, we examine which characteristics best distinguished tree species that were most negatively affected by a recent drought in Kruger National Park, South Africa. Woody-encroaching, root-suckering species, which tended to have relatively low investments in non-structural carbohydrates, had the highest mortality rates. Of particular interest was Dichrostachys cinerea, a notable contributor to widespread woody encroachment across southern African savannas, which suffered high drought mortality. Using rainfall maps and vegetation survey data, we show that D. cinerea distributions in Kruger National Park are limited to areas with higher mean annual rainfall and lower past drought frequency, and stem densities are lower where droughts have occurred more frequently, suggesting that past droughts have had lasting impacts on the distribution and abundance of this species. Woody encroachers, such as D. cinerea, may face a trade off between traits promoting proliferation vs. drought tolerance, and more frequent and severe droughts could increasingly limit their spread. [ABSTRACT FROM AUTHOR]

Increasingly frequent and severe droughts under climate change are expected to have major impacts on vegetation worldwide. However, research to date has focused on tree vulnerability to drought in forests. Less is known about trees and drought in savannas, where a sparse tree layer coexists with grass. These tree–grass interactions (often mediated by fire and herbivory) shape savanna tree ecology, and confound predictions of how strongly drought might affect trees. On the one hand, drought is physiologically stressful, which could harm trees and be exacerbated by herbivore impacts; on the other hand, trees adapted to semiarid savannas might be relatively drought tolerant, and the considerable impacts of drought on grass could even benefit trees via reduced grass competition and fire risk, especially in the year following a drought. Here, we sought to understand the net effects of severe drought on the savanna tree layer, and how fire and herbivory mediate these effects. We monitored tree growth, mortality, and community structure for 2 yr within existing long‐term fire and herbivory experiments across a drought‐severity contrast, following a major drought in Kruger National Park, South Africa. Overall, severe drought was a major stressor for trees. Tree mortality rates in most species increased by an order of magnitude in the year following drought, and slower growth rates for some persisted for 2 yr. At the community level, this translated into substantial decreases in tree densities. Herbivory and fire did little either to mitigate or exacerbate drought effects on trees, and overall, drought swamped effects of herbivory and fire that have otherwise been observed. However, species differed in their responses to drought, with some dominant encroaching species especially vulnerable. We suggest that increasing drought frequency and severity could drastically alter savanna vegetation by repeatedly killing off trees. [ABSTRACT FROM AUTHOR]

• Monospecific woodland groves were associated with soil forms and flooding regimes in a floodplain. • Extreme floods removed trees and created substrates for woodland regeneration. • Damage by elephants and meso-browsers affected regeneration dynamics. • Flooding and herbivory interactively generated a shifting woodland mosaic anchored by soil. Our study addressed the regeneration dynamics of the patch mosaic of monospecific woodland groves in the floodplains of the Luvuvhu and Limpopo rivers following the removal of human settlements to create the Makuleke Contractual Park falling within the northern Kruger National Park. We anticipated that subsequent increases by elephants and meso-browsers like impalas would deflect or even prevent the re-establishment of these groves through the damage that they inflicted on plants. We compared the responses exhibited in groves of fever trees (Vachellia xanthophloea) associated with layered clay alluvium with those of ana trees (Faidherbia albida) associated with deep sandy deposits. Multi-decadal changes in the woodland mosaic were identified using aerial photographs and Google Earth imagery supported by onsite inspections and related to flood events and soil features. Woodland regeneration following recent major floods was tracked using onsite inspections over five years recording sapling growth and disappearance along with the breakage, bark removal and defoliation imposed by elephants, medium-sized browsing ruminants and porcupines. Fever trees were concentrated near the confluence of the two rivers where floodwaters inundated this region. Extreme floods carried away flanking ana trees while leaving sandy deposits open for recolonization. Elephants pushed over tall fever trees but not the ana trees, while regenerating saplings of both tree species were broken or uprooted. Browsing impalas defoliated juvenile trees contributing to their demise. These impacts contributed to the contraction or even disappearance of some fever tree groves and suppressed the growth and establishment of ana tree saplings. While these herbivores contributed to the shifting patch mosaic of these monospecific woodlands, the local distribution of these woodlands was associated with the soil features that they favoured. [ABSTRACT FROM AUTHOR]