PS 25-82 - Woody recovery after exceptional drought in African savannas

Abstract: African Savannas are important natural ecosystems full of ecological, economical, and cultural significance. They have a unique structure that features the codominance of grasses and trees in a balance that is kept in check by annual precipitation, grazing, and fire. However, the Earth’s changing climate is leading to more frequent and severe natural events that threaten this structure. One of these extreme events took place in southern Africa in 2016 in the form of an exceptional drought that changed plant community structure across many savanna ecosystems including those in Kruger National Park near the Mpumalanga province of South Africa. Our goal in this study was to identify changes in woody plant persistence and growth following this major drought, and how these trends interacted among three fire frequencies – in areas burned annually, triennially, or unburned – and under different levels of herbivory – full herbivory, large herbivore exclusion, and all herbivore exclusion. These data were collected from 2015 to 2022, excluding 2016, in long term experimental plots in Kruger National Park, South Africa.

We assessed counts and diameters of all woody species to assess initial impacts of the 2016 extreme drought, as well as the recovery from 2017-2022. These data were analyzed in R to see interactions between year, burn treatment, and herbivory level on stem diameter and number of living individuals. We found that severe drought initially caused a decrease in the number of individuals, but an increase in average diameter of trees across most fire and herbivory treatments. This was due to smaller woody plants having greater mortality rates during drought compared with larger, more established plants. In annually burned areas, total stem diameter was low and neither drought nor herbivory treatments had substantial effects. In plots burned triennially or unburned, herbivory substantially reduced total stem diameter and drought effects were much more pronounced. These findings highlight the importance of interactive effects in determining recovery trajectories after extreme climatic events, and potential management strategies that may aid the resilience of savanna ecosystems.