The effects of floods, droughts and elephants on riparian tree mortality in a semi-arid savanna

Abstract

Riparian forests have declined around the world over the past century. This is mostly attributed to alteration of river flow regimes and abstraction of alluvial groundwater, although in Africa damage from elephants is often considered the primary cause. Large stands of riparian forest along the Limpopo River in Mapungubwe National Park (South Africa) were transformed to woodland between 1990 and 2005, and water stress resulting from reduced river flows and abstraction, an extreme flood and canopy infestation by climbing plants were identified as contributing causes. Recent recolonization of the area by elephant has created an additional potential driver of mortality. To determine if tree losses have continued and gain further insights into the causes - particularly the relative importance of water stress versus debarking by elephant - tagged trees were surveyed annually from 2011 to 2022. Rates of mortality were higher than in the previous decade for four of the five dominant tree species. Bank erosion from flooding remained a cause but accounted for far less mortality than in the past. For individuals not lost to floods, survival analysis indicated that low rainfall was the primary driver of mortality. Canopy infestation by climbers did not decrease survival, and while elephant damage was common for all species and most likely killed some individuals, it had far less effect than our measures of water availability. Isotopic analysis of xylem, soil, river and groundwater revealed few differences in source water use between species, and marginal contributions from the alluvial aquifer. All species relied on deep soil water, illustrating a key linkage between rainfall and survival under the current hydrological regime. Persistence of the remaining large trees at this site is highly unlikely, considering the altered flow regime of the river and likelihood that future climate change will exacerbate water stress. Given the degree of ecosystem change over the past four decades, future conservation should shift focus to the recruitment and establishment of other large tree species, particularly those tolerant of water stress and elephant browsing.