Locally high, but regionally low: the impact of the 2014-2016 drought on the trees of semi-arid savannas, South Africa

Drought is thought to be a factor limiting the abundance of shrubs and trees in savannas, but this has rarely been tested empirically. The severe drought of 2014-2016 provided an opportunity to document the impact of drought on the mortality of woody plants in semi-arid savannas of South Africa. Annual data from long-term tree monitoring plots were combined with opportunistic sampling in areas of high mortality to determine the extent and severity of mortality across an area of approximately 10 000 km2. Although mortality was not extensive, certain landscapes suffered mortality similar to the highest values reported for semi-arid ecosystems. Rainfall deficit was identified as a key determinant of spatial variation in mortality at the regional scale, which varied from 3% to 49% over two years. Spatial variation at the landscape scale was also high, and affected primarily by species composition and height of the woody plants, with higher mortality for taller individuals. An effect of topographic position and unknown edaphic factors was also evident. Species that suffered high mortality included a common bush encroachment species, but also many non-encroachers. Additional study is needed to confirm that the observed mortality will cause a long-term reduction in woody plant density.