Persistence of the African baobab (Adansonia digitata L.) in a system experiencing chronic utilization by elephants

Abstract

Chronic utilization of woody vegetation by African savanna elephants may transform woodland vegetation to open savanna and threaten the extirpation of targeted species. This situation has arisen in Gonarezhou National Park (GNP), Zimbabwe, where a large elephant population (>2 km⁻²) threatens the persistence of the long-lived baobab (Adansonia digitata) tree, through elevated mortality levels. This study investigated whether complete or partial refugia from elephant utilization existed for baobabs, and whether protection of trees by management, using wire netting or log- or rock-packs around a tree base, had been effective. A total of 563 baobabs were sampled across northern GNP. Elephants had debarked 99% of trees to some degree, and gouged holes in the stem of 22% of trees. The probability, and severity, of debarking and gouging decreased with increasing distance to permanent water, increasing slope, and the amount of boulder cover. Slope gradient had the greatest effect on elephant impact, with trees located on steep slopes being less affected by elephants than those on flat ground. Distance to water had an effect despite the farthest tree, at 9 km from water, being within the 15 km foraging range of bull elephants. Large boulders partly protected trees even on gentle terrain. None of these factors provided a complete refuge from elephant impact within GNP, but, individually and collectively, provided a partial refuge at current elephant densities. Protection measures doubled tree survival over approximately 5 years, with fencing wire wrapped around baobab stems proving to be the most effective method. Assessment of the future security of baobabs in GNP requires knowledge of their distribution and that of the refugia provided by distance from permanent water, slope, and boulder cover.