Elephant herbivory, frost damage and topkill in Kalahari sand woodland savanna trees

Abstract

Questions: Which factors best predict the probability of elephant and frost damage in Kalahari sand woodland savanna? What is the association between tree mortality and the disturbance regime? Location: Western Zimbabwe. Methods: Elephant and frost damage, topkill, and whole-plant mortality were quantified in ten common tree species in a Kalahari sand savanna in Zimbabwe. Individual trees were tagged in 20 plots and monitored over a two-year period. A model selection approach was used to test the association between the probability of damage and size, prior damage, and neighbourhood effects, and to investigate the effect of damage on mortality. Results: Elephant damage differed strongly among species, and was not influenced by neighbourhood effects or prior disturbance. Frost damage also varied across species, and declined as a function of stem size and neighbourhood tree cover, and - against expectations - prior disturbance. Topkill increased as a function of elephant and frost damage, but was lower in previously damaged than in undamaged trees. Conclusions: Frost and elephant damage are influenced by community composition, and frost damage is also correlated with community structure and prior disturbance. Frost is an important and generally overlooked disturbance agent in southern African woodlands, where it may play a key role in association with other disturbance factors - such as elephant herbivory - that reduce woodland canopy cover.