Large herbivores and season independently affect woody stem circumference increment in a semi-arid African savanna

Abstract

Seasonality is a characteristic feature of savannas. Assuming plants are dormant in the dry season, long-term browsing by large herbivores (or their exclusion) in savannas should affect stem growth in the wet season alone. In addition, plant survival should improve with exclusion. A total of 236 dendrometers on six woody species in three large (>25 ha) plots (all large herbivores excluded, elephants excluded, control—no large herbivores excluded) were observed from 2006 to 2013 in Kruger National Park, South Africa. Average monthly increment (AMI) per dendrometer was calculated and the interaction between exclusion and season (dry or wet) was tested species-wise. The AMIs of Acacia exuvialis, Combretum apiculatum, Dichrostachys cinerea and Grewia flavescens were generally reduced by herbivore exclusion, while the unpalatable Euclea divinorum was unaffected. Survival rates of C. apiculatum and E. divinorum were not significantly associated with exclusion, while other species’ survival rates benefitted from exclusion. The dry-season AMIs of all species were significantly lower than wet-season AMIs, but dry-season AMIs were positive for two species common on footslopes (A. grandicornuta and E. divinorum), while being almost zero for species common on midslopes and crests, possibly because of hydrological variation on the catena. The results suggested that stem circumference growth of prominent woody species in semi-arid savannas is prone to reduction by exclusion of large herbivores, except for generally unpalatable species. Large herbivores may affect woody stem growth in semi-arid savannas by altering competition among plants.