The effect of an increasing elephant population on the allometry of trees

Abstract

African savannas, which support a diverse range of plant species, from evergreen trees to deciduous shrubs, are regulated by complex interactions between climatic, fire, soil and herbivory processes. African elephants (Loxodonta africana) exert substantial top-down control over vegetation dynamics by utilising large amounts of plant biomass, as well as through various forms of non-consumptive effects. However, how much elephants affect the pattern of proportional growth between different tree components such as roots, stems or branches (known as allometry), remains understudied. Using vegetation data collected in 2000-2001 and 2019 for more than 3500 individual trees inside and outside Madikwe Game Reserve, South Africa, we determined the long-term effects of an increasing elephant population on allometric relationships in 13 tree species. The presence of elephants reduced logged canopy volume, and increasing elephant population in Madikwe reduced logged tree height and logged initial growth rate of the allometric relationship between tree height and tree diameter, without modifying asymptotic change in growth rate. Although not significantly, elephants seemed to increase asymptotic change in growth rate of canopy volume according to tree diameter, suggesting that compensatory mechanisms for the loss of height might happen in the longer-term. Although the species-specific elephant use index increased over the 20 years of the study, no consistent effect on allometric coefficients was detectable. Our study emphasises the necessity of long-term, species-specific studies to improve our understanding of how elephants affect allometric relationships in trees, which will ultimately guide management of savanna systems with increasing elephant numbers.