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

Abstract

African elephants (Loxodonta africana), in conjunction with the community of browser species, exert substantial top-down control over the woody vegetation in savannas by utilising large amounts of plant biomass, as well as through non-consumptive effects. However, how much browsers affect the pattern of proportional growth between different tree components remains understudied. Using vegetation data collected in 2000-2001 and 2019 for more than 3,500 trees inside and outside Madikwe Game Reserve, South Africa, we determined the long-term effects of an increasing elephant population, in conjunction with the community of meso-browsers, on structural relationships in 13 tree species. The number of trees utilised by elephants increased between 2000 and 2019, but individuals were not more intensively utilised. After almost two decades of use by elephants, we observed a reduction in the logged initial growth rate of the structural relationship between tree height and stem diameter, without modification of the asymptotic change in growth rate. Despite species-specific variability, tree height was overall reduced for a given stem diameter. Canopy area, as well as its structural relationship with stem diameter remained mostly stable. We suggest that elephants are responsible for hedging by reducing tree height. Together with impala (Aepyceros melampus), the dominant species in this meso-browser community, they could stimulate regrowth by browsing the canopy of the vegetation maintained in the browsing trap. Our study emphasises the necessity of long-term, species-specific studies to improve our understanding of how the browser community, and elephant in particular, affect structural relationships in trees.