Browsing and fire interact to suppress tree density in an African savanna

Abstract

Disturbances from fire and herbivory strongly affect savanna vegetation dynamics. In some savannas, fire especially may be instrumental in preserving the coexistence of trees and grasses. The role of herbivory by large mammals is less clear; herbivory has been shown variously to promote and to suppress tree establishment. Here we ask how interactions between herbivory and fire act to shape savanna vegetation dynamics via their effects on tree populations in Hluhluwe iMfolozi Park in KwaZulu Natal, South Africa, a savanna with a full complement of native large mammals. We examined the effects of herbivore exclusion on tree growth, mortality, and seedling establishment from 2000 to 2007 at 10 sites located in areas of low and high herbivore pressure throughout the park. Results were analyzed statistically and using Leslie matrix models of population dynamics. Herbivory and fire acted primarily to suppress sapling growth rather than on sapling mortality or seedling establishment. This indicates that browsing, like fire, suppresses tree density by imposing a demographic bottleneck on the maturation of saplings to adults. Model results suggest that, while browsing and fire each alone impacted growth, a combination of browsing and fire had much greater effects on tree density. Only fire and browsing together were able to prevent increases in tree density. These results suggest that, while soil resources, including nutrients and moisture, are probably instrumental in determining tree growth rates, disturbances from fire and herbivory may be instrumental in limiting tree cover and facilitating the coexistence of trees and grasses in savannas.