Fire limits generalized woody encroachment in a South African savanna
Woody encroachment is a concern in many grassy ecosystems worldwide, where increases in biomass and cover of trees and shrubs can drastically alter ecosystem services and carbon dynamics. The causes of woody encroachment are widely debated, with explanations ranging from changes in top-down control (e.g. fire suppression, browser population crashes, overgrazing) to shifting atmospheric and climatic conditions (e.g. rising carbon dioxide concentrations, changing precipitation patterns). We examined recent vegetation change in savannas in Hluhluwe-iMfolozi Park in South Africa through a comparison of 163 plots, each 40m x 10m, sampled in 2007 and 2014. Plots were distributed across broad gradients in mean annual rainfall, herbivore use intensity, and fire frequency, on a variety of soil types, allowing us to examine different potential drivers of change. In each plot we identified to species and measured the heights of all trees, estimated grass cover and biomass at 2 m intervals along the long axis of the plot, and counted dung piles as a proxy for herbivore use. In 2014 we also collected soil samples that were analyzed for total C, N, and P and soil texture. We used fire frequency and precipitation records maintained by park management.
From 2007 to 2014, trees in the medium size class (2-4 m in height) increased in number in our plots, while on average there was no change in small trees (0.3-2 m) or large trees (>4 m). This finding agrees with anecdotal reports of woody encroachment in the park, but the magnitude of change also varied substantially across environmental gradients, with some sites seeing neutral or negative changes in number of medium-sized trees. Sites with the greatest increase in medium-sized trees tended to have lower recent fire frequencies, higher grazer use, and higher soil sand content. Grass biomass also increased overall from 2007 to 2014, and was anti-correlated with increases in trees, suggesting the presence of competitive effects or fire feedbacks limiting one at the expense of the other. Our findings indicate that Hluhluwe-iMfolozi Park is experiencing a generalized increase in both tree cover and grass production, possibly attributable to rising CO2 levels, but that woody encroachment can still be somewhat controlled with frequent fire. Local feedbacks such as fire and herbivory will continue to operate in grassy ecosystems in the face of global change, potentially acting as a limited buffer against the direct responses of plants to novel conditions.