Unraveling Fire and Herbivore Interactions to Manage Tree Cover in an African Savanna
Savanna ecologists continue to debate the relative importance of fire and herbivory disturbances in affecting local savanna tree cover and applied issues such as "bush encroachment". Contemporary grass-tree co-existence models and a large body of literature emphasize the importance of both top-down disturbance types. However, no fully replicated experiments have simultaneously manipulated fire and herbivore regimes to directly examine potentially synergistic interactions controlling tree demography. We hypothesized that the “fire trap” wherein trees are repeatedly top-killed by fire and prevented from transitioning to taller fire resistant height classes, is further reinforced by the negative effects of ungulate browsing on tree height. Conversely, grazing by livestock or the absence of all herbivory was expected to increase height and biomass of re-sprouting trees after fire. We used the Kenya Long Term Exclosure Experiment (KLEE), which for the past 20 years has restricted access by six different combinations of mega-herbivores (i.e., elephant and giraffe), meso-wildlife (e.g. gazelle, oryx, cape buffalo), and cattle. Within each of 18 four-ha KLEE plots a 30 X 30 meter prescribed burn was implemented in 2013. We used linear regression models to compare pre-fire tree height and post-fire tree re-sprout height and morphology relationships among the six different herbivore treatments. One and a half years after the fires, the relationship between pre-fire tree height and post-fire tree height was dependent upon both the herbivore treatment and colonization by ant mutualists that defend trees against browsing. In the presence of wildlife (i.e. browsers) trees compensated for lost tissue by increasing the number of lateral basal stems. We expect that these shorter multi-stemmed growth forms will have prolonged susceptibility to future fires reinforcing the negative effects of fire on tree cover. These results highlight that long term changes in tree cover may be dependent upon the stocking rate/ density of both wild and domestic herbivores following disturbance.