Fire Herbivore Trap: Effects Herbivory on Post-Fire Tree Re-sprout Size and Morphology in Acacia drepanolobium Savanna.

In savanna ecology, grass-tree co-existence models emphasize the importance of two top-down disturbances herbivory and fire in controlling tree cover. However, no fully replicated experiments have simultaneously manipulated both fire and herbivory regimes to examine potentially synergistic interactions controlling tree demography. We hypothesized that a 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 trees top-killed by fire. The Kenya Long Term Exclosure Experiment (KLEE) is a replicated factorial experiment that manipulates access by six different combinations of mega-herbivores (i.e., elephant and giraffe), wildlife (e.g. gazelle, oryx, cape buffalo), and cattle. Within each of 18 four-ha KLEE plots 30 X 30 meter prescribed burns were implemented in 2013. We used a generalized linear models to compare pre-fire tree height and post-fire tree re-sprout height and morphology relationships among six different herbivore assemblages. One and a half years after fire, the relationship of pre-fire tree size to post fire total stem length (biomass proxy) did not depend on herbivore treatment, however the relationship between pre-fire tree size and post-fire tree height did vary with treatment. Browsed trees compensated for lost tissue by growing laterally and increasing branch length. These shorter growth forms are more susceptible to future fires reinforcing the negative effects of fire on tree height. These results highlight that post fire vegetation responses are highly dependent upon the type and stocking rate/ density of both wild and domestic herbivores. Understanding how fire and herbivory interact to affect tree cover (i.e. bush encroachment) has important applications for wildlife habitat management, livestock production, and pastoral livelihoods in East Africa