In semi-arid savannas tree cover is limited by a key demographic transition from juvenile to adult stage, which can be delayed when herbivory or fire events remove live aboveground tissues. Compensatory growth (in response to repeated browsing) and post-fire resprouting are adaptations whereby trees rapidly replace aboveground tissues. Repeated browsing can increase carbohydrate allocation to belowground storage but it is unknown if long-term browsing legacy in turn affects the capacity of trees to resprout post-fire. We asked does post-fire resprout size increase with increasing levels of pre-fire (“legacy”) browsing intensity. We investigated post-fire size responses of a widespread monodominant East African tree Acacia drepanolobium in six different herbivore “legacy” treatments using the Kenya Long Term Exclosure Experiment (KLEE). The herbivore treatments included factorial combinations of cattle, meso-wildlife species (large ungulates 15–1000 kg), and mega-herbivores (elephants and giraffes). After 20 years of exposure to herbivory treatments we applied three nested treatments using a burn-bin to top-kill individual trees and steel cages to protect individuals from post-fire browsing: burn-caged, burn-uncaged, control. Two years post-fire treatment we assessed survival, height, diameter, main stem length, and total stem length among burned-caged and uncaged trees within each herbivory level.
Results/Conclusions
Trees that were exposed to the highest level of pre-fire herbivory prior to burning (megaherbivores, meso-wildlife, and cattle) were taller and had higher total stem length than trees that had not been browsed for 20 years pre-fire. Trees protected from post-fire herbivory were taller than unprotected trees in the three plots with the highest levels of herbivory. Tree mortality was low (<16% all trees) and not significantly different among herbivory levels. Mortality was highest in burned trees (burned-caged 9%, burned 6%, unburned control 1%), and most mortality occurred the second year following fire treatments during a drought period. Our results suggest that the high levels of pre-fire browsing conferred positive compensatory growth response in burned-caged trees. In uncaged trees, post-fire herbivory reduced resprout size, suggesting that post-fire browsing offset the positive effects of compensatory growth. Our results suggest that pre-fire browsing confers compensatory growth that may help trees persist following fire. This is consistent with the concept of a persistence niche and demonstrates that above ground size may not be the best indicator of demographic stage in juvenile size (~1m) savanna trees with contrasting browsing legacies.