Trees Facilitate Hibiscus meyeri , a Common Savanna Forb, via Multiple Mechanisms

Questions Facilitation is a common phenomenon that underlies the local abundance and distribution of many plants. Facilitation can arise via multiple mechanisms, and classes of mechanistic effects have the potential to modulate one another's effects. We quantified two mechanisms by which trees may facilitate understory plants: amelioration of abiotic conditions and herbivory reduction. Location Kenya. Methods We quantified the spatial association of a common forb, Hibiscus meyeri, with tree canopies in an East African savanna in the presence and absence of large mammalian browsers across an aridity gradient. Namely, we compared the observed proportion of H. meyeri under a tree canopy and their distance to the tree canopy to null distributions derived from simulated landscapes. We also used a spatially naïve linear model approach to test for site and exclosure treatment effects on the probability of being under a tree canopy and on distance to canopy. Results Both our randomization and linear model approach suggested that both aridity and herbivore presence increased H. meyeri individuals' association with the tree canopy, suggesting trees facilitate H. meyeri via both abiotic habitat amelioration and herbivory reduction. In addition, herbivore presence more strongly increased H. meyeri 's association with the tree canopy at the arid site, suggesting that facilitative effects of abiotic habitat amelioration could be modulating those of herbivory reduction. Conclusions We found that trees facilitate H. meyeri via multiple facilitative mechanisms. Our results suggest that the strength of one facilitative mechanism can depend on whether other facilitative mechanisms are also operating.