COS 62-1 - Disaggregating direct and indirect impacts of large herbivores on African savanna tree and herbaceous structure: an experimental test combining high-resolution field and LiDAR data

Abstract: Large mammalian herbivores exert strong top-down control on vegetation structure in African savannas. Recent herbivore declines have underscored this linkage, most notably the irreplaceable role of megaherbivores in regulating tree cover and preventing woody encroachment. However, herbivore impacts on vegetation comprise both direct and indirect interactions and differ substantially with body size (which itself varies >1,000-fold within many herbivore assemblages). Plant responses to herbivory are similarly variable across growth forms and species. Understanding how herbivores affect vegetation structure thus requires disaggregating the effects of herbivores and the response of plants, as well as more completely accounting for indirect interactions that affect vegetation structure. We combined high-resolution Light Detection and Ranging (LiDAR) with field measurements from size-selective herbivore exclosures in a Kenyan savanna to determine how herbivore impacts on vegetation structure vary with body size, how different plant types and species respond to herbivore exclusions, and whether herbivore impacts on plant diversity and plant-plant interactions act as indirect mechanisms to alter vegetation structure. We found that herbivore impacts varied substantially as a function of body size and plant type: megaherbivores were the only size-class to affect tree structure, while medium- and small-bodied species had distinct effects on the structure of herbaceous vegetation (which did not respond to the exclusion of megaherbivores alone). Plant species responses were similarly varied: of the five numerically dominant tree species at our study site, only three exhibited significant individual morphological variation across exclosure treatments, and the subset of herbivores responsible for these changes varied across tree species. Herbivores tended to reduce plant diversity, and the strength of this effect varied as a function of both herbivore body-size and plant type; more diverse tree and herbaceous communities tended to be more structurally complex. Herbivores also altered the strength and direction of tree-herbaceous interactions, which shifted from facilitative in the presence of herbivores to competitive in their absence. Collectively, these results suggest that herbivore-induced variation in savanna vegetation structure is driven by several factors, including direct and indirect interactions between plants and herbivores, which both vary as a function of herbivore body size and plant identity.