The relative influence of fire and herbivory on savanna three-dimensional vegetation structure

The relative importance of fire and herbivory on vegetation structure has been the subject of much debate in savanna ecology. Fire regime and herbivore numbers are two key variables that managers of protected areas can manipulate to meet their conservation objectives. We deployed a new airborne remote sensing system (Carnegie Airborne Observatory) to the Kruger National Park (KNP), South Africa, to map a unique herbivore/fire exclusion experiment on basaltic soils. We collected high resolution (56 cm) three-dimensional (3-D) vegetation structural data over areas that have been protected from herbivores (34 yr) and/or fire (7 yr), as well as those exposed to both disturbance agents. Canopy height distribution, as well as the distribution of foliage within the vertical canopy profile, differed significantly between all treatments and between each, treatment and the control area (Kolmogorov-Smimov, p < 0.001). Herbivory exerted a greater influence on vegetation 3-D structure and heterogeneity than did fire. At the broad scale, total percentage woody cover was 36 times greater in areas protected from herbivores, compared to the control area. At a finer scale, areas protected from herbivores contained 5 times more tall tree canopy (> 9 m) and up to 66 times more small tree canopy (3-6 m). Fire restricted growth of vegetation in the 0-3 m height range, both in the absence and presence of herbivores. Our findings highlight the active role that conservation managers can play in modifying vegetation structure and heterogeneity through herbivore and fire management, as well as the value of 3-D remote sensing for the assessment of conservation management outcomes. (C) 2009 Elsevier Ltd. All rights reserved.