Human impacts on the functioning of African savannas

Abstract

Changes in rainfall regimes in combination with rising temperatures are projected to lead to longer and more frequent droughts in savanna ecosystems. Concurrently, human impacts in African savannas alter ecosystem functioning through changing disturbance regimes, which calls for a better understanding of how increasing human activities affect the resilience of savannas. The 'resistance-recovery' framework has become a general concept to quantify the response of ecosystems to perturbation. While conceptual and theoretical progress has improved the understanding of the links between ecological change and functional diversity, experiments in natural ecosystems are now needed to independently quantify resistance and recovery at the landscape level. Here, we used a field-based experiment in an East-African savanna ecosystem to simultaneously measure both resistance to drought and recovery from above-ground biomass removal of herbaceous vegetation in a protected area (PA) and adjacent pastoral village lands (VL). Drought conditions were imposed on the vegetation using a novel method which involves the redirection of run-on rainfall water on a slope through gullies while pulse perturbations were performed by removing all aboveground standing biomass using a brush cutter. We considered the role of tree-grass interactions in altering resistance and recovery in each land use by measuring greenness underneath tree canopies and in paired inter-canopy plots before and during drought (resistance), and directly after and during a two month phase following pulse perturbation (recovery). We found that herbaceous vegetation in VL was resistant to experimental drought, while vegetation greenness was significantly reduced in the PA. Conversely, vegetation greenness recovered fast in the PA compared to the VL from experimental pulse disturbance. Trees facilitated recovery of sub-canopy herbaceous vegetation, but only in the VL. Trees did not show any effect on resistance of sub-canopy vegetation to drought for both land-uses. Our findings demonstrate that increasing human activities in the form of livestock grazing decrease the recovery potential, but may increase the resistance of savanna plant communities. This points towards a trade-off in resistance versus recovery at the landscape level, where a change in disturbance regime has likely shifted traits associated to coping with pulse disturbance (fire) to coping with press disturbance (herbivory and drought). We conclude that the use of the resistance-recovery framework is a promising way to experimentally assess the impacts of human activities on ecosystem functions and to identify elements (such as nurse trees) in the landscape that maintain response diversity. High impact grazing pushes savannas from high recovery to high resistance systems 27 CHAPTER 2