What can drought experiments tell us about the ecology and management of dryland vegetation?

Background/Question/Methods

About 41% of Earth’s terrestrial landmass is covered by drylands, comprising arid, semi-arid and dry subhumid ecosystems. In these ecosystems, plant growth is mainly limited by low and variable precipitation, which constrains human activities mainly to livestock production. Projected global change will increase the frequency and intensity of drought events. At the same time, drylands are facing considerable population growth, leading to increased demand for ecosystem services from vegetation, which itself might negatively feedback on ecosystems functioning and ecosystem service (ES) provision. However, combined effects of grazing and drought are still poorly understood, as few experimental studies have considered both drivers simultaneously. Hence, it remains an open question how an adequate grazing management could potentially mitigate drought effects on the functioning and ES provision from dryland vegetation.

Results/Conclusions

We present first results from DroughtAct, a field experiment combining drought and grazing treatments in semi-arid South African savanna. It simulates centennial-scale drought (intercepting 66% of rainfall for several years) in conjunction with resting schemes. It is part of Drought-Net, a global network to assess terrestrial ecosystem sensitivity to drought. According to the ‘hierarchical-response framework’ (Smith et al. 2009), we expected a temporal hierarchy of responses starting with individual responses, followed by species reordering within communities, and finally species loss and immigration. In the first treatment year, we thus concentrated on low-hierarchy responses, and recorded how within-species trait variability and population dynamics of key species responded to drought (yes/no) and grazing (continued/excluded), and how these responses were translated into aboveground net primary production (ANPP). Results show that ANPP was considerably reduced under centennial-scale drought conditions, while within-species trait responses varied with traits assessed. Future treatment years will also show if severe droughts could provide an opportunity for rangeland restoration, e.g. by opening up establishment windows for desirable species.

Literature cited: Smith, M.D., Knapp, A.K., Collins, S.L., 2009. A framework for assessing ecosystem dynamics in response to chronic resource alterations induced by global change. Ecology 90, 3279-3289.