Tropentag, September 10 - 12, 2025, Bonn "Reconciling land system changes with planetary health"

Land-use history mediates responses to global change drivers in a semi-arid rangeland

¹University of Bonn, Inst. Crop Sci. and Res. Conserv. (INRES) - Plant Nutrition, Germany
²University of Potsdam, Inst. of Biochemistry and Biology, Biodiversity Research / Syst. Botany, Germany
³Namibia University of Science and Technology, Dept. of Agriculture and Natural Resources Sciences, Namibia
⁴Fraunhofer-Institute for Computer Graphics Research IGD, Competence Center Bioeconomy, Germany
⁵University of Namibia, Dept. of Environmental Science, Namibia
⁶University of Bonn, Inst. Crop Sci. and Res. Conserv. (INRES) - Soil Science and Soil Ecology, Germany

Abstract

Increasing drought frequency and land-use intensification are key global change drivers threatening dryland ecosystems, with significant implications for forage provision and livelihoods, particularly in the Global South. However, the combined effects of drought and grazing on rangeland productivity remain poorly understood.
We investigated (1) how drought and consecutive grazing affect aboveground net primary productivity (ANPP), and (2) whether land-use history mediates these effects. To address these questions, we established a full-factorial field experiment in a semi-arid rangeland in Namibia. Rainfall reduction was simulated using passive rain-out shelters (–66% of ambient rainfall), while grazing was mimicked through repeated clipping at three intensities (low, moderate, high). Land-use history was represented by two sites differing in grazing pressure due to their proximity to a watering point and consequently different levels of degradation. We quantified ANPP through biomass harvests and assessed vegetation composition to estimate functional group-specific productivity (perennial/annual grasses, forbs, legumes).
Our results from the 2021/22 vegetative season demonstrate that the effects of one-year drought and two years of grazing were strongly mediated by land-use history. At the more degraded site, grazing significantly increased ANPP (F = 4.37, p = 0.02), but this positive effect disappeared under drought combined with moderate grazing (interaction: F = 3.44, p = 0.04) leaving ANPP unaffected compared to low grazing. In contrast, the less degraded site showed no significant changes in ANPP under any treatment. Shifts in plant functional group composition appeared to buffer productivity losses at both sites. We compared ANPP values with the ones from the previous season under ambient conditions and identical clipping treatments and found the relative year-to-year changes diverged significantly between sites: productivity increased by 40% on average at the healthier site, while it declined by 27% at the degraded site, despite above-average rainfall. These findings highlight not only the interactive nature of global change drivers but also the crucial role of past land-use in shaping ecosystem responses. Understanding such dynamics is vital for sustainable rangeland management under increasing environmental stress.

Keywords: ANPP, cattle grazing, degradation, drought, field experiment, Namibia, savannah ecosystem

Contact Address: Lisa-Maricia Schwarz, University of Bonn, Inst. Crop Sci. and Res. Conserv. (INRES) - Plant Nutrition, Karlrobert-Kreiten-Str. 13, 53115 Bonn, Germany, e-mail: lisa.schwarzuni-bonn.de