The structuring influence of wetlands in semi-arid environments: From water availability dynamics to landscape structures in a southern African savanna

Soutenance de thèse
 06/11/2025
 14:00:00
 Alexis Roy, ESE
 IDEEV - Salle Rosalind Franklin

This thesis was co-supervised by Florence Hulot and Kamel Soudani ESE   .

Abstract Arid zones cover approximately 40% of the Earth’s surface and comprise a group of ecosystems where limited water resources are a key factor in their ecology. In these arid climatic zones, there are ecosystems where water is more or less intermittent, known as wetlands, which are both biodiversity hotspots and important terrestrial carbon reservoirs. In arid environments, they are also a major source of hydration for wildlife and are home to specific species. While the contribution of climatic parameters to large-scale spatial dynamics has been well studied, the relationship between these parameters, the dynamics of wetland filling and drying, and the spatial organization of vegetation at different scales has so far been little studied. However, our hypothesis is that these parameters are closely linked by the limitation of water availability. This is the case in southern Africa, which is undergoing particularly intense climate change, and whose impacts on wetlands are still poorly understood and described. In this context, how do water availability and dynamics in wetlands structure the landscape and functioning of southern African savannas?

To this end, we focused on Hwange National Park in Zimbabwe, where human impacts other than climate change are limited. Monitoring temporary wetlands in arid environments is a complex task. First, we confirmed a significant increase in temperatures since 1986. Using remote sensing methods, we were able to document the seasonal dynamics of filling and drying between 1986 and 2020, and quantify the frequency of water presence. We thus highlighted a two-month lag between peak precipitation and maximum wetland filling, marking the distinction between climatic aridity (characterized by precipitation and temperature) and ecological aridity (actual water availability). The analysis also revealed that the transition from diesel to solar pumping increased the presence of water during the dry season at artificial water points. This increased availability promotes the formation of piospheres, gradients of degraded vegetation around water points, due to the concentration of wildlife during periods of drought. Using a saturation model of a vegetation index, we showed that the size of piospheres is directly dependent on the frequency of surface water availability. Finally, a spectral unmixing method enabled us to detect a significant reduction in water surface area in intermittent wetlands, correlated with the rise in temperatures observed since 1986. Although vegetation cover was not directly influenced by climate, it was significantly linked to water surface area, which itself was decreasing in the context of global warming within temporary wetlands. We have thus demonstrated that while climate plays a crucial role in hydrological variations in wetlands, water availability is the primary factor influencing landscape dynamics in their vicinity, with contrasting effects depending on the frequency of surface water presence.

Composition of the jury

• Prof. Daniel Gilbert, University of Franche-Comté, Rapporteur and examiner
• Dr. Annelise Tran, University of Montpellier, Rapporteur and examiner
• Prof. Ludwig Jardillier, University of Paris-Saclay, Examiner
• Prof. Gema Parra, Universidad de Jaén, Examiner