PhD defence: Wim Verbruggen

Wim Verbruggen defends his thesis,

Dryland ecosystems of the Sudano-Sahel: a vegetation model perspective

Video link

Professor Hans Verbeeck, Ghent University
Dr Félicien Meunier, Ghent University
Associate professor Guy Schurgers, IGN
Associate professor Stéphanie Horion, IGN

Assessment Committee:
Professor Anja Rammig, TU München
Professor Diego Miralles, Ghent University
Professor Wim Cornelis, Ghent University
Associate professor Martin Rudbeck Jepsen, IGN
Professor Eveline Volcke (chair), Ghent University

Drylands form a major component of our Earth's land surface. These ecosystems encompass several biomes, such as dry forests, savannas, grasslands, shrublands and deserts. More than 30% of the global human population lives in dryland ecosystems and depends on the ecosystem services that drylands provide. However, these ecosystems are subject to climate extremes that are projected to increase in frequency and severity under most future climate scenarios. Such extremes can have a devastating impact on the ecosystems and livelihoods of global drylands, as well as amplify pressure on fragile economic structures. Drylands account for a large fraction of the total land carbon sink, and have been shown to dominate its trend and year-to-year variability. Despite their global importance, drylands remain severely understudied and especially a detailed optimization of vegetation models is lacking.
In this thesis I contributed to resolve this problem. Based on data from our own field work, I parameterized two dynamic vegetation models (LPJ-GUESS and ED2.2) to dryland conditions, specifically the Sudano-Sahel region. My optimized parameterization enables these models to realistically simulate carbon and water fluxes that were measured at several fluxtower sites across the Sahel. Using the LPJ-GUESS model I then studied how the distribution of rainfall over the rainy season can impact dryland ecosystems at the site level. Using the ED2.2 model I studied how access to the perennial soil moisture layer can influence dry season water use by deep rooted trees.
In a next step I upscaled one of these models (LPJ-GUESS) to the regional level and I evaluated its simulations of the Sudano-Sahel vegetation against satellite data products. Using this regional model I then studied how soil texture can influence dryland leaf cover and ecosystem composition. I found that at the ecosystem scale the model was relatively insensitive to soil texture. In contrast, at the plant level there was a large impact, as soil texture shifed the competitive balance between evergreen and deciduous woody species. In another chapter I studied the impact of rainfall variability and found that an increased year-to-year rainfall variability can decrease dryland ecosystem productivity. This study also showed that woody trends in drylands are mainly caused by carbon dioxide fertilization. However, I also showed that rainfall variability can strongly offset these gains, especially for southern regions.
By focusing on drylands and by parameterizing vegetation models to realistically represent dryland ecosystems, this study provides a unique insight into dryland ecosystem functioning.

A digital version of the PhD thesis can be obtained from the PhD secretary Mikala Heckscher at