A spatiotemporal analysis of desert ecosystems with phytogenic mounds (nebkhas)

Date: 15 September 2015

Venue: UAntwerp - Campus Drie Eiken - Building Q - Promotiezaal Q0.02 - Universiteitsplein 1 - 2610 Antwerp (Wilrijk)

Time: 10:00 AM

Organization / co-organization: Faculty of Science, Department of Biology

PhD candidate: Jan Quets

Principal investigator: Ivan Nijs & Stijn Temmerman

Short description: PhD defence Jan Quets - Faculty of Science, Department of Biology


Climate change and human land management can cause desertification. The latter phenomenon is considered a huge environmental issue affecting most part of all drylands on Earth. During the last several decades many regions were affected by desertification, and, at the moment of writing, various other areas are at risk of becoming so. Desertification implies hysteresis, meaning that there is no simple turning back by merely changing the environmental parameters to what they were during pre-desertified conditions; this highlights the robustness of desertified states. A widespread outcome of desertification is the occurrence of landscapes with phytogenic mounds, or so-called nebkhas (i.e. sediment-fixing plants buried by eolean sediment, thereby forming vegetated mounds).

Indeed, nebkhas often arise in degraded grasslands, thereby replacing the latter by landscapes dominated by nebkhas. However, nebkhas can also be regarded as ecosystems susceptible to further desertification leading to landscapes void of vegetation. Most research on nebkhas has been focusing on the causing principles underlying grassland transformation into nebkhaland, and on resource heterogeneity inside landscapes with nebkhas. However, much is still uncertain about the robustness of the nebkha landscape against further land degradation.

This dissertation contributes to the identification of biotic and abiotic factors, which affect the demography of nebkha landscapes, and how strong these factors quantitatively act. This thesis can be regarded as fundamental research, but also delivers information about the resistance and resilience of nebkha landscapes against further desertification, and also brings perspectives for restoration. Most of the data in this thesis were acquired through self-made high-resolution aerial imagery in a nebkha landscape of Saudi Arabia. However, also Google Earth imagery was used to study a New Mexican and a Libyan nebkha landscape. Experiments about seed limitation were performed as well.

Our results indicate that the most humid studied nebkha landscape (i.e. the study site in New Mexico subjected to an arid climate) is much more shaped by competition than the hyper-arid nebkha fields in Libya and Saudi Arabia, the latter of which are under high influence of changing topography due to mobile sediment. As a result, both seedlings and adult nebkha plants are highly affected by topography in the Saudi Arabian nebkha landscape, in terms of emergence, growth and shrink and survival. Therefore changes in topography also go along with changes in spatial emergence patterns of seedlings, and growth and shrink processes in adult nebkhas. The aforementioned changes in topography of the hyper-arid nebkha fields are not solely abiotically driven, as we discovered an interaction between topography and vegetation cover, leading to a novel mechanism of self-organization in hyper-arid nebkha fields.