Earth, wind, and fire: the impact of burning on potential dust emissions from partially vegetated dunes in the southwest Kalahari

Thesis

Sand dunes are not typically considered to be a significant contributor to atmospheric dust loading due to coarse grain sizes and a lack of observed emission events. In vegetated dune systems, dust emission is rarer due to plant cover preventing the wind from reaching the surface. However, disturbed vegetated dunes have the potential to emit dust through the release of resident fine grains. Fire represents one such significant disturbance factor, forcing rapid shifts in the ecosystem state resulting in an increased propensity for aeolian erosion. Accordingly, this thesis investigates potential dust emissions after de-vegetation of dunes through burning in the southwest Kalahari Desert.

First, satellite imagery is used to assess the spatiotemporal extent of burning in the dune field for 2000-2023. Fires were common in the region and occurred most frequently during or just after La Niña extreme events which provide moisture for biomass build up. However, fire is limited by anthropogenic modulation of the fire regime. Different land uses have significant differences in fire occurrence and size, with more, but smaller, burns occurring on privately owned land with larger, but less frequent, burns occurring within the National Park. National Park burns also had longer durations.

Post-fire vegetation recovery was also tracked from space and found to be swift after burning, with cover returning to unburned levels within two years of being burned, limiting the period where bare ground is available for wind erosion. This finding was confirmed by ground surveys which identified that in a wet year, vegetation can recover within 10 months of burning. However, burned plots are more vulnerable to further disturbance, i.e. through drought, than unburned plots.

Field studies assessed the grain size distribution of the dunes and found a proportion of dust-sized material (<62.5 μm). Portable wind tunnel experiments found that this material can be mobilised in the form of dust, but the burned sites had a higher erosion threshold than the unburned sites. Surfaces were further disturbed by removing the top 2 cm of sediment, and fluxes of dust from these mechanically disturbed surfaces could exceed 77 times those of both the unburned and burned surfaces. This finding indicates the importance of biocrusts, which were found at 95% of the interdune plots, and how their survival limits post-fire dust emissions.

Results presented in this thesis show that currently there is little potential for dust emission in the post-fire de-vegetation period. Yet, this potential might be high under different environmental conditions such as droughts or high grazing intensities, where reduced biocrust cover unlocks the resident fine sediment. But post-fire conditions have quick vegetation recovery and the protective effect of biocrusts surviving fire limits the availability of fine sediment for wind erosion.

Authors: Huck, RA

• DOI: 10.5287/ora-9gngabjeq
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Aeolian processes; sand dunes; dust; biogeomorphology; Kalahari; fire
• Subjects: Eolian processes; Geography; Geomorphology