The role of airborne dust in the growth of tree islands in the Okavango Delta, Botswana

The Okavango Delta, situated in the Kalahari Desert (Botswana), is host to extensive areas of seasonal swamp that are characterised by the presence of numerous tree-covered islands. Islands have been shown to play an important role in the landscape through the creation of habitat diversity, focusing of nutrients, and sequestration of salts. Islands are thought to grow through the subsurface precipitation of carbonate and silica, although recent work has suggested that other factors may be involved, notably airborne dust accumulation. In this study, we investigate the role of airborne dust in the maintenance and growth of tree islands in the seasonal swamps of the Okavango Delta. Chemical and grain size analyses indicate that whilst the channels and floodplains in the Okavango are dominated by well-sorted Kalahari sand that covers the entire region, material on the surface of islands is distinctly different. Island soils are enriched in Al2O3 and are characterised by higher proportions of poorly sorted, fine-grained material that we attribute to the addition of airborne dust. Large quantities of material that circulate in anticyclonic systems over southern Africa represent a potentially significant source of particulate sediment to the Okavango, whilst peat fires and the desiccation of the surrounding floodplains during the dry season are also considered to be important sources of local dust. The data suggest that varying proportions of Kalahari sand, dust, and chemical precipitate give rise to the range of compositions found on and within the islands of the Delta. Dust typically accounts for between 20 and 60% of material found on the surface of islands, whilst dust and chemical precipitate dominate the subsurface material. Islands thus appear to grow through a combination of ongoing surface and subsurface processes that result in considerable heterogeneity in soil composition. In both instances vegetation, especially trees, is the main driving force behind island development, not only causing the subsurface accumulation of CaCO3 and SiO2 in island soils, but also trapping airborne dust on the surface of islands. Our study suggests that dust fallout is an equally or possibly even more important contributor to the local topographic irregularities and thus habitat diversity in the Okavango Delta. Despite the potential importance of airborne material to the biogeochemistry and development of tree islands in wetland systems, our knowledge regarding these processes remains poor.