Land Degradation in Drylands

Drylands across the globe are dominated by the presence of biological soil crusts (Belnap and Lange, 2003). They form a surface layer consisting of an assemblage of cyanobacteria, lichens, fungi, algae sediment and organic matter (Belnap et al., 2003). Biological soil crusts are an important factor in stabilising soil in drylands from erosion where organic matter in soil is characteristically low. Cyanobacteria are particularly influential in the stabilising of the soil surface, their filament form entangles sediment grains, reducing the susceptibility of soil to the effects of wind erosion (Thomas and Dougill, 2007).

The notable "Dust Bowl" period that occurred across 1930s USA saw how poor land management resulted in elevated dust release to the atmosphere, reducing precipitation in a positive feedback mechanism (Worster, 1979). Land management is becoming increasingly significant in dryland systems as a means of stabilising sediments and reducing the risk of land degradation (Ravi et al., 2010). Despite this, in African countries such as Botswana, grazing intensities- and with it, soil crust destruction- are more dependent upon socio-economic factors rather than long term landscape stability.

Over-grazing of rangelands in African dryland areas directly causes vegetation change (Ravi et al., 2010). As the more palatable grasses are grazed at a rate greater than they can re-grow, woody shrub encroachment over the cleared grassland occurs (Eldridge et al., 2011). Field et al. (2011) found that enhanced levels of trampling from cattle can cause destruction of biological soil crusts, leaving loose soil vulnerable to removal by aeolian and fluvial processes.

  References

Belnap, J., Lange, O.L., 2003. Structure and functioning of biological soil crusts: a synthesis. In: Belnap, J., Lange, O.L. (Eds.), Biological Soil Crusts: Structure, Function and Management. Springer-Verlag, Berlin, pp. 471–479.

Belnap, J., Büdel, B., Lange, O.L., 2003. Biological soil crusts: characteristics and distribution. In: Belnap, J., Lange, O.L. (Eds.), Biological Soil Crusts: Structure, Function and Management. Springer-Verlag, Berlin, pp. 3–30.

Eldridge, D. J., Bowker, M. A., Maestre, F. T., Roger, E., Reynolds, J. F., Whitford, W. G., 2011. Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis. Ecology Letters. Vol 14 (p709-722).

Field, J. P., Breshears, D. D., Whicker, J. J., Zou, C. B., 2010. Interactive effects of grazing and burning on wind- and water- driven sediment fluxes: rangeland management implications. Ecological Applications. Vol X, (pX-X).

Ravi, S., Breshears, D. D., Huxman, T. E., D'Odorico, P., 2010. Land degradation in drylands: Interactions among hydrologic-aeolian erosion and vegetation dynamics. Geomorphology. Vol 116, (p236-245).

Thomas, A. D., Dougill, A. J., 2007. Spatial and temporal distribution of cyanobacterial soil crusts in the Kalahari: Implications for soil surface properties. Geomorphology. Vol 85, (p17-29).

Worster, D., 1979. Dust bowl: the Southern Plains of 1930s. Oxford University Press, New York.