Dryland degradation: Measurement and effects on ecosystems

Land degradation is frequently described as a global crisis, affecting large areas globally and large numbers of people. Nevertheless, the location and severity of degradation globally with a resolution relevant to human activities is unknown. Beyond the direct stress of degradation on human livelihoods, there are important effects on the physical and biological environment. Examples include loss of potential primary production, changes in the surface water and energy balances, erosion, sediment transport and lofting of dust aerosols. Globally , degradation is mainly associated with drylands, such as the US dustbowl of the 1930s and, supposedly, ongoing loss of crop and livestock production in desert margins on all continents. The alarm over loss of land to deserts, particularly early 1980s in the African Sahel, led to the adoption of the term "desertification". Such degradation is said to have two components; a physical environment that reduces productivity; and human land use that exceeds the resilience of the land. Ecological theory suggests that land can exist in multiple stable states with transitions between them. Some experimental evidence suggests that one such state is degradation from which there can be no recovery. Clearly the occurrence of such stable degradation, where land is unable to recover when the physical conditions such as rainfall and human land use are ameliorated, is of theoretical and practical importance. The aim of this work is to contribute to the resolution of two issues: (i) are there significant areas in which land has been degraded by human actions and, (ii), have any of these areas entered a stable degraded state? Detection of the human component necessarily requires control of the physical component of degradation. We have developed a technique to detect areas that are at their potential production and to assess other areas relative to these. Satellite measurements of vegetation indices are used as a surrogate for Net Primary Production (NPP) which, in turn, is used as a surrogate for land condition. The land surface is classified into homogeneous land capability units based on the physical environment and the maximum NPP found in the unit is used as the best estimator of the potential NPP. Other parts of the unit are scaled relative to the maximum. This technique, known as Local NPP Scaling (LNS), has been applied in southern Africa, the Sahel , and in various stages of completion in SW USA, Negev and Queensland. The effects of the degradation on surface fluxes of water, energy and carbon dioxide have been assessed in the Sahel. The provisional conclusions are that there are, indeed areas of drylands that are performing below expectation from their physical environment and that limited parts of these areas have entered into a long-term, maybe permanent degraded state. Furthermore, a key factor is the loss of soil moisture.