Monday, August 3, 2009 - 3:40 PM

COS 1-7: Challenging desertification paradigms: Towards an objective quantification of land degradation and the Charney effect in African ecosystems

Niall P. Hanan1, Lara Prihodko1, and Christopher A. Williams2. (1) Colorado State University, (2) Clark University


Global environmental changes, in the form of changing climate, human land use and other anthropological perturbations, present challenges to the ecological integrity and function of ecosystems in Africa, as they do in other parts of the world. In particular, anthropogenic impacts through grazing of domestic animals, fire and agricultural expansion has frequently been identified as a leading cause of soil and vegetation degradation (or ‘desertification’); periodic drought has been responsible for loss of vegetation cover; and both anthropogenic and climate-related vegetation decline and increasing albedo have been identified as potentially leading to positive feedback loops that may reduce atmospheric uplift, cloud formation and rainfall (i.e. the Charney Hypothesis). On the other hand, recent trends in vegetation production in African drylands show signs of recovery in vegetation conditions following the droughts of the 1970s and 1980s. These observations suggest resilience of vegetation in African ecosystems and that production may respond simply to rainfall fluctuations with no clear signal of either human-induced desertification or albedo-induced declines in rainfall.


In this study we use long-term observations of global scale climate indices, rainfall and vegetation across the African continent to ascertain the extent to which regional changes in vegetation condition may alter rainfall patterns, thus quantifying the Charney effect, and assess relative changes in primary production response to rainfall (rain use efficiency) to quantify spatial and temporal trends in vegetation degradation or desertification. We identify regions of apparent change in vegetation condition and compare our results for the African continent to recent global-scale desertification assessments. We discuss the extent to which vegetation production in African ecosystems, particularly the semi-arid and seasonal savannas, may be resilient to anthropogenic perturbations; the importance of positive feedbacks between vegetation condition and rainfall; and the value and validity of the commonly cited, but infrequently tested, desertification paradigms.