Recent trends in vegetation dynamics in the African Sahel and their relationship to climate
Introduction
The African Sahel, a semi-arid grass- and shrubland region bordering the Sahara desert to the south, is a dynamic ecosystem that responds to fluctuations in climate and anthropogenic land use patterns. Contrary to largely anecdotal assertions of widespread irreversible ‘desertification’ in the Sahel (e.g. Lamprey, 1975, reprinted in 1988), recent findings based on analyses of satellite images report an increase in greenness over large areas of the Sahel since the mid-1980s, which, at a coarse scale, is well correlated with an overall increase in rainfall and has been interpreted as a recovery of the vegetation from the great Sahelian droughts in the 1970s and 1980s (Tucker and Nicholson, 1999; Eklundh and Olsson, 2003) However, the greening trend is not uniform, suggesting that factors other than rainfall may have contributed to a differential greening response, with greening taking place in some areas but not in others.
Although its actual meaning on the ground has not yet been firmly established, the observed greening trend has challenged notions of irreversible damage inflicted on the Sahelian ecosystem (Dregne, 1983; Middleton et al., 1997), revived debates about the concept of desertification, and triggered re-assessments of its nature, scale and extent, facilitated by progress in remote sensing technology as a tool for environmental monitoring and analysis. However, while studies based on long time series of satellite and ground data have confirmed the dynamic nature of the Sahelian ecosystem and its susceptibility to change, they have not resulted in a consensus on either the direction of changes or its underlying causes.
Section snippets
Background
The Sahel (Arabic for ‘shore’) is a transition zone between the arid Sahara in the north and the (sub-) humid tropical savannas in the south, and is marked by a steep north–south gradient in mean annual rainfall (Le Houerou, 1980). The rainfall gradient is expressed on the ground in a continuum of change in vegetation species and life forms from the Saharan biome with very sparse vegetation cover—thorny shrubs interspersed between annual and perennial grasses—to the Sudanian and Guinean biomes,
Objectives and rationale
In light of the ongoing debate about the driving forces of vegetation dynamics and land degradation in the Sahel and the still insufficient distinction made between the effects of drought and ‘desertification’, the objectives of this research were to (1) further explore the relationship between climatic and anthropogenic causes of land degradation at a coarse resolution, (2) break down trends in precipitation and vegetation dynamics into spatial patterns and (3) identify hotspots of potentially
Normalized Difference Vegetation Index
The NDVI was employed in this study as a proxy for vegetation greenness. The data is derived from measurements made by the Advanced Very High Resolution Radiometer (AVHRR) instrument on board the National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellite series. The NDVI, a normalized ratio of the near-infrared and red spectral reflections (NIR−red/NIR+red), is sensitive to the presence, density and condition of vegetation and is correlated with absorbed photosynthetically
Methodology
The spatial–temporal analysis of the dynamics and trends in rainfall and vegetation greenness during the study period 1982–2003 is based on a sample size of 264 months, whereas the sample size of the shorter period 1998–2003 (validation period) amounts to 72 months. A definition of the Sahel region was derived from a 20-year-average NDVI rather than average annual precipitation as in Tucker et al. (1991), i.e. the Sahel was delineated by a minimum NDVI of 0.15 and a maximum of 0.4, which
Overall trends
For the period 1982–2003, the overall trend in monthly maximum NDVI is positive over a large portion of the Sahel region (Fig. 1), reaching up to 50% increase in the average NDVI in parts of Mali, Mauritania and Chad. It is understood, however, that averages are not very meaningful in this highly dynamic environment where considerable seasonal fluctuations around the mean are the norm. This result confirms previous regional-scale findings for the period 1982–1999 by Eklundh and Olsson (2003)
Conclusions
This research adds to a series of coarse-resolution studies on the Sahel which refute claims of widespread human-induced land degradation at a regional scale, e.g., Prince et al. (1998), Tucker and Nicholson (1999); Hellden (1991) and Eklundh and Olsson (2003). Rather, a greening of the Sahel expressed in positive trends in NDVI indicates a net increase in biomass production during the period 1982–2003, which challenges the notion of irreversible desertification in the Sahel. Whether this
Acknowledgements
Funding for this research was provided through the NASA Graduate Student Summer Program hosted by Goddard Earth Sciences and Technology Center. We would like to thank Henry David Snyder, Jorge Pinzon, Molly Brown and Daniel Slayback for their assistance with the data processing.
The GPCP combined precipitation data were developed and computed by the NASA/Goddard Space Flight Center's Laboratory for Atmospheres as a contribution to the GEWEX Global Precipitation Climatology Project. The TRMM 3B43
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