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While Normalized Di erence Vegetation Index (NDVI) images are widely used for assessing vegetation vigour and extent, much remains to be known regarding both how the images themselves and their interannual variation correspond to ground-level reality. This work addresses these issues through a discussion of a series of six north± south transects in the West African Sahel. In some cases along these transects, vegetation spatial aggregation and density vary directly with the NDVI coe cient of variation (CoV), while in others community dynamics confound identifying such relationships. Analysis of CoV data infers the existence of a signal for rapid, dynamic land degradation (describable by the term`deserti® cation') in both the northern and southern Sahel.

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