Abstract
We assess 32 years of vegetation change in the West African Sudano-Sahelian region following the drought events of the 1970s and 1980s. Change in decadal mean rain use efficiency is used to diagnose trends in woody vegetation that is expected to respond more slowly to post-drought rainfall gains, while change in the slope of the productivity–rainfall relationship is used to infer changing herbaceous conditions between early and late periods of the time series. The linearity/non-linearity of the productivity–rainfall relationship and its impact on the interpretation of overall greening trends, and specific woody and herbaceous vegetation trends, is also examined. Our results show a mostly positive association between productivity and rainfall (69% of pixels), which can be best described as linear (32%) or saturating (37%). Choosing the ‘best’ model at a specific location using Akaike Information Criterion has no discernible effect on the interpretation of overall greening or herbaceous trends, but does influence the detection of trends in woody vegetation. We conclude that widespread recovery in woody vegetation is responsible for the post-drought greening phenomenon reported elsewhere for the Sahel and Sudanian sub-regions. Meanwhile, trends in herbaceous vegetation are less pronounced, with no consistent indication towards either herbaceous degradation or recovery.
Highlights
The West African Sudano-Sahelian (WASS) region is an extensive semi-arid, drought-seasonal environment south of the Sahara Desert with an approximate range in mean annual precipitation (MAP) of 150–900 mm and savanna vegetation featuring both woody and herbaceous functional types
A close, positive relationship is anticipated in semi-arid and arid landscapes and is a necessary precursor for using changes in rain use efficiency as an indicator of long-term trends relating to post-drought recovery, or the degradation and recovery dynamics caused by climate variability and/or anthropogenic disturbance [11,34]
In this study we revisited the discussion on long-term vegetation trends in water-limited environments, and we used our previously developed framework for diagnosing pixel-level trends in woody and herbaceous vegetation in the WASS region
Summary
The West African Sudano-Sahelian (WASS) region is an extensive semi-arid, drought-seasonal environment south of the Sahara Desert with an approximate range in mean annual precipitation (MAP) of 150–900 mm and savanna vegetation featuring both woody and herbaceous functional types. Herbaceous vegetation, by contrast, is a primary grazing resource for livestock and wildlife [2], and provides wild harvest for humans [3] and ecosystem services such as erosion control [4]. The long-term balance between woody and herbaceous cover is, important for savanna ecosystem maintenance, and an indicator of natural resource sustainability and food security [5]. Tropical drylands are mostly sensitive to inter-annual and decadal-scale variation in precipitation [6], as well as human-induced activities (e.g., agriculture, fire, wood harvesting, and grazing) that can lead to land degradation and possibly desertification [7]. The last 2–3 decades have seen a revision of this picture by studies that show little or no evidence of a drastic negative shift such as desertification [8], but rather evidence of vegetation recovery or so-called re-greening in the post-drought era [9,13,14]
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