On regreening and degradation in Sahelian watersheds

Abstract

Over many decades our understanding of the impacts of intermittent drought in water-limited environments like the West African Sahel has been influenced by a narrative of overgrazing and human-induced desertification. The desertification narrative has persisted in both scientific and popular conception, such that recent regional-scale recovery ("regreening") and local success stories (community-led conservation efforts) in the Sahel, following the severe droughts of the 1970s-1980s, are sometimes ignored. Here we report a study of watershed-scale vegetation dynamics in 260 watersheds, sampled in four regions of Senegal, Mali, and Niger from 1983-2012, using satellite-derived vegetation indices as a proxy for net primary production. In response to earlier controversy, we first examine the shape of the rainfall-net primary production relationship and how it impacts conclusions regarding greening or degradation. We conclude that the choice of functional relationship has little quantitative impact on our ability to infer greening or degradation trends. We then present an approach to analyze changes in long-term (decade-scale) average rain-use efficiency (an indicator of slowly responding vegetation structural changes) relative to changes in interannual-scale rainfall sensitivity (an indicator of landscape ability to respond rapidly to rainfall variability) to infer trends in greening/degradation of the watersheds in our sample regions. The predominance of increasing rain-use efficiency in our data supports earlier reports of a "greening" trend across the Sahel. However, there are strong regional differences in the extent and direction of change, and in the apparent role of changing woody and herbaceous components in driving those temporal trends.Sahel | desertification | rain-use efficiency | drylands | West Africa

Desertification, resilience, and re-greening in the African Sahel – a matter of the observation period?

“…Regarding the aspect of topographic position, Kaptué et al (2015) confirmed an increasing woody cover at the watershed scale in the majority of their samples. The drier more northerly Sahelian watersheds in Senegal and eastern Mali appear to show stronger reforestation trends than the more mesic region of western Mali and the highly populated area near Niamey, Niger.…”

“…Based on these techniques, changes in vegetation pattern and density were quantified and mapped in the form of change detection maps showing "winners" (predominantly agriculture) and "losers" (vegetation) as a basis for planners and decision makers. The method was established in 1999 and has been further developed from 2001 on (Kusserow, 2001;Kirsch-Jung and Kusserow, 2002;Kusserow, 2010Kusserow, , 2014.…”

“…Other studies from Senegal and Mali confirmed these results (Maranz, 2009;Miehe et al, 2010;Herrmann and Tappan, 2013;Brandt et al, 2014bBrandt et al, , 2015Brandt et al, , 2016aSpiekermann et al, 2015). Kaptué et al (2015) refer to a post-drought tree population recovery and present evidence of a decline in populations of economically and culturally important trees and shrubs despite the increase in woody cover. For Burkina Faso (Oursi dune), Rasmussen (1999) noted that the species currently invading the live dunes -created in the 1970s -are not the same as those dominating before.…”

Woody Vegetation Die off and Regeneration in Response to Rainfall Variability in the West African Sahel

“…The rich and diverse scientific literature, covering various temporal and spatial scales, often disagrees with the overall trends of environmental change [4] but recent research suggests the Sahel is an unstable but resilient ecosystem which is steadily recovering from prolonged droughts and below average rainfall conditions [5]. The potential of recovery is especially favoured in areas of low human pressure and population density [5], and after droughts in activity over time (see Figures A1-A6 and Table 1). Soils play an important role in defining the contrasting character of the western and eastern parts of the study region.…”

Farmlands Feature Greater Woody Cover than Savannas in Semi-Arid West Africa

“…These studies did not, however, allow separation of the contributions from the herbaceous and woody vegetation components. Only recently has this been achieved 18,19 revealing that the greening may be partly attributed to an increase in woody cover. The coarse spatial and limited temporal resolution of the satellite images used and the complexities of the methods applied imply that such assessments of vegetation change in the Sahel do not necessarily form a robust basis for estimating trends in woody cover locally, and leave considerable room for speculations regarding the nature of the woody vegetation changes.…”