On regreening and degradation in Sahelian watersheds

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.The Sahel (Fig. S1) extends east-west across Africa between the Sahara desert to the north and the humid savanna to the south. It is one of the world’s largest water-limited environments (WLE). The region is often considered to be particularly vulnerable to climate change and human activities (1, 2). Herders and farmers in the Sahel have long recognized the importance of short-term rainfall variability on farm and livestock production, with drought being the principal cause of food insecurity (3). Rainfall largely controls net primary productivity (NPP), forage availability, and livestock carrying capacity (1), and is a primary driver of carbon cycling (4, 5). Climate variability (within and between seasons) in WLE such as the Sahel modifies the structure, composition, and diversity of vegetation via changes in NPP and recruitment-mortality dynamics of woody and herbaceous plants (6, 7).Open in a separate windowFig. S1.Averages of (A) seasonal iNDVI and (B) seasonal iR across the Sahel from July to October for the period 1983–2012. The figure also shows the location of the southwestern Niger [1], eastern Mali [2], western Mali [3], and northern Senegal [4] regions selected for further investigation.The degradation of the Sahel has been greatly debated within the scientific community. In the 1930s, after a visit to West Africa, Stebbing was one of the first to conclude that the Sahara desert was expanding south into the Sahel and that the cause of degradation was human activity (8). With above-average rainfall in the region in the 1950s and 1960s, these concerns diminished for a while. In the 1970s and 1980s, however, severe drought and famine in the Sahel, coincident with influential ideas on the potential for human mismanagement of common land (9) and the potential for amplifying feedbacks between grazing and drought (10), led to widespread acceptance in both scientific and popular imagination of Sahelian “desertification” as a pervasive and irreversible process (11, 12). In the 1990s, however, an alternative picture emerged as political ecologists and others began to question the “received wisdom” of human-induced desertification (e.g., refs. 13 and 14), perhaps motivated by recognition of the positive role of social institutions in the management of common pool resources (15) and by the recognition that degradation and recovery of Sahelian vegetation is a normal consequence of drought cycles, with or without human agency (16–19).In the last 20 y, remote-sensing studies have documented an apparent increase in vegetation productivity in the Sahel using satellite measurements of vegetation greenness (i.e., normalized difference vegetation index, NDVI) as a proxy for NPP (20–22). Independent studies have also documented farmer strategies to restore landscape-scale function, in particular through planting of trees and soil conservation actions (23, 24). Other authors, however, have used both satellite and field data to argue that the apparent “regreening” of the Sahel may still hide real degradation in the form of ecosystem ability to use available rainfall (i.e., rain-use efficiency), or in the structure and species composition of the vegetation (25–29).Meanwhile, in the popular press and often in the environmental and development literature, the reports of recovery are sometimes forgotten (30–32), to the extent that popular opinion in the West—and indeed very often in Africa—holds fast to pessimistic images of overgrazing, degradation, sand storms, and sand-dunes “marching” south from the Sahara towards the sea. The differences in perception of recent changes highlight the need to quantify the extent of recovery (or otherwise) in Sahelian systems since the droughts of the 1970s and 1980s and the extent to which vegetation changes in the Sahel respond proportionally to climate variations.Many studies have shown that the interannual variability of NPP in WLE is positively correlated with the interannual variability of rainfall (33–35). In addition, the rain-use efficiency (RUE; the slope of the relationship between NPP and rainfall) quantifies an ecosystem’s ability to use rainfall and may therefore be a useful indicator of ecosystem health or degradation. However, the use of RUE has been dominated by two contrasting schools of thought. Many authors assume that RUE will be constant with interannual variation in rainfall if ecosystem conditions are constant. In this situation, changing RUE can be used directly to diagnose changes in vegetation, reflecting long-term degradation or recovery (20, 21). Meanwhile, based on physical and physiological logic, others argue for an underlying nonlinear relationship between NPP and rainfall, with NPP initially increasing with rainfall before saturating at higher rainfall as light or nutrients become limiting (25, 27). In this situation, we no longer expect a constant RUE (calculated simply as NPP/rainfall), which compromises (or at least complicates) its utility as a degradation indicator. In all cases, however, if the functional relationship between NPP and rainfall is known, or can be derived from observations, then temporal trends in residuals from the fitted relationship can be used to infer changing ecosystem condition. This is the basis for the “residual trend” approach used by several authors to diagnose changing ecosystem health in the Sahel and other regions (36, 37).The aim of this study is to quantify the changes in vegetation condition in the Sahel based on assessments of long-term satellite-derived greenness sensitivity to rainfall. We explore changes at watershed scales in four test regions across the Sahel and examine the form (linear, nonlinear) of functional relationships between this proxy for NPP and rainfall. We then propose a new approach to the diagnosis of trends in vegetation condition that relates short-term and long-term NPP–rainfall sensitivity to changes in herbaceous and woody plants, respectively. We address the following research questions: (i) What is the shape of the rainfall–NPP relationship and its relevance to the ongoing debate of regreening of the Sahel? (ii) How has NPP varied as a function of rainfall since the 1980s drought in the Sahel? (iii) Can changes in short- and long-term NPP sensitivity to rainfall inform our understanding of changing vegetation structure in different regions of the Sahel?We based our study on a 30-y (1983–2012) African Rainfall Climatology (ARC) rainfall database (38) in combination with a merged NDVI dataset from Moderate Resolution Imaging Spectroradiometer (MODIS) (39) and Advanced Very High Resolution Radiometer (AVHRR) (40). Annual integrals of NDVI during the growing season (from June to October) are used as a proxy for NPP (20–22) over 260 watersheds located in four regions of Senegal, Mali, and Niger (Fig. S1).