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

Abstract

Abstract:The greening in the Senegalese Sahel has been linked to an increase in net primary productivity, with significant long-term trends being closely related to the woody strata. This study investigates woody plant growth and mortality within greening areas in the pastoral areas of Senegal, and how these dynamics are linked to species diversity, climate, soil and human management. We analyse woody cover dynamics by means of multi-temporal and multi-scale Earth Observation, satellite based rainfall and in situ data sets covering the period 1994 to 2015. We find that favourable conditions (forest reserves, low human population density, sufficient rainfall) led to a rapid growth of Combretaceae and Balanites aegyptiaca between 2000 and 2013 with an average increase of 4% woody cover. However, the increasing dominance and low drought resistance of drought prone species bears the risk of substantial woody cover losses following drought years. This was observed in 2014-2015, with a die off of Guiera senegalensis in most places of the study area. We show that woody cover and woody cover trends are closely related to mean annual rainfall, but no clear relationship with rainfall trends was found over the entire study period. The observed spatial and temporal variation contrasts with the simplified labels of "greening" or "degradation". While in principal a low woody plant diversity negatively impacts regional resilience, the Sahelian system is showing signs of resilience at decadal time scales through widespread increases in woody cover and high regeneration rates after periodic droughts. We have reaffirmed that the woody cover in Sahel responds to its inherent climatic variability and does not follow a linear trend.

Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

“…The increase is more pronounced in protected areas ("forêts classées") than in reforestation areas such as the Great Green Wall. As previously observed by Hiernaux et al [85], Gardelle et al [48], Dardel et al [13] in Eastern Mali, Brandt et al [15] showed that in Senegal too, lower vegetation recovery occurred over shallow soils (here in ferrugineous plateaus of the Ferlo), where they observed "mass dying of trees on lateritic soils during droughts". Bamba et al [90] observed that the increasing trend in vegetation greenness is extending from the South to North.…”

“…This demonstrates the resilience of Sahelian ecosystems, although this trend is generally observed in places with low human pressure. Brandt et al [15] further identified the pastoral area of Senegal as the area with the largest increase in woody cover within the Sahel. The increase is more pronounced in protected areas ("forêts classées") than in reforestation areas such as the Great Green Wall.…”

“…Almost all remote -based vegetation maps demonstrate a dominant increase in NDVI signal since the early 1980s [10][11][12][13][14][15][16]. These studies did not always determine the respective contribution of the annual and perennial vegetation in this progression.…”

“…These studies did not always determine the respective contribution of the annual and perennial vegetation in this progression. However, most of the re-greening is probably due to herbaceous recovery, although there is also evidence of significant natural increase in tree cover over the last 15 years [15].…”

“…Figure 5. Classification of rivers basins according to the five described types (Volta River was added, despite the biases introduced by Akosombo dam, in Type 1, as the literature well describes its behavior); Sokoto is type 2, since an increase in RC is observed, but data are very sparse [2,3,15,49,61,63,64,68,72]; Komadugu has the same behavior as the Sahelian Basin (personal communication with Pierre Genthon, IRD). Figure 5.…”