Trends in Woody and Herbaceous Vegetation in the Savannas of West Africa

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.

Identifying Ecosystem Function Shifts in Africa Using Breakpoint Analysis of Long-Term NDVI and RUE Data

“…The trend reversals occurred around the early 2000s (Figure 2c), coinciding with early reports of a slowdown in vegetation gains around the millennium [29]. 'Increasing' pixels in the North are in the more arid regions, which have seen the largest increases in rainfall in the post-drought years, leading to increases in woody plant cover [59]. 'Decreasing' pixels show an alignment with regions of high and rapidly increasing human population.…”

Toward Operational Mapping of Woody Canopy Cover in Tropical Savannas Using Google Earth Engine

“…In northern Senegal (the region known as the Sandy Ferlo), low %WCC cover was predicted by our model (mostly < 10%, Figure 5), consistent with the expectation that low rainfall (MAP ∼300 mm or less) limits the establishment and maintenance of woodland systems. However, despite the low cover, other timeseries studies have inferred long-term gains in woody vegetation in this area, mostly as recovery from 1970s/80s drought events (Kaptué et al, 2015;Anchang et al, 2019), but also due to relatively low human influence in areas that are largely not suitable for agriculture (Brandt et al, 2017). Our ability to detect low cover of trees and shrubs in this region can play a key role in supporting such conclusions in future studies.…”

“…Senegal has a bio-climatic gradient ranging from the arid Sahel in the north (mean annual precipitation or MAP < 300 mm, with low %WCC and shrubs and trees generally 1-2 m tall), through the semi-arid Sudano-Sahelian zone (300 mm < MAP < 900 m, a blend of woodland and grassland savannas, with shrubs and trees 2-6 m tall), to the humid Guinean savanna/forest mosaic south of The Gambia (MAP > 900 mm, trees up to ∼10 m tall) . Geomorphologically, Senegal is a mostly flat country with a maximum elevation less than 200 m above sea-level (Diouf et al, 2015;Anchang et al, 2019). The northern, central and eastern parts of Senegal are the main zones of pastoral activity and have been divided into 4 ecological zones reflecting soil and land use characteristics: Sandy Ferlo (north), Ferruginous Ferlo (north east), mixed agro-pastoral zone (central) and savanna-woodland transition zone (East) (see Figure 2, as well as Diouf et al, 2015).…”

Detecting phenological changes in plant functional types over West African savannah dominated landscape

“…Conceptionally, plant functional types (PFTs) are groups of species that show similar structural, physiological and/or phenological features, responses to environmental conditions or have similar impacts on ecosystems (Gitay and Nobel 1997;Ustin and Gamon 2010). Recently, there has been increased interest in monitoring land surface phenological changes of PFTs over various spatial and temporal scales (de Bie et al 1998;Zhang et al 2003;Whitecross, Witkowski, and Archibald 2016;Julius et al 2019). In savannah dominated landscape, phenological changes are highly sensitive to climatic fluctuations due to ecosystem diversity and high interannual climate variability (e.g.…”