Changes in vegetation greenness related to climatic and non-climatic factors in the Sudano-Sahelian region

Abstract

The potential drivers of vegetation changes in the Sudano-Sahelian region of Africa remain poorly understood due to complex interactions between climatic and anthropogenic processes. In this study, we analyzed the vegetation greenness trends in relation to rainfall variability that we considered the essence of climatic effects on vegetation in a well-known water-limited environment by using time series of satellite data in the Sudano-Sahelian region during 2001–2020. We quantified in more detail the relative contributions of rainfall variability (climatic factor), land use/land cover (LULC) change, and fire occurrence change (non-climatic factors) to vegetation greenness trends in selected sub-regions. The results showed that vegetation greening was widespread (26.9% of the total study area), while vegetation browning was more clustered in central West Africa (5% of the total study area). About half of the vegetation greening area can be explained by long-term rainfall variability during 2001–2020, but most of the area characterized by a browning trend was unrelated to rainfall variability. An analysis of the relative importance showed that LULC changes had significant local effects on vegetation greenness and that these changes were characterized by a strong spatial heterogeneity in specific sub-regions. Gains in cropland and natural vegetation related to positive land management were probably the dominant drivers of greening in Senegal and Ethiopia. Also, the combined impacts of rainfall variability and LULC changes contributed to greening trends in the arid zone, particularly in Mali and Sudan. In contrast, vegetation browning in central West Africa appeared to be driven by cropland gain and natural vegetation loss associated with extensive agricultural production activities. Furthermore, we found that repeated fires for agricultural expansion in central West Africa intensified vegetation browning. These results advanced our understanding of vegetation dynamics in response to climatic and non-climatic factors in Sudano-Sahelian drylands characterized by increasing pressures on land resources.

Hydrological Factor and Land Use/Land Cover Change Explain the Vegetation Browning in the Dosso Reserve, Niger

“…The fractional abundance of each class within each 250 m pixel was then calculated using the GLC_FCS30D data, denoted as Crop, Forest, Grass, Shrub, Wetland, Water, Settlement, and Bare. In addition, the "no change" LULC types and paired LULC change types during 2001-2020 were identified based on the total change of each LULC fraction in each pixel, following our previous paper [16]. Here, if the total changes of fractions in all LULC classes in a 250 m pixel during 2001-2020 fell within a range of ±5% (to reduce classification error), then "no change" type in that pixel was defined, and the class with the maximum fraction was assigned as the main LULC class of that pixel; otherwise, a paired LULC change type was defined according to the maximum gain (maximum positive total change value) and the maximum loss (minimum negative total change value).…”

“…The topic of vegetation dynamics has been explored previously in the Sahel-Sudan region, but most studies focus on vegetation greenness changes at the sub-continental scale [9][10][11][12][13][14][15][16], lacking detailed analyses of the specific protected areas. Recent studies have highlighted significant land use/land cover changes and vegetation browning in the Sahel region of West Africa [14,17,18], strongly linked to human activities, resulting in extensive loss and fragmentation of protected areas and surrounding habitats [19,20].…”

“…Several factors explain the trends of regional vegetation greenness, and the main drivers and mechanisms of the vegetation greenness in the Sahel are still under debate. Precipitation is considered a key climatic factor influencing vegetation dynamics in the Sahel-Sudan region, but recent studies have shown that its role is weakening [14,16]. Even in water-limited drylands, precipitation only explains vegetation greenness trends in less than half of the region [16].…”

“…Precipitation is considered a key climatic factor influencing vegetation dynamics in the Sahel-Sudan region, but recent studies have shown that its role is weakening [14,16]. Even in water-limited drylands, precipitation only explains vegetation greenness trends in less than half of the region [16]. In addition to precipitation, other climate-hydrological factors, such as air temperature, solar radiation, atmospheric vapor pressure deficit (VPD), and soil moisture, also play important roles in influencing the spatial heterogeneity of vegetation dynamics through their regulation of plant biophysical processes [30][31][32].…”

“…In addition, changes in land use/land cover (LULC) associated with human activities, such as urbanization, overgrazing, afforestation, and agricultural land management, also affect vegetation dynamics at multiple spatial and temporal scales. Recent studies have highlighted the importance of local LULC changes and their impacts on vegetation in the Sahel region [13,14,16,[38][39][40]. Therefore, it is necessary to further investigate the impacts of multiple factors on vegetation greenness.…”