Drought tolerant species dominate as rainfall and tree cover returns in the West African Sahel

- Tree cover decreased 1968-1984 but had increased again to pre-drought levels by 2006.
- Despite increasing rainfall, recovering tree species were drought tolerant native and exotic species.
- Tree cover increased more close to residential areas than in other land classes.
- Land use changes are suggested to be a main driver of change in vegetation.

After the severe droughts in the 1970s and 1980s, and subsequent debates about desertification, analyses of satellite images reveal that the West African Sahel has become greener again. In this paper we report a study on changes in tree cover and tree species composition in three village landscapes in northern Burkina Faso, based on a combination of methods: tree density change detection using aerial photos and satellite images, a tree species inventory including size class distribution analysis, and interviews with local farmers about woody vegetation changes. Our results show a decrease in tree cover in the 1970s followed by an increase since the mid-1980s, a pattern correlating with the temporal trends in rainfall as well as remotely sensed greening in the region. However, both the inventory and interview data shows that the species composition has changed substantially towards a higher dominance of drought-resistant and exotic species. This shift, occurring during a period of increasing annual precipitation, points to the complexity of current landscape changes and questions rain as the sole primary driver of the increase in tree cover. We propose that the observed changes in woody vegetation (densities, species composition and spatial distribution) are mediated by changes in land use, including intensification and promotion of drought tolerant and fast growing species. Our findings, which indicate a rather surprising trajectory of land cover change, highlight the importance of studies that integrate evidence of changes in tree density and species composition to complement our understanding of land use and vegetation change trajectories in the Sahel obtained from satellite images. We conclude that a better understanding of the social-ecological relations and emerging land use trajectories that produce new types of agroforestry parklands in the region is of crucial importance for designing suitable policies for climate change adaptation, biodiversity conservation and the sustainable delivery of ecosystem services that benefit local livelihoods in one of the world's poorest regions.

A conceptual model illustrating different tree cover pathways in relation to species composition and rainfall variability. The diagram could be interpreted along any, even small, shifts along these gradients. Wet and dry habitat indicates species thriving under relative wetter to dryer conditions. Brown and green refers to a sparse and dense woody vegetation cover, respectively.114