Presentation short abstract

Presumed increases in carbon sequestration have driven afforestation investments in drylands like the Sahel, excluding residents from needed resources. Using growth and survivorship data of vegetation in savannas and tree plantations, these increases are shown to be significantly overestimated.

Presentation long abstract

In Africa's drylands, despite the limited benefits and real costs of afforestation to rural residents, programs of mass afforestation, such as the Great Green Wall in the Sahel, are popular with donors from the Global North. A major motivator is the presumed carbon sequestration benefits of tree planting on what are seen as empty and denuded landscapes. The core scientific premise of these efforts is that tree planting significantly increases terrestrial carbon stocks in drylands since trees store carbon over a number of years and herbaceoous vegetation does not. In reality, both types of vegetation are ephemeral in drylands with trees being less ephemeral but slower growing. We empirically evaluate this premise by estimating the relative importance the carbon stored in herbaceous and lignaceous vegetation at rangeland sites and in plantations of Senegalia senegal (Acacia senegal), the most favored plantation tree in the region. These estimates are generated through the development of quantitative relationships of survivorship and growth, as affected by mean rainfall and edaphic condition. Treating terrestrial carbon stores in vegetation as not as a static stock but integrated across time (tons of carbon months), the replacement of a savanna/steppe ecological community with a tree plantation, not only has biodiversity and social justice implications but may result in much lower increases in (integrated) carbon storage than assumed. The results call into question the large investments funding treed exclosures across the region and illustrate the strong role played by national forestry services in their promotion.