An unexpectedly large count of trees in the West African Sahara and Sahel

Abstract

A large proportion of dryland trees and shrubs (hereafter referred to collectively as trees) grow in isolation, without canopy closure. These non-forest trees have a crucial role in biodiversity, and provide ecosystem services such as carbon storage, food resources and shelter for humans and animals1,2. However, most public interest relating to trees is devoted to forests, and trees outside of forests are not well-documented3. Here we map the crown size of each tree more than 3m2 in size over a land area that spans 1.3million km2 in the West African Sahara, Sahel and sub-humid zone, using submetre-resolution satellite imagery and deep learning4. We detected over 1.8billion individual trees (13.4trees per hectare), with a median crown size of 12m2, along a rainfall gradient from 0 to 1,000mm per year. The canopy cover increases from 0.1% (0.7trees per hectare) in hyper-arid areas, through 1.6% (9.9trees per hectare) in arid and 5.6% (30.1trees per hectare) in semi-arid zones, to 13.3% (47trees per hectare) in sub-humid areas. Although the overall canopy cover is low, the relatively high density of isolated trees challenges prevailing narratives about dryland desertification5-7, and even the desert shows a surprisingly high tree density. Our assessment suggests a way to monitor trees outside of forests globally, and to explore their role in mitigating degradation, climate change and poverty.