Environmental sciences: Estimating the carbon stocks of trees in African drylands

An inventory of nearly 10 billion trees in semi-arid sub-Saharan Africa provides detailed estimates of the amount of carbon stored in this ecosystem, reports a paper published in Nature. These data, collected from more than 300,000 satellite images, can help to improve our understanding of the carbon cycle, and may be a useful resource for scientists, policymakers, dryland restoration practitioners and farmers.

Trees offer a range of benefits to their ecosystems: they provide shelter and food for animals, contribute to economies, and have important roles in carbon cycling and the climate system. These roles are especially important in dryland ecosystems. Detailed information about dryland trees is important for efforts in climate mitigation, carbon accounting and the protection and restoration of ecosystems, but such data have been lacking.

Addressing this gap in the record, Compton Tucker and colleagues present an assessment of more than 9.9 billion trees within semi-arid Sub-Saharan Africa north of the Equator, covering nearly 10,000,000 km2 of land. Machine learning was used to scan 326,523 high-resolution satellite images to identify and map individual trees, providing an opportunity to estimate the amount of carbon stored in the foliage, wood and roots of each tree. Values were divided into different rainfall zones: hyper-arid (0-150 mm of rain per year), arid (150–300 mm per year), semi-arid (300–600 mm per year) and dry sub-humid (600–1,000 mm per year). The average carbon stock for a single tree is 51 kilograms of carbon (kg C) in the hyper-arid, 63 kg C in the arid, 72 kg C in the semi-arid and 98 kg C in the sub-humid zone. Comparisons with previous models for the same area indicate that most attempts have overestimated the carbon stocks of trees.

“Tucker and colleagues’ study is a big step forward for efforts to generate accurate data on dryland carbon stocks,” which could inform future dryland management, note Jules Bayala and Meine van Noordwijk in an accompanying News & Views.