Wood, foliage and root carbon of 9,947,310,221 trees with crown area >3 m2 in 9.7 million km2 were mapped. a, Our study covered the southern Sahara, the Sahel and the northern Sudan zone of Africa and showed total carbon density (foliage + wood + root) per hectare for 9,947,310,221 tree crowns from year 0–1,000 mm−1 average rainfall area. Isohyets mark 150, 300, 600 and 1,000 mm year−1 precipitation zones (from north to south). b, Example showing the woody carbon stock of each individual tree for an agroforestry area in Senegal. c, Mean tree carbon density at the 5th, 25th, 75th, and 95th percentiles along the precipitation gradient for wood, foliage, and root carbon. d, Average carbon stock of individual trees at the 5th, 10th, 25th, 75th, 90th, and 95th percentiles along the rainfall gradient. Our definition of a tree is a green leaf crown >3 m2 with a relative shadow. Credit: Nature (2023). DOI: 10.1038/s41586-022-05653-6
A NASA-led research team used satellite imagery and artificial intelligence methods to map billions of distinct tree crowns at a scale of 50 centimeters. The images included a large swath of arid northern Africa, from the Atlantic to the Red Sea. Allometric equations based on previous tree sampling allowed the researchers to convert the images into estimates of tree wood, foliage, root size and carbon sequestration.
The new NASA estimate, published in the journal Nature, was surprisingly low. While the typical estimate of an area’s carbon stock might be based on counting small areas and extrapolating the results upward, the technique demonstrated by NASA only counts the trees that are actually there, down to the individual tree. Jules Bayala and Meine van Noordwijk published a News & Views article in the same magazine commenting on the NASA team’s work.
The initial expectation of measuring each scattered tree, in areas that previous models often represented with zero values, was erased by large overestimates in other areas of previous estimates. In previous attempts to use satellites, cropland and ground vegetation negatively affected visual images. If radar was used, topography, wetlands, and irrigated areas affected radar backscatter, predicting higher carbon stocks than current NASA estimates.
Mapping based on deep learning
The researchers applied deep learning-based tree mapping, manually trained on approximately 90,000 trees, to a dataset of nearly 300,000 satellite images to count more than 9.9 billion woody plants that exhibited a shade and crown area greater than 3 square meters. Only features that showed a distinct crown area and associated shadow were selected, which allowed the team to exclude small bushes, grass clumps, rocks and other misleading features.
The plotted areas were correlated to reflect four precipitation bands. super-arid, arid, semi-arid and dry sub-arid—as rainfall affects carbon uptake and storage. While foliage represents only 3% of total dry mass, it was used as an indirect measure to quantify total mass. The ratio of the root mass is, on average, 15-20% of the total mass and was also derived based on the foliage.
Want to visualize your large tree mapping dataset in an interactive browser format? The researchers did too, so they created a competent viewer to work with and made it publicly available here.
The ability to monitor the effectiveness of carbon sequestration could become of global importance in the fight against climate change. Reforestation is a leading method by which the world’s nations have committed to offsetting their carbon footprint.
However, the practicality of these pledges has been tested by a team of 20 researchers working with the University of Melbourne’s Interdisciplinary Climate Research Initiative. They added up the obligations and found that it would require planting trees on nearly 1.2 billion hectares, an area larger than Europe or the US and about the amount of land currently used for crops worldwide.
Compton Tucker et al, Subcontinental-scale carbon stocks of individual trees in African drylands, Nature (2023). DOI: 10.1038/s41586-022-05653-6
Jules Bayala et al, estimated that the carbon stocks of billions of individual African dry trees, Nature (2023). DOI: 10.1038/d41586-023-00531-1
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Reference: NASA Captures Sequestered Carbon of 9.9 Billion Trees with Deep Learning and Satellite Images (2023, March 5) Retrieved March 5, 2023, from https://phys.org/news/2023-03-nasa-captures- sequestered-carbon-billion .html
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