by Wood, foliage and root charcoal from 9,947,310,221 trees with crown area >3 m2 over 9.7 million km2 were pictured. a, Our study covered southern Sahara, Sahel, and northern Sudanese zone of Africa and showed the aggregate carbon density (foliage + wood + root) per hectare for 9,947,310,221 tree canopies from years 0–1,000 mm−1 Medium precipitation area. The isohyets mark the 150, 300, 600, and 1,000 mm years−1 Rain zones (north to south). b, Example showing the charcoal 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, Mean carbon stock of individual trees at the 5th, 10th, 25th, 75th, 90th, and 95th percentiles along the precipitation gradient. Our definition of a tree is a green leaf crown >3 m2 with associated 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 discrete tree canopies down to a 50cm scale. The images covered much of arid North Africa, from the Atlantic to the Red Sea. Allometric equations based on previous tree samples allowed the researchers to convert 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 a region’s carbon stock might be based on counting small areas and extrapolating the results upwards, 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 counting every scattered tree in areas that previous models often represented by null values was dashed by large overestimates in other areas of the previous assessments. In previous attempts using satellites, cropland and ground vegetation adversely affected the optical images. When radar was used, topography, wetlands, and irrigated areas affected radar backscatter and predicted higher carbon stocks than current NASA estimates.
Deep learning based mapping
The researchers applied deep-learning-based tree mapping, which was manually trained on about 90,000 trees, to a dataset of nearly 300,000 satellite images to measure more than 9.9 billion trees that have a shade and canopy area of more than 3 square meters exhibited. Only features that had a distinct crown area and associated shadow were selected, allowing the team to exclude small bushes, tufts of grass, rocks, and other misleading features.
The mapped regions have been correlated to reflect four zones of precipitation; hyperarid, arid, semi-arid, and arid subhumid—since rainfall affects carbon uptake and storage. While foliage accounts for only 3% of total dry matter, it was used as an indirect measure to quantify total mass. The proportion of the root mass is on average 15-20% of the total mass and was also derived from the foliage.
Would you like to visualize the large tree mapping data set in an interactive browser format? The researchers did, so they created a nifty viewer to work with and made it publicly available here.
The ability to track the effectiveness of carbon sequestration could be of global importance in the fight against climate change. Reforestation is a leading practice that the world’s nations have committed to offsetting their carbon footprint.
However, the practicality of these commitments has been under scrutiny by a team of 20 researchers collaborating with the University of Melbourne’s interdisciplinary climate research initiative. They summed up the commitments, noting that it would require planting trees on nearly 1.2 billion hectares, an area larger than Europe or the US and roughly equal to the area currently used for cultivation worldwide.
More information:
Compton Tucker et al., Subcontinental carbon stocks of individual trees in African drylands, Nature (2023). DOI: 10.1038/s41586-022-05653-6
Jules Bayala et al., Carbon stocks of billions of individual African dryland trees estimated, Nature (2023). DOI: 10.1038/d41586-023-00531-1
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Citation: NASA captures sequestered carbon from 9.9 billion trees using deep learning and satellite imagery (2023, March 5), retrieved March 5, 2023 from https://phys.org/news/2023-03-nasa-captures- sequestered-carbon-billion.html
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