A recent study has mapped the locations of 9.9 billion trees across Africa’s drylands, a region below the Sahara Desert and north of the equator.The research, which combined satellite mapping, machine learning and field measurements, led to an estimate of 840 million metric tons of carbon contained in the trees.This figure is much lower than the amount of carbon held in Africa’s tropical rainforests.However, these trees provide critical biodiversity habitat and help boost agricultural productivity, and this method provides a tool to track both degradation and tree-planting efforts in the region. For the first time ever, researchers have plotted out the locations and tabulated the carbon stocks of more than 9.9 billion individual trees spread across the dry belt of land stretching between the southern edge of Africa’s Sahara Desert, and the wetter savannas and tropical forest closer to the equator.
Until now, estimates of the carbon content in these dryland trees relied on lower-resolution satellite images, or on models that use algorithms to predict where they were located and how much carbon they hold. These estimates came with a lot of uncertainty, which made it difficult to account for carbon stocks or to track programs, such as Africa’s Great Green Wall of the Sahara and the Sahel, which aims to boost arable land in this part of the continent by planting trees.
“No one knows if they’re really planted and what is the carbon stock,” said Martin Brandt, an associate professor and physical geographer at the University of Copenhagen in Denmark. “You could not monitor these things.”
Now, thanks to research Brandt and his colleagues published in the journal Nature March 1, it is possible to more accurately track the progress of these types of initiatives.
The new method they developed to pinpoint the trees employs a type of artificial intelligence known as machine learning that mapped individual trees found in more than 326,000 NASA satellite images. Then, by coupling these data on billions of trees with on-the-ground weights and measurements, the team was able to link canopy size to the biomass contained in each tree’s roots, trunk, branches and leaves, providing the most accurate estimates to date of their carbon stocks.
A road through the savanna in north Sudan. Image by Rita Willaert via Flickr (CC BY-NC 2.0).
The analysis covered nearly 10 million square kilometers (about 3.9 million square miles) of a belt of land comprising the southern edge of the Sahara, the acacia-dotted transition grasslands called the Sahel, and the “Sudanian” savanna where precipitation tops out at about 1,000 millimeters (40 inches) of rainfall per year.
They estimate that trees in this region contain roughly 840 million metric tons of carbon — in general, a lower figure than those reported in most previous studies.
Edward Mitchard, a professor in the school of geosciences at the University of Edinburgh in the U.K., who was not involved in the research, called the new methodology “amazing.”
“I believe the carbon numbers far more than any other map for this region,” he added.
In fact, the fine-scale resolution mapping in this study forms a stark contrast with what Mitchard himself has used in the past. The current study used a resolution of 0.5 meters (about 20 inches) per pixel. But his 2013 study, looking at the fluxes in vegetation in African savannas and woodlands, employed 8-kilometer (5-mile) pixels. As a result, Mitchard’s estimates were based on thousands of trees, along with a lot of other objects that could throw off the analysis, including grasses, shrubs and even houses. (Advances in computing power and the resolution of available images helped make this work possible, Brandt said.)
Brandt and his colleagues have made their data available on a public viewer so that anyone can look at the attributes of the trees. Now, individual farmers can zero in on the trees on their own land, even if it just holds a handful of them, to determine what their carbon stocks are, Brandt said.
“That would allow even smallholder farmers to be included in these kinds of [conservation] programs and to receive carbon credits,” he added.
A baobab in the Sahel. Image by Eric Montfort via Flickr (CC BY-NC-ND 2.0).
At a broader scale, countries can include these data in their national carbon inventories. Brandt said a recent analysis in Rwanda that was similar in design is helping that country’s leaders in its carbon accounting.
Still, this method doesn’t work as well in more densely forested regions, like Africa’s tropical rainforests, Brandt said.
“In a humid forest, it’s a very heterogeneous arrangement of different layers … You don’t see the all the trees because they are covered by other trees,” he added. “It’s very difficult.”
The authors write that their maps “should be used with caution” in areas with more than 800 mm (31.5 in) of annual rainfall, and they call for more research comparing what they’ve found with estimates in wetter parts of the continent.
Mitchard noted another important finding of the new study: The carbon content of the drylands doesn’t stack up well against that stored by Africa’s tropical rainforests. So it may be that conservation initiatives, rather than offering carbon credits, should instead offer water or biodiversity credits.
“This is not a high carbon stock area of the world,” he said.
Even the highest estimated carbon densities estimated by Brandt and his colleagues maxed out at no more than 10 metric tons per hectare, Mitchard said. Carbon densities in humid tropical forests can be 10 to 20 times that amount.
However, the importance of these dryland trees extends far beyond carbon, he added, noting that the authors also address this point. Trees provide habitat for other species. They anchor the soil and prevent erosion, and they help infuse it with the nutrients necessary for crops and other plants to grow. The leaves are also fodder for livestock.
Those ecosystem services, along with the carbon the trees sequester, are under threat. Though tree cover is increasing in some parts of the drylands region, in others degradation is occurring, caused by charcoal production and other human uses, Mitchard explained. Repeating the analysis over time could yield important insights into what’s happening in the drylands of Africa, he said.
“For this region, it’s a great way of monitoring biomass, and if you could redo this every few years, you could really see the growth of individual trees and which of these trees had disappeared,” Mitchard said. “It’s really a game-changing method for dry regions.”
Banner image: Trees in the plains of Sahel. Image by Jay Sterling Austin via Flickr (CC BY 2.0).
John Cannon is a staff features writer with Mongabay. Find him on Twitter: @johnccannon
Why keep Africa’s dryland forests alive?
Mitchard, E. T., & Flintrop, C. M. (2013). Woody encroachment and forest degradation in sub-Saharan Africa’s woodlands and savannas 1982–2006. Philosophical Transactions of the Royal Society B: Biological Sciences, 368(1625), 20120406. doi:10.1098/rstb.2012.0406
Mugabowindekwe, M., Brandt, M., Chave, J., Reiner, F., Skole, D. L., Kariryaa, A., … Fensholt, R. (2022). Nation-wide mapping of tree-level aboveground carbon stocks in Rwanda. Nature Climate Change, 13(1), 91-97. doi:10.1038/s41558-022-01544-w
Tucker, C., Brandt, M., Hiernaux, P., Kariryaa, A., Rasmussen, K., Small, J., … Fensholt, R. (2023). Sub-continental-scale carbon stocks of individual trees in African drylands. Nature, 615(7950), 80-86. doi:10.1038/s41586-022-05653-6
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