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In fact, there are around 1.8 billion trees in the Sahara and Sahel deserts of West Africa, also known as semi-wetlands. These new and never-before-found results disprove previous assumptions about such potential habitats, the researchers say.
“We were very surprised that so many trees grew in the Sahara desert,” Martin Brandt, lead author of the study, told AFP.
“Of course there are huge areas without trees, but there are still areas where there is a high density of trees and even some sand here and there between the sand dunes,” added the associate professor of geography at the University of Copenhagen. M. Brandt.
These data could help guide efforts to combat deforestation and more accurately measure terrestrial carbon.
“This data is very important in determining the baseline,” said Jesse Meyer, a programmer at NASA’s Goddard Space Flight Center who worked on the study.
“In a year, two or ten years, the study could be repeated to see if efforts to revitalize and reduce deforestation are effective or not,” Jesse Meyer told NASA in a press release.
Finding and counting trees was not an easy task. In areas rich in trees, satellite images, even at low resolution, show relatively clear and thick growth clumps that can be easily distinguished from bare ground. However, where they are most prevalent, satellite images may be too low in resolution to distinguish individual trees or small groups of trees.
Now there are higher resolution images, but problems still persist: it is almost impossible to count individual trees, especially over large areas of the site.
Brandt and his team came up with a solution that combines very high-resolution satellite imagery with deep learning, essentially teaching a computer program to do the job for them. But that doesn’t mean they can just sit back and wait for the results.
Before the deep learning program was launched, you needed training. It was a difficult process in which Brandt individually counted and tagged nearly 90,000 trees. It took years.
“The level of detail is very high and the program needs to know what different trees look like in different landscapes,” he said.
“I did not accept the incorrect classifications and also taught the program when I saw the incorrectly classified trees.”
He said it was worth the effort, allowing the program to count trees in just a few hours, when millions of people would have needed to do so for years.
“Other studies are based on estimates and extrapolations, here we see and directly count each tree, this is the first study of its kind.”
The study, published in the journal Nature, covered an area of 1.3 million square kilometers and analyzed more than 11,000 images.
The technique suggests that “it will soon be possible, with some limitations, to calculate the location and size of each tree around the world,” wrote Niall P. Hanan and Julius Anchang, researchers in the Department of Plant and Environmental Sciences at the State University of New Mexico.
Accurate information on vegetation in deserts and other arid areas is “important for understanding global ecology, biogeography, and the biogeochemical cycles of carbon, water, and other nutrients.”
More accurate information can help determine how much carbon is stored in these places, which are not normally included in climate change models, doc. M. Brandt.
However, it is too early to say whether exact tree counts will affect understanding of climate change and its acceleration, he added.
Now he hopes to use the technique elsewhere to map more than the 65 million square kilometers of arid regions of the world that have been hidden until now.
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