Surprise! satellites counted more trees west of the Sahara and the Sahel than previously thought

Data collected by satellites enabled researchers to inventory trees west of the Sahara and the Sahel with a level of detail never equaled over such a large area. And against all odds, they listed more than they had imagined. Soon, thanks to artificial intelligence, the location and size of every tree on Earth can be mapped.

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Southern Sahara and the Sahel are home to more thantrees previously thought, playing a “crucial role” for the biodiversity and the lives of populations, according to a study published Wednesday in the revue Nature.

The international team of researchers has developed a pattern recognition program by artificial intelligence for count trees with a plant surface of more than three square meters, from more than 11,000 images satellite High Definition.

Over an area of ​​1.3 million km2 (i.e. two and a half times the area of ​​metropolitan France) in the south of the Sahara, the Sahelian strip (semi-arid zone south of desert) and sub-humid areas in Africa from the west, they were able to count more than 1.8 billion trees. Or an average of 13.4 trees per hectare, with a median plant cover of 12 m2.

An astonishing density of trees

This vegetation, certainly sparse, ” plays a crucial role for biodiversity and forecosystem as storage of carbone, food resources and shelter for human and animal populations, note the researchers. Although the total vegetation cover is low, the relatively high density of isolated trees challenges the prevailing idea of desertification dry areas, and even the desert offers a surprising density of trees ».

The density increases as it descends towards the wetter zones in the South, from 0.7 trees per hectare in the “hyper-arid” zones, to 9.9 in the arid zone, 30.1 in the semi-arid zone 47 trees per hectare in a sub-humid zone.

AI to identify every tree on Earth

In addition to this census, the study offers a new method for studying the presence of trees outside dense forest areas, and in particular their possible role in matter of climate change and therefore potentially poverty, through their contribution to the agricultural systems, the authors say.

« This kind of data is very important to establish a base. And in two or ten years, we could repeat the study to see if efforts to revitalize [la végétation] are effective “, Explained in a press release one of the researchers, Jesse Meyer, of the American space agency. Nasa.

The AI ​​technique used further suggests “ that it will soon be possible, within certain limits, to map the location and size of all trees, [une connaissance] fundamental to our understanding ofecology worldwide Said Niall P. Hanan and Julius Anchang of the American University of New Mexico, in an analysis of the study commissioned and published by Nature.

This video allows you to see the scientific work. © NASA

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