Drylands greener with forests than previously thought

New research has, for the first time, made use of very high resolution images of Earth’s drylands, revealing that they hold a much larger portion of the world’s forests than previously thought.

Employing a new method only possible because of the fine-grained quality of these images and the frequency at which they were taken, a team of scientists from 15 different institutions has found 467 million hectares (1.8 million square miles) of dryland forests that has escaped our notice until now. The findings, reported Thursday in the journal Science, notch up estimates of the world’s forest cover by 9 percent or more – high enough to impact global carbon budgets and open new doors to conservation in arid regions.

Jean-François Bastin, a remote sensing ecologist at the Food and Agriculture Organization (FAO) of the United Nations in Rome and the lead author of the study, said the team was “surprised and stunned” to see the bump in dryland forest cover from their assessment. Globally, 1,079 million hectares (4.166 million square miles) of forest covers this arid biome – from  40 to 47 percent above previous estimates.

These sections of the Earth, where more water is lost to evaporation and transpiration by plants than falls as precipitation, account for roughly 41 percent of land on Earth. They stretch from the tropics to the upper latitudes of the Northern and Southern Hemispheres. But ecologists have often paid more attention to tropical rainforests or boreal forests because they cover larger areas than dryland forests.

“When you have big decisions and we are trying to think about a way to do forest restoration, to do forest conservation, basically drylands were always put to the side,” Bastin said in a Skype interview.

It turns out, however, that the new calculations of dryland forest area put them in the same range as tropical forest area. Tropical forests covered 1,156 million hectares (4.463 million square miles) in 2000.

Part of the reason for the underestimation of dryland forests in earlier studies has to do with methodology. The “classical” approach for quantifying forest cover relies on vegetation “signals” from a few selected points that indicate the presence of forest. Then, scientists use models to estimate how much land area is forest.

This approach works well for more uniform areas of forest, but with drylands, assessments until now have been understood to be “limited,” Bastin said.

For this study, the team examined 0.5-hectare sample plots – 213,795 of them, to be exact – and counted individual trees in the canopy using images with resolutions of 1 meter (about 39 inches) or less. That allowed them to include sections of forest that had previously slipped past the model-based approach.

In addition to forest and tree cover, the researchers assessed 70 different attributes, including the locations of roads, settlements and croplands.

“The most striking information that we had was about the difference in terms of dryland forest cover,” Bastin said. Being able to tease apart areas that were true forest from areas that just appeared to be forests at lower resolutions – such as agroforestry plots, for example – allowed the team to produce an even more accurate tabulation of global forest cover.

The data detailing the other attributes will no doubt be useful in future research, he added, especially since the their findings demonstrate the importance of forests found in drier parts of the world.

“There is a lot to do in those regions, and we should be more focused about what is happening there,” Bastin said. For example, “What is their role … for global carbon cycling?”

The new estimate of dryland forest cover could mean that global carbon stocks are 20 percent greater than we thought they were.

The authors also write that a more prominent role for dryland forests could lead to solutions “to mitigate climate change, combat desertification, and support the conservation of biodiversity and ecosystem services that underpin human livelihoods.” Tropical dry forest was recently found to be the most endangered biome on the planet.

“These are very arid regions,” Bastin said. “You don’t have a lot of competition for the lands.” He pointed to efforts such as the Great Green Wall aimed at creating a 7,000-kilometer (4,350-mile) barrier of vegetation against drought and desertification across the African Sahel, home to some of the world’s poorest countries.

“It could be a win-win solution for many people.”

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CITATION

• Bastin, J., Berrahmouni, N., Grainger, A., Maniatis, D., Mollicone, D., Moore, R., … Mamane, B. (2017). The extent of forest in dryland biomes, Science, 356(6338), 635–638.