Some Dryland Forests Have Been Hiding in Plain Sight

This dry-season baobab forest in Senegal is the type of forest that was likely underestimated in earlier satellite studies of global forest cover. | FAO/Faidutti

A new estimate of the extent of dryland forests suggests that the global forest cover is at least 9% higher than previously thought. The study , published in the May 12 issue of Science, will help reduce uncertainties surrounding how much carbon dioxide plants absorb from the atmosphere globally. As carbon dioxide is a key driver of climate change, the study's results are important for climate modeling.

Given the vastness of land across the globe, researchers rely on satellite data to estimate the amount of forest cover. Yet dryland biomes — as their name suggests — are arid ecosystems where precipitation is outweighed by evaporation, making them particularly difficult places to spot and measure via satellite.

"First, the vegetation is quite sparse, so the signal is often a mix between vegetation and non-vegetation, like soil or even tree shadow," explained Jean-François Bastin of the United Nations Food and Agriculture Organization and co-author of the study. "Second, the vegetation in drylands is quite particular. To adapt to arid conditions, and therefore limit evapotranspiration, trees are leafless most of the year which makes it difficult to detect with classic mapping approaches."

This uncertainty can have huge implications for quantifying global forest cover, as dryland biomes cover about 40% of the Earth's land surface. It also raises questions about the real contribution made by forests to the global carbon cycle.

To attain a more thorough analysis of dryland biomes, Bastin and his colleagues acquired detailed satellite data from Google Earth that captured more than 200,000 plots of land from around the globe. Rather than depending on an algorithm to parse out plots that qualify as "dryland," team members painstakingly identified each plot themselves.

Their new estimate of dryland forest is 40% to 47% higher than previous estimates, corresponding to 467 million hectares of forest that have never been reported before. This increases current estimates of global forest cover by at least 9%, and will help researchers better quantify the role of forests as a global carbon sink.

Oceania, which includes Australia and the surrounding Pacific islands, and Africa were regions where dryland forests had been substantially underreported in previous analyses. These areas contain a very high proportion of open forest, making it more difficult to classify their nature compared to areas with a more full and lush green cover.

So, might other types of forest also be substantially underestimated? Bastin does not think so. He cited previous research suggesting that the biggest disparities in quantifying forest coverage involve dryland biomes.

Importantly, Bastin noted that in dryland biomes there is no competition for other activities such as intensive farming. "It means that these areas consist of great opportunities for forest restoration. Our data will help here to assess areas suited for forest restoration, to combat against desertification and therefore to combat climate change," he said.