Forests come in many different formats, but they're all hugely important to life on Earth — humans included. Yet as deforestation continues to shrink woodlands around the world, these eminent ecosystems are overdue for some good news.
And a new study obliges: Using satellite imagery, scientists have discovered global forest cover is at least 9 percent higher than previously thought. Because forests help absorb some of the carbon dioxide emissions that drive climate change, this could have big implications for climate modeling. More broadly, it's also just a helpful reminder of how much natural heritage still exists for humanity to preserve.
Published in the journal Science, the study sheds light on dryland biomes — places where precipitation is offset by evaporation from surfaces and by transpiration in plants, leaving a scarcity of available water. It offers a new estimate for how much dryland forest exists on Earth, including an astounding 467 million hectares (1.1 billion acres) of dryland forests "that have never been reported before."
That's larger than the Congo Basin, home to the second-largest tropical forest on Earth, and it's roughly two-thirds the size of the Amazon. These newly reported dryland forests are scattered all over the world, but taken together, this is sort of like discovering a "second Amazon," as Patrick Monahan writes in Science Magazine.
Missing the forest for the trees
Because Earth has so much ground to cover, scientists often use satellite imagery to estimate forest area. But as study co-author Jean-François Bastin explains in a statement, dryland forests can be hard to find 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," says Bastin, a remote-sensing ecologist with the United Nations' Food and Agriculture Organization (FAO). "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."
Since dryland biomes cover about 40 percent of Earth's land surface, that difficulty was kind of a big deal. To clear things up, Bastin and his colleagues obtained high-res satellite data featuring more than 200,000 plots of land around the world. Instead of relying on an algorithm to figure out which plots qualify as dryland, the researchers did the grunt work themselves, meticulously identifying each individual plot.
Dryland forests had been underreported across parts of Africa and Oceania, including Australia and various Pacific islands, the study found. Many of these areas have lots of open forest, which — along with quirks of dryland trees — can make them harder to identify in satellite images than the canopies of fuller, greener forests.
The researchers doubt that other forest types have been similarly underreported, noting previous research had already indicated that dryland forests likely account for the biggest discrepancies in global forest-cover estimates.
A forest to be reckoned with
The critically endangered palila inhabits dry forests on the Big Island of Hawaii. (Photo: HarmonyonPlanetEarth/Flickr)
The new study's insights should give scientists a clearer picture of how much carbon dioxide Earth's forests are absorbing from the atmosphere, and thus clarify how much they'll help us with climate change in the years and decades ahead.
Forests alone may not save us from our own greenhouse gas emissions, but their carbon-hogging trees are some of our best allies in this struggle.
Many dryland forests are also sanctuaries for biodiversity, so this may be good news for the fight against global mass extinction, too. In Hawaii, for example, more than 40 native plant species grow in dryland forests, including the endangered kauila, uhiuhi, koki‘o, ‘aiea and halapepe trees. More than 25 percent of Hawaii’s endangered plant species are found in dryland forests, according to the nonprofit Ka'ahahui 'O Ka Nāhelehele, and these ecosystems are also home to rare birds like the ‘amakih and the palila, an endangered Hawaiian honeycreeper.
And while many forests face pressure from humans who'd like to use the space for farmland, pasture or other purposes, Bastin points out that dryland forests' arid environments don't invite quite the same degree of competition.
"It means that these areas consist of great opportunities for forest restoration," he says. "Our data will help here to assess areas suited for forest restoration, to combat against desertification and therefore to combat climate change."