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How Do You Count All Those Trees, Anyway?

Like many scientists, Jean-François Bastin and colleagues had a question. A question that on its surface seems like it may have an obvious answer, or at least, an obvious way to find out the answer. But looks can be deceiving and things are not always as easy as they seem. But thanks to 210,000 sample plots, a ton of satellite imagery, and new geospatial tools from Google Earth they found an answer.


They wanted to know just how many trees there are in drylands.


The answer? A whole lot more than we thought.
In a recent paper in the journal Science, Bastin and colleagues show that the total forest cover in drylands is up to 47% greater than previously thought for a total of 1.3 billion hectares (~3.2 billion acres) of forest cover. Much of this “new” forest cover was found in Africa, both along the Mediterranean and in Southern Africa. This puts dryland forest cover on par with tropical moist forests which covered nearly 1.1 billion hectares (~2.9 million acres) in 2000.


That’s a lot of trees.


In fact, it bumps up the total estimate of global forest cover by 9%–to 5 billion hectares (12 billion acres) and increases the estimate of global carbon stored in forests by somewhere between 2 to 20%. Unsurprisingly, this paper generated a lot of news as well 1, 2, 3, 4.


Forest distribution in drylands, from Bastion et al. 2017. Areas of green are forests, while areas of yellow are not.


But despite some hyperbolic claims of “lost forests”, since these forests were here all along, the real question here is how are there more than we thought there were? Just how do you count these trees?
Drylands get their name from the fact that they are, as you probably guessed, really dry. Scientifically, an area is classified as a dryland if it has an aridity index below 0.65. The aridity index is the ratio of how much moisture comes into the system to how much would be expected to leave through evaporation and transpiration, or the potential evapotranspiration which itself is calculated as a function of temperature and day length. If you know what is coming in and what is going out, well, now you know what to expect.
One of the problems with studying drylands is here are a lack of large-scale, on-the-ground studies.. Much of these areas are quite remote and, shall we say, “climatically hostile.” This makes field work challenging. This has resulted in a high degree of uncertainty in what is actually out there. Researchers have to turn to remote sensing techniques such as satellite and airborne imagery.


Using Google Earth imagery, the group designed a stratified sampling strategy to visually identify trees and tree cover in very high resolution images. Then, with the help of scientists and students from over 15 organizations world-wide, used this framework to process 213,795 of these images.


Trees can be picked out from shrubs by looking at the “crown shadows” produced by trees in the imagery. Trees are taller than shrubs, and produce a tell-tale shadow on the ground that can be picked up visually.


An example from Google Earth. Here I zoomed into the Sahel region of Africa and reproduced an approximate plot to what researchers would have used. Note that within each 0.5 hectare square, you can pick out tree and shrub cover.


Each image examined had an area of half a hectare—a square about 70 by 70 meters (~230 feet), but a very fine resolution, think on the order of a meter or so where individual trees could be picked out. The group then compared results from visual inspection to satellite data from MODIS—NASA’s Moderate Resolution Imaging Spectroradiometer. MODIS is a pair of satellites, named Terra and Aqua, which orbit around the earth every couple of days producing images with a resolution, at best, of 250 meters (~820 feet) on either side or about 63 hectares in area.


When the group compared the results from their high resolution, visually inspected data to the much lower resolution MODIS data, they found that MODIS greatly underestimated the number of trees and the amount of forest cover.


On the left, the Flatiron mountains above Boulder, Colorado from Google Earth taken October, 2015. You can see the individual crowns of each tree and trace ecosystem details. To the right, an image from the MODIS observation platform showing vegetation density. Each pixel is 250 by 250 meters.


In 2000, researchers from Oxford University and Conservation International published a paper in Nature, identifying 25 biodiversity hotspots for conservation priority with an eye to promote a “silver bullet” strategy to focus on these hotspots.


Seven of the hotspots included in this survey were in drylands. These often disregarded biomes cover more than 40% of the global land surface and are among the most threatened ecosystems on the planet—facing the squeeze not only from changing climate, but from human activity as well.
Climate projections show that the total area of drylands may expand by over 20% by the end of this century. This would mean that over half of the land surface would be classified as drylands. As drought increases in likelihood with changing climate and human pressures on these systems mount, desertification—the conversion of the landscape to a desert biome–becomes more likely. For the nearly 1 billion people who live in sub-Saharan Africa, this is an ever-present issue.


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