Drylands are one of the more important ecosystems in the world, comprising fully 40 percent of the Earth’s land surface. And now, an alarming new study in the Proceedings of the National Academy of Sciences says the impact of a warming climate on these ecosystems could be much worse than expected — comparable to humans trampling the landscapes underfoot or driving off-road vehicles across them.
“Contrary to our expectations, experimental climate change and physical disturbance had strikingly similar impacts,” wrote the researchers, led by Scott Ferrenberg of the U.S. Geological Survey’s Southwest Biological Science Center in Moab, Utah. Ferrenberg conducted the work with two Geological Survey colleagues.
Ferrenberg and his colleagues ran a long term experiment at Utah’s Upper Colorado Plateau, a “cool desert” region that receives less than 10 inches of rain per year. Here, the ground is covered by a complex group of organisms collectively called “biocrust” — a combination of mosses and lichens that are in effect glued together by photosynthetic microorganisms called cyanobacteria, which provide structure to the landscape through the carbohydrate molecules they secrete.
This structure, in turn, allows the more complex organisms like mosses and lichens to grow — and when it’s all assembled, the biocrust then holds the soil in place and prevents dust storms and erosion. “Things would blow away without biocrust,” says Ferrenberg.
“There’s sort of this expression the crust community uses: ‘Got dust? You need crust,’” he continues. “It’s sort of a nerd joke in this community.”
At the same time, the growth of mosses and lichen, supported by the cyanobacteria, allows these landscapes to store a considerable amount of carbon which might otherwise wind up in the atmosphere. Overall, drylands store 25 percent of the carbon found in the planet’s soils.
A desert landscape like the Upper Colorado Plateau, however, is highly vulnerable to destruction if humans or livestock tread across it or especially if vehicles tear it up. And if that happens enough, then not only are dust storms more likely to blow up the loose sand, but there will be less storing of carbon and the ecosystem may be set up for what researchers call “succession” — transition into a more extreme desertified state, without as many lichens or mosses and with only cyanobacteria hanging on.
So what does climate change add to this dynamic? To find out, the researchers conducted a long term experiment in which plots of Colorado Plateau biocrust were subjected to either 10 years of warmer temperatures or more rainfall, or 15 years of literal human trampling.
In the climate change scenario, plots of ground were continually warmed by infrared heaters several degrees Celsius above the temperature surrounding them. In the stomping scenario, by contrast, a team of volunteers walked heel to toe, twice, across a plot of land — once per year for 15 years.
The researchers found that in all cases, the effect was more or less the same — a severe blow to the lichens and mosses of the community, leaving behind only the algae or “cyanobacteria,” which proceeded to show an increase that the researchers called “dramatic.”
For instance, while algae made up 81 percent of the biocrust community prior to human trampling, afterward it made up 99 percent. Warmer temperatures had a similar effect.
“We were really surprised,” said Ferrenberg. “We know that walking on them or driving on them kills them. We were really surprised that giving them a bit of extra heat would kill them, and it did.”
The researchers proceeded to observe that while you can protect drylands from humans, vehicles, or livestock — at least to an extent — you can’t switch off global warming. “The effects of warming described here are a great cause for concern, as increasing annual temperatures are a near certainty across dryland ecosystems,” they wrote.
The experiment only looked at one dryland ecosystem, but according to the researchers, biocrust is prevalent in drylands across the globe. “We think this heralds pretty bad news for biocrusts on a global scale,” Ferrenberg said.
And if that’s right, then it doesn’t just mean drylands may not hold in as much carbon any more (which means more of it goes to the atmosphere). It also means they may produce worse dust storms in the future.
So, no, drylands and biocrusts don’t get a lot of attention. But maybe now you can see why, in their paper, the researchers use words like “disconcerting” and “alarmingly” to describe what could happen to them.