The biocrust conundrum
By destroying biocrust communities, climate change may be making arid lands more reflective — which could slow down warming.
In the high desert of Castle Valley outside of Moab, Utah, famous redrock spires and mesas tower against bright blue sky. Not everyone notices another set of strange formations stretching up from the desert floor: biological soil crusts, or “biocrusts,” tangled masses of mosses, lichens and cyanobacteria. Because the most dramatic biocrusts top out at a few inches tall, people are more likely to step on biocrusts than see them. Yet tiny lives can sculpt ecosystems: Biocrusts are the stalwart architects of dryland soils, digging in where many plants can’t survive. Biocrusts glue the soil together, slowly building its fertility by pulling nitrogen from the air and converting it to a form usable by plants, as well as storing atmospheric carbon. Biocrusts also soak up water like sponges when it rains, slowing down runoff and helping store water for desert plants.
Now, new research published in the peer-reviewed journal Scientific Reports has introduced a bizarre twist into the hidden world of biocrusts: A changing climate may be killing the mosses and lichens in biocrusts. Their demise, in turn, may actually slow down climate change by making drylands reflect more sunlight back into space.
Drylands, which cover more than 35 percent of the Earth’s land, are places that get so little rain or snow, it doesn’t replace the amount that evaporates away. “Over time, there’s chronically less moisture than comes in,” Scott Ferrenberg, a U.S. Geological Survey biologist and a collaborator on the study, said. Drylands cover most of North America and the West. To the south, they include deserts like the Sonoran; to the north, they include the colder Colorado Plateau, where this biocrust research took place.
Biocrusts live across those arid ecosystems, taking hold where plants struggle. “Biological crusts can often cover just as much ground or more as plants do,” said Matthew Bowker, a soil ecologist and biological crust expert at Northern Arizona University. “If it hasn’t been walked on, driven on, or over-grazed, chances are good you’ve been around it.”
For the dry places biocrusts live, they behave a bit like a dark shirt in the hot sun, absorbing sunlight and heating up the desert that they cover. This adds to global warming overall. Scientists call a surface’s reflectivity its “albedo.” High albedo is important for keeping the planet cool because the sun’s rays are reflected back. A desert decked out in biocrust has lower albedo than bare desert soil, because it’s darker and more textured than crust-free soil. And more complex biocrusts like those found on the Colorado Plateau have lower albedo than simpler ones.
Austin Rutherford, a graduate student at the University of Arizona, along with colleagues including Ferrenberg, published the results of a 10-year experimental study that found the mosses and lichens that give biocrusts their darker colors and textured surfaces may not make it through the next century. As they die out, the albedo of the drylands where those biocrusts live may increase by more than 30 percent, which could be enough to slow down climate change.
The team set up experimental plots in the desert, creating a futuristic climate scenario with temperature and rainfall modeled after what’s predicted to happen in the region, then watched what unfolded in the biocrust world at their feet. This meant, ultimately, warming some plots by as much as about 7 degrees Fahrenheit – the projected increase for the next 100 years – and giving some biocrusts short bursts of 1.2 millimeters of simulated rain twice each week.
That’s to reflect the fact that rainfall is becoming shorter and more frequent in the region, Rutherford explained. “You’re still going to get your monsoon, but in shorter, more intense rainfall.”
As it turns out, biocrust creatures can be picky about how their water gets delivered – especially mosses. Within the first year of the study, mosses died out in plots that got extra water, or extra water with warming. Over the long term, as treatments continued, mosses died out in the plots that were only warmed, too.
To explain why the mosses died, Ferrenberg points to USGS researcher Sasha Reed’s work on moss biology.
Mosses often dry out and then come back to life. To do that, dried out mosses store energy that they made while wet so that the next time it rains, they have the resources to get back to photosynthesizing and living.
But mosses are ancient types of plants. “They don’t have stomata or waxy cuticles like grass. When you wet a moss it immediately becomes active, ” Ferrenberg said. If it rains long enough to wake them up but not to let them build up energy reserves, it’s as though they keep walking to the grocery store and finding that it just closed. Eventually, if the store doesn’t stay open long enough – if the rain doesn’t last long enough – the mosses will starve to death.
The researchers found that as the mosses died, cyanobacteria grew in their place. Albedo in the plots increased by more than 30 percent, reflecting more of the sun’s energy back into space.
There’s another potential feedback loop in Colorado Plateau biocrusts, which develop particularly rough textures in part because of freezing and thawing of the soil. As the region’s climate warms, less freeze-thaw will further accelerate the move to smoother, lighter and more reflective biocrusts, which absorb less of the sun’s energy. This may also slow down climate change.
But the full consequences of changing biocrusts on the warming climate are likely more complicated.
“While at the surface it might look like, ‘Hey, let’s go out and drive all over the biocrust to help stop climate change,’ it turns out that it could put us right back where we were, maybe worse,” Ferrenberg cautioned. For example, without a network of biocrust holding the Colorado Plateau’s soil together, wind erosion could throw enough dust onto nearby mountains to cover snowpacks, which are themselves important reflectors of sunlight. That could cancel out the cooling from the smoother, more reflective biocrusts. Plus, dead biocrusts will release of all the atmospheric carbon that they once stored as they grew.
In the end, a conversation about climate change mitigation that includes losing biocrusts ends up where it started. If Westerners don’t cut back on climate-warming emissions, we may end up grasping for… well, not straws. Or moss. Maybe cyanobacteria.
Maya L. Kapoor is an associate editor with High Country News.
This story has been updated to correct that the research was published in Scientific Reports, an affiliate of Nature, and that temperatures are predicted to warm by as much as 7 degrees F.