For fragile drylands, climate change is crushing

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Plant cover is sparse in dryland ecosystems, but land that looks bare can be far from lifeless. Up to 70 percent of the ground in drylands is covered by biocrusts, unshowy but vital communities of microbes, mosses, and lichens at the surface of the soil.

Biocrusts are the foundation of dryland ecosystems: they fix carbon and nitrogen, regulate water flow, and hold soil in place, preventing erosion and dust storms. Researchers have long known that these crusts are vulnerable to trampling by humans, livestock, or vehicles. Now, researchers with the U.S. Geological Survey have found that climate change affects biocrusts in a similar way and to an equally dramatic degree as physical disturbance.

The insight comes from a long-term field experiment conducted in the high desert of the Colorado Plateau in Utah, where researchers marked off a series of 2-meter-by-2.5-meter experimental plots. In some, they used heat lamps to warm the soil by 2 degrees Celsius for three years and then by 4 degrees Celsius for the next seven. Other plots got misted with water from hand-held sprayers to simulate frequent, small rainfall events – a pattern predicted for the region under many global climate models. Some plots were subjected to both warming and watering, while control plots received neither treatment. The researchers compared the biocrusts in these plots to those in another set of 2-meter-by-5-meter plots that that were trampled annually between 1996 and 2011.

In healthy, mature biocrusts, strands of microbes called cyanobacteria glue soil particles together. On top of and among the cyanobacteria grow a rich array of lichens and mosses, as well as fungi and other organisms. But both trampling and climate change disrupt these biocrust communities, and in similar ways, the researchers reported yesterday in the Proceedings of the National Academy of Sciences.

Trampling and climate change alike dramatically reduced mosses and lichens in the biocrusts, the researchers found. Meanwhile, cyanobacteria spread to fill in areas from which the other organisms had disappeared. Mosses and lichens covered about 19 percent of the ground area in control plots, but just 0.5 percent in experimental plots.

This pattern of extreme dominance by cyanobacteria, and scarcity of mosses and lichens, is considered characteristic of early successional biocrusts. These early-stage biocrusts don’t absorb water as well, leave the land more vulnerable to erosion, and store less carbon and nitrogen than mature crusts.

The findings are especially concerning because dryland ecosystems cover 40 percent of the Earth’s land surface and may account for up to one-quarter of the carbon stored in soil. Biocrusts play a big role in that. So while biocrusts may not look like much, their effects are globally significant. And people can take steps to mitigate or avoid physical trampling in dryland ecosystems, but warming is almost certain. —Sarah DeWeerdt | September 15, 2015

Source: Ferrrenberg S. et al. “Climate change and physical disturbance cause similar community shifts in biological soil crusts.” Proceedings of the National Academy of Sciencess DOI: 10.1073/pnas.1509050112

Header image: A trail carved through biocrust in Utah. Credit: Jason Hollinger via Flickr.