Biocrusts mediate new mechanism for land degradation under a changing climate

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As a team of scientists living and working on the Colorado Plateau in the western United States, we are situated in a region that is warming at a more rapid pace than the global mean. Here, the landscape is home to some of the most abundant, diverse, and functionally important photosynthetic soils called biological soil crusts. Using a one-of-a-kind in situ dryland climate manipulation experiment and a long-term mechanical disturbance experiment, we tracked how biocrusts recover from both physical and climate disturbances, and how warming temperatures affect the pace of biocrust recovery.

Biological soil crusts (biocrusts) are fundamental communities in drylands worldwide composed of photosynthetic soil organisms, including mosses, lichens, and cyanobacteria. Biocrusts provide a variety of important ecosystem functions, such as increasing soil fertility through nitrogen fixation and decreasing soil erosion.  We know biocrusts are sensitive to disturbance, but we know much less about how they recover. For example, critical but poorly understood questions are: How quickly do biocrusts recover following disturbance, do they return to their previous state or follow a new trajectory, and do warming temperatures affect the rate and direction of recovery?

Our study examined the legacy effects of disturbance and the effect of warming on the composition of the biocrust community, but also the downstream effects on an important ecosystem function, erosion control. Intact biocrusts greatly decrease erosion potential by increasing soil stability and acting as a “living skin” on the soil surface. Soil erosion is a major concern in dryland ecosystems, as dust production can reduce soil fertility, decrease air quality, cause vehicle accidents, and compromise human health.

We combined ~15 years of data from our climate and mechanical disturbance experiments to understand how biocrusts recover from two types of perturbation: a mechanical disturbance conducted via human trampling annually for 15 years, and a precipitation disturbance that increased the frequency of small rain events (< 1.5 mm) during the summer months. An increased frequency of these small precipitation events, which are common in the southwestern U.S., has been shown to kill biocrusts due to the stress of the wet up and dry down cycles. In addition to exploring recovery from these mechanical and climate disturbances under current conditions, we examined how warming temperatures (4° C above ambient) affect biocrust communities and their recovery following the cessation of the precipitation disturbance.

Our study suggests that biocrust states that take a longer time to develop (intermediate and later successional states) recover much faster than previous literature suggests. We observed significant biocrust recovery ~8 years after disturbance ended. However, warming combined with the legacy of altered precipitation halted recovery completely and warming alone decreased the presence of mosses. Mosses are known to fix relatively large amounts of carbon and confer the highest amount of soil stability compared to other biocrust types. In conjunction with loss of mosses under warming we saw decreases in soil stability, suggesting that warming has strong potential to increase soil erosion though biocrust degradation. Additionally, the combination of warming with the legacy of the precipitation disturbance led to even more severe moss reduction than warming alone.

Climate forecasts for most drylands suggest increased severity and frequency of drought, greater variability in precipitation, and, unswervingly, increased temperatures. The Intergovernmental Panel on Climate Change (IPCC) highlights desertification and climate change as critical concerns, both individually and concomitantly, because they reduce ecosystem functions and health in drylands. Drylands cover almost half of Earth’s terrestrial surface, support the livelihoods of billions of people, and are predicted to expand drastically in the coming decades. Yet our understanding of how climate change will drive state shifts in dryland systems remains exceedingly poor, particularly for ecologically important biocrusts. This work takes a first step in understand the implications of climate change and disturbance on biocrusts and the functions they provide.   

Our study suggests high vulnerability of key biocrust types to warming, coupled with our observations that warming halts recovery following disturbance, and highlights the potential of rising temperatures to act as a new mechanism for land degradation through losses of soil stability and fertility. Biocrusts are incredibly common in drylands worldwide and can often be the dominant cover type. Our work illustrates that they may also serve as a bellwether for predicting changes to ecosystem function in drylands under a changing climate.

Photo of biocursts on the Colorado Plateau. Photo Credit:  C.Lauria

Michala Phillips

Ecologist, United States Geological Survey