Climate and Physical Disturbance Effects on the Spectral Signatures of Biological Soil Crusts: Implications for Future Dryland Energy Balance

Abstract:

Drylands comprise ≈40% of the terrestrial Earth surface and observations suggest they can respond markedly to climate change. A vital component of dryland ecosystems are biological soil crusts (biocrusts) - a network of surface soil lichens, mosses, and cyanobacteria – that perform critical ecosystem functions, such as stabilizing soil and fixing carbon and nitrogen. Yet, our understanding of the role biocrusts play in dryland energy balance remains poor. Changes in climate can rapidly affect biocrust communities and we have long known that biocrusts respond dramatically to physical disturbance, such as human trampling and grazing animals. Associated changes in biocrust cover often result in increased bare soil; creating higher surface reflectance. We used spectral solar reflectance measurements in two manipulative experiments to compare the effects of climate and physical disturbance on biocrusts of the Colorado Plateau We measured reflectance at two heights: at crust surface and 1 m above. The climate disturbance site has four treatments: control, warming (4°C), altered precipitation, and warming plus altered precipitation. The physical disturbance site was trampled by foot annually since 1998. At the climate experiment, the largest change in reflectance was in the altered precipitation treatment (35% increase) at the surface-level, and the smallest difference was in the warmed (17% increase) at the meter-level. Physical disturbance differences were 10% at meter-level and 25% at surface-level. Unexpectedly, these results suggest that, via effects on biocrust communities, climate change could have a larger effect on dryland energy balance relative to physical disturbance, and result in more radiation from drylands returned to the atmosphere. Biocrusts cover large portions of the Earth’s surface and, to our knowledge, these are the first data showing climate-induced changes to biocrust reflectance, with negative feedback in the global energy balance.