Same shift, different disturbance: Experimental warming, altered precipitation, and physical disturbance lead to a similar alternate state in biological soil crust communities
The potential for ecological state changes in response to climate change and physical disturbance is a widespread and growing concern. In dryland ecosystems, disturbance of biological soil crusts (biocrusts)—communities of mosses, lichens, cyanobacteria, and heterotrophs living at the soil surface—can dramatically alter biogeochemical processes, hydrology, surface energy balance, and vegetation cover. Because biocrusts are fundamental components of dryland ecosystem structure and function, understanding how these communities will respond to perturbation is central to predictions of dryland response to change. In addition to longstanding concern over the impacts of physical disturbances on biocrusts (e.g., trampling by foot, vehicles, and livestock), there is increasing concern over the potential impacts of climate change on biocrust community structure. With the goal of understanding how biocrusts will be affected by ongoing global change pressures, we used long-term data sets from the Colorado Plateau, USA, to compare the effects of experimental warming (2-4°C above ambient via infrared heaters), altered precipitation (1.2mm of simulated rainfall twice per week during the summer monsoon), and physical disturbance (replicated human trampling) on biological soil crust community structure.
Surprisingly, the long-term effects of both warming and watering were similar to the effects of trampling, as all three led to increased cover by cyanobacteria dominated crusts, and decreased moss cover, with more variable effects on lichens. In addition to verifying previous reports that water additions had a rapid negative effect on moss cover, we also found a slower, but equally negative impact of warming on moss cover. Lichens had a more nuanced response to treatments, with warming and warming+watering leading to decreased lichen cover, while lichen cover in watering only treatments did not differ from control plots. Our results suggest that, over time, climate change will have impacts on biological soil crusts that are of a similar scale to those observed with physical disturbance, and that all three disturbance types will lead to a similar crust community alternate state. However, given the different rates at which these disturbances alter soil crust communities, variation in their ultimate effects on ecosystem processes are likely and require additional research.