Physiological Ecology of Dryland Biocrust Mosses

Abstract

Soil biocrusts are assemblages of cyanobacteria, lichens, and mosses ubiquitous to arid and semi-arid (dryland) systems that offer an array of ecosystem services. Soil crust mosses are taxonomically diverse, account for up to 30 % of crust cover, and offer large contributions to crust biogeochemical functionality, yet remain the least understood component of the community. Because of selective pressures of their growth environment, such species are highly desiccation tolerant, with the ability to withstand the loss of most cellular water for extended periods of time, during which metabolism is suspended. Biocrust mosses can also tolerate larger ranges of temperature, light, and cellular water content than mesic species, yet still remain sensitive to certain aspects of environmental alteration. For one, changes in precipitation regime are likely to heavily influence survival in dryland mosses. Rainfall, occurring as discrete periods of hydration in dryland systems, causes mosses to undergo wet-dry cycles that result in either a positive or a negative carbon balance. Carbon balance can be used as a measure of performance during individual rainfall events, and is a metric for long-term viability. Recent work suggests rainfall event magnitude plays a large role in carbon balance, as does the frequency and seasonality with which events fall. Biocrust mosses are stimulated by elevated CO2, yet may not acclimate photosynthetically to long-term enrichment. Interestingly, elevated CO2 may favor stress tolerance at the expense of growth in biocrust moss, particularly at high temperatures. Finally, despite low annual growth rates, nitrogen appears to place physiological limitations on reproductive biology of biocrust mosses. High levels of nitrogen deposition, however, have been shown to cause toxicity, competitive exclusion by vascular plants, and can reduce cyanosymbioses.