Climate change in the form of increased temperature, elevated CO2, and changes in global hydrology is likely to have global ecological ramifications. Arid and semi arid regions are though to be particularly sensitive to these changes because they are comprised of highly adapted species that are often operating at their physiological limits. Due to the delicate hydrological balance of arid regions, shifts in precipitation regime (including changes to rainfall frequency and intensity) may play a large role in determining desert plant growth, distribution, and survival under future climate scenarios. In arid regions of the Southwest USA, some of the most ecologically important organisms are located in biological soil crusts (BSCs), soil-surface communities consisting of cyanobacteria, lichens, and moss. BSCs increase water-holding capacity of soil, prevent erosion, aid in seedling establishment, and often are the primary nitrogen source for plants. Ascertaining the vulnerability and response to BSC organisms to potential changes to precipitation regime is therefore essential in predicting arid ecosystem response to global change. Crust moss may be particularly sensitive to changes in water availability since they are dependent on hydration for C acquisition and growth, and because the rehydration phase of each wet-dry cycle is associated with significant respiratory losses. The present study sought to 1) examine the physiological response (in terms of net carbon gain or loss) of the common BSC moss Syntrichia caninervis to an array of rainfall event sizes; then 2) determine the degree to which rainfall frequency affects C gain per event. 
Results/Conclusions

Rainfall size had a significant effect on net C gain (P<0.05). Specifically, moss receiving 1/4 average rainfall amount suffered net loss of C per event because respiratory losses during the rehydration phase were not compensated for during the wet, photosynthetically active phase. In all other precipitation amounts (up to 4x average), moss achieve a net positive C balance, though this affect appeared to plateau when excptionally large rainfall amounts may have caused waterlogging. The length of dry time prior to an event only marginally affected total C gain per event, with the most profound effects occurring only at the shorter end of the dry time scale. These data suggest that future climate scenarios involving smaller, high frequency events may cause moss decline due to a progressively negative seasonal C balance, and larger events interspersed by drought may be more favorable.