Climate models offer an array of predictions for changes in global hydrology that include alterations in the magnitude and timing of precipitation. Arid systems will be particularly responsive to these changes because water exerts considerable control on the physiological functioning of organisms in this biome. In the Southwestern US, changes in rainfall patterns are likely to play a large role in the growth and survival of bryophytes, a key component of desert biocrust communities. Bryophytes in this region, most commonly the moss Syntrichia caninervis, rely on hydration from individual rainfall events for physiological functioning, thus long-term survival is a function of cumulative performance over repeated events. Performance during an individual event is best characterized by the difference between carbon gained from photosynthetic fixation and carbon lost from respiration (i.e., carbon balance). We have shown that carbon balance increases as a function of rainfall amount and that less frequent events result in a reduction in total carbon gain. However, one complication to the general applicability of this assessment is that the physiological state of moss varies throughout the year, likely altering this response. The objectives of the current work were to (1) examine the effect of season on carbon balance per rainfall event, and (2) elucidate the physiological drivers of carbon acquisition potential by season.

Results/Conclusions

Under constant measurement conditions, S. caninervis exhibited the highest carbon balance per rainfall event in the fall and winter, and the lowest in the summer (P<0.05). When S. caninervis was exposed to low (1/4 average) rainfall event sizes, and long (>5 day) periods between events, summer samples entered carbon deficit, yet samples from other seasons showed net gains. By examining four parameters that can affect carbon balance during an event (maximum photosynthetic and respiratory rate, A_max, R_max; time to A_max; drying time) as a function of season and rainfall amount, we determined the physiological factors influencing the seasonal differences in carbon balance. A_max was the largest in fall and winter and explained much of the variation in carbon balance (P<0.01). Drying time, longest in the winter and shortest in the summer, only explained differences in carbon balance when interacting with rainfall amount (P<0.001), as did R_max. Time to A_max had less predictive value of seasonal variation observed and was primarily affected by rainfall amount (P<0.05). Our results reveal seasonal changes in the photosynthetic apparatus of S. caninervis that may influence carbon fixation under future precipitation scenarios.