Deserts are likely to experience some of the most dramatic and unpredictable shifts in precipitation patterns over the next century, yet we still know very little about how vegetation in these systems will respond physiologically and ecologically to such changes. In particular, changes in intra-annual precipitation parameters are likely to influence photosynthetic performance and biomass accumulation in constituents of soil biocrusts (assemblages of cyanobacteria, lichen and moss that influence soil fertility and stability in arid systems). Because of the ecological importance of biocrusts in deserts, changes in structure and function that result from altered performance of crust members could have implications for ecosystem function on large spatial and temporal scales. Using the common biocrust moss Syntrichia caninervis, we constructed a predictive model for cumulative annual carbon balance (a proxy for biomass accumulation) based on meteorological data from Southeastern Utah coupled with physiological data on moss photosynthetic response to rainfall. Our goals were: (1) to develop a model that accurately predicts both biomass accumulation and mortality in moss; and (2) to use a series of simulations to determine the influence of changes in rainfall event size, frequency, and seasonality on long-term moss performance.
Results/Conclusions
Running the simulation with past meteorological data, our model predicted cumulative carbon balance with 95% accuracy based on the range of predicted carbon balance values from analyses of moss growth rates in the field. Additionally, when we used data from a dry year (shown to cause moss biomass decline and mortality in some populations) as the input, our model correctly predicted cumulative carbon losses and 20-50% mortality after five years, consistent with field observations. Forecasting simulations that included a 15% reduction in rainfall event magnitude caused reductions in moss biomass ranging from 5-12% per year, and simulations that reduced rainfall frequency by 15% resulted in a 4-10% biomass reduction per year. These results were due to changes in seasonal carbon balance in S. caninervis, where reductions in water availability in cooler months drove long-term trends. Here we show that even small changes in precipitation patterns can have compounded, dramatic effects on long-term carbon balance and viability of S. caninervis. Because of the biogeochemical significance of biocrust mosses in arid systems, predicted changes in precipitation are likely to play a large role in desert soil stability and fertility in a future climate.