The potential effects of changing climate on biocrusts

Background/Question/Methods

Dryland ecosystems comprise approximately 40% of all terrestrial surfaces and are Earth’s fastest growing landscapes.  These vast ecosystems are predicted to be disproportionately susceptible to changes in climate, especially precipitation.  Biological soil crusts (biocrusts) – soil communities composed of lichens, mosses and cyanobacteria – are a fundamental component of dryland ecosystems and contributing significantly to carbon and nitrogen inputs, soil stability, water infiltration and vascular plant recruitment.  Biocrusts can be the dominant living cover in dryland ecosystems and often cover the large interspaces between plants.  Nevertheless, we know surprisingly little about how biocrust community composition and function will respond to global change.  To enhance this understanding, we conducted numerous studies throughout the Southwest USA to investigate the effect of increased atmospheric CO₂ concentrations, increased temperature, altered precipitation patterns, changes in land use, and invasion of exotic grass species on biocrusts.  In particular, we were interested in the effects these changes would have on biocrust cover and species composition, effective quantum yield (a proxy for photosynthetic capacity) and pigment concentrations (a measurement of cyanobacteria biomass and metabolic activity).

Results/Conclusions

Our initial findings suggest that precipitation consistently had the greatest effect on biocrust cover, yield and pigment concentration.  Declines in total precipitation resulted in a significant decrease in cover and photosynthetic yield.  Long term exposure to elevated CO₂ (at the desert FACE experiment) did not mitigate water stress, as biocrust cover and yield decreased in response to drought to the same extent regardless of CO₂ treatment.  We observed that an increase in small frequent precipitation events had a drastic impact on biocrust species composition: while cyanobacteria cover was not negatively affected, moss cover was completely eliminated.  This suggests that moss cover could respond rapidly and dramatically to altered precipitation patterns and that, when event totals are below a certain threshold, increased precipitation could actually result in moss mortality.  Interestingly, an increase in temperature had no observable impact on biocrust composition or cover.  Biocrust cover and composition was significantly correlated with both land use and ecosystem type.  In particular, summertime grazing and exotic grass species reduced moss and lichen cover, while increasing cyanobacteria cover.  Taken together, these findings suggest that changes in precipitation totals and frequency, changes in land use, and exotic plant invasion all significantly affect biocrusts in dynamic ways.