Declining biocrust lichen cover suggests warming tipping point has already been met in a protected grassland in Canyonlands National Park, Utah

Rebecca Finger Higgens, Michael Duniway, Erika Geiger, Stephen Fick, David L. Hoover and Jayne Belnap, Southwest Biological Science Center, U.S. Geological Survey, Moab, UT, David L. Hoover, Rangeland Resources and Systems Research Unit, USDA-ARS, Fort Collins, CO, Alix A. Pfennigwerth, Great Smoky Mountains National Park, National Park Service, Gatlinburg, TN

1)Background/Question/Methods
Biological soil crusts (biocrusts), comprised of mosses, lichens, and cyanobacteria, are key components to many dryland systems globally. Climate change and other anthropogenic disturbances are assumed to cause a decline in late-successional biocrusts cover; yet, few long-term studies exist to track potential shifts in these sensitive soil-surface communities. Using a globally unique long-term observational dataset from a grassland on the Colorado Plateau located within Canyonlands National Park, we examine 23+ years of observed environmental variation and Bromus tectorum invasion on biocrust communities in an area free from wildfire and never grazed by livestock. Study plots were established using a factorial design within two dominant bunchgrass types, Hilaria jamesii and Stipa spps., both with and without Bromus tectorum invasion (n=3 for each cover type for a total of 12 plots). Using local weather data, we explored potential climate drivers as predictors for biological soil crust cover variation and explored potential ecological thresholds using breakpoint analysis. Additionally, for the Bromus tectorum invaded plots, we evaluated if invasion had longer-term effects on subsequent biocrust cover dynamics.
2) Results/Conclusions
Analysis of long-term climate records indicate that mean annual temperatures at our study site have steadily increased from 1966-2019, at a rate of approximately 0.27 °C (±0.05 °C) per decade (Fig. 1; τ=0.388, p<0.001). Concurrent with warming trends, annual observations of biocrusts show a steep decline in N-fixing cyanolichen cover (dominated by Collema species) between 1996 and 2002, with Collema communities never able to recover to pre-2000 levels. Declines in chlorolichen cover were also observed, both in the number of species present and by total cover, which was attributed to increasing summertime temperatures. Conversely, mosses gradually gained in cover over the survey years, especially following a large Bromus tectorum invasion at the study onset (ca. 1996-2001). These results support a growing body of studies that suggest climate change to be a key driver in changes to late-successional biocrust communities. Importantly, results here suggest that warming can partially negate decades of protection from anthropogenic disturbance. The accelerated rate of ongoing warming occurring across the Colorado Plateau captured in this 23-year study has already resulted in the loss of biocrust lichen cover and diversity, which could have long-term impacts for this large dryland ecosystem.