Decline in biological soil crust N-fixing lichens linked to increasing summertime temperatures

Authors: FINGER-HIGGINS, REBECCA - Us Geological Survey (USGS); DUNIWAY, MICHAEL - Us Geological Survey (USGS); FICK, STEPHEN - Us Geological Survey (USGS); GEIGER, ERIKA - Us Geological Survey (USGS); Hoover, David; PFENNIGWERTH, ALIX - National Park Service; VAN SCOYOC, MATTHEW - National Park Service; BELNAP, JAYNE - Us Geological Survey (USGS)

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2022
Publication Date: 4/11/2022
Citation: Finger-Higgins, R., Duniway, M., Fick, S., Geiger, E., Hoover, D.L., Pfennigwerth, A., Van Scoyoc, M., Belnap, J. 2022. Decline in biological soil crust N-fixing lichens linked to increasing summertime temperatures. Proceedings of the National Academy of Sciences (PNAS). 119(16). Article e2120975119. https://doi.org/10.1073/pnas.2120975119.
DOI: https://doi.org/10.1073/pnas.2120975119

Interpretive Summary: Across many global drylands, biocrusts form a protective barrier on the soil surface and serve many critical roles in these harsh yet fragile environments. Previous short-term research suggests that climate change and invasive plant introduction can damage and alter biocrust communities, yet few long-term observations exist. Using a globally unique long-term record of continuous biocrust surveys from a rare never-grazed and protected grassland on the Colorado Plateau, USA, we found lichen species diversity and cover to be negatively correlated to increasing temperatures, while moss species showed more sensitivity to variation in precipitation and invasive grass cover. This portends less N for these ecosystems. These results suggest that dryland systems may be at a critical threshold where ongoing warming is resulting in soil crust degradation.

Technical Abstract: 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 thought 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 temperate dryland with initial observations dating back to 1967, we examine the effects of 53 years of observed environmental variation and Bromus tectorum invasion on biocrust communities in a grassland never grazed by domestic livestock. Annual observations show a steep decline in the N-fixing cyanolichen cover (dominated by Collema species) from 1996-2002 during a period of extended drought, with Collema communities never able to recover to pre-2000 levels. Declines in chlorolichen species were also observed, both in the number of species present and by total cover, which was attributed to increasing summertime temperatures. Conversely, moss species 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 suggests climate change is a key driver in changes to late-successional biocrust communities. Importantly, results here suggest that warming can partially negate decades of protection from disturbance with biocrusts communities reaching a vital tipping point. The accelerated rate of ongoing warming captured in this study has already resulted in the loss of biocrust lichen cover and diversity, which could have long-term implications for temperate dryland ecosystems globally.