A comparative study of lichen and bryophyte communities on sandstone and ultramafic bedrocks along a maritime gradient in central California

Posted on by

Prepared by Michael Mulroy and Nishanta Rajakaruna

Left: quadrat from a coastal sandstone rock outcrop in San Luis Obispo County. We found 14 lichen species, including the yellow cobblestone lichen, Acarospora socialis. Right: the rock nipple lichen, Thelomma mammosum, grows among mosses and other lichens on a coastal serpentinite outcrop in San Luis Obispo County. The hand lens diameter is approximately 1.4 centimeters. Photo credit: Michael Mulroy

Lichens and bryophytes may be small, but they are among the dominant life forms inhabiting rock outcrops. These exposed surfaces are typically too harsh for most vascular plants, such as trees, shrubs, ferns, grasses, and forbs, yet they support remarkably diverse and specialized assemblages of lichens and bryophytes.

Our study set out to explore how these communities vary among rock outcrops and what environmental factors shape their composition. We focused on sandstone and serpentinite, two contrasting rock types, distributed along a 70-kilometer gradient from the foggy, thermally stable California coastline to the drier, more thermally variable interior. This design allowed us to capture how both geology and climate influence lichen and bryophyte communities.

We sampled 16 outcrops (eight sandstone, eight serpentinite) using 20 × 20 cm quadrats. Species were identified in the field with hand lenses and later confirmed in the lab. Across 128 quadrats, we recorded 132 lichen species and seven moss species. Our results showed clear ecological patterns: sandstone and serpentinite communities were distinct, and both varied substantially across the coastal–inland gradient. Interestingly, the contrast between sandstone and serpentinite communities was greatest along the coast, while inland, the two rock types hosted more similar communities.

The first author adopting a variety of poses to detect cryptic lichens and bryophytes on the rock surface. Photo credit: Jason Dart

One particularly unexpected finding was the higher abundance and richness of cyanolichens on serpentinite. Unlike most lichens, cyanolichens contain nitrogen-fixing cyanobacteria, allowing them to capture atmospheric nitrogen and enrich terrestrial ecosystems. Our data suggest that cyanolichens favor cracks, ledges, and overhangs—features more common on serpentinite—because they help channel and retain the liquid water they need to photosynthesize. Together, these results highlight how both rock type and climate interact to shape specialized lichen and bryophyte communities. They also suggest the potential role of serpentinite outcrops as reservoirs of nitrogen-fixing lichens, small organisms with potentially significant roles in ecosystem functioning. More broadly, our findings help explain how rock outcrop communities respond to environmental variation and highlight the remarkable biodiversity contained within them. Understanding these ecological patterns is an important step toward conserving rock outcrop communities and safeguarding a major source of biodiversity across the landscape.

This is a plain language summary of the paper of Mulroy et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.70072)