Estimate that for 90% or more of my encounters with Ochrolechia upsaliensis it has been on Selaginella wallacei. Wonder what characteristic of the Selaginella is so attractive to O. upsaliensis. (Posted some photos from diverse locations I’ve visited where they are common - within Hells Canyon in ID, near the Columbia River in north central WA, Washington Park on Fidalgo Island WA, and high on Mt Townsend WA.) The only description of O. upsaliensis I find that mentions Selaginella is the one in McCune’s Miscellaneous Keys, along with soil, moss and plant debris. Looking at scattered collection locations on the CNALH interactive map the most common substrate for O. upsaliensis seems to be moss, with the rest including soil, humus, rock, Selaginella, bryophytes, and (my favorite) vegetabilia. About half of the O. upsaliensis photos online at Sharnoff Photos and Ways of Enlichenment have S. wallacei - all of these are from WA. Don’t see it on photos from other sites I looked at, e.g. The Lichen Herbarium, University of Olso. Anyway, this just piqued my interest.