Why is the rainforest lichen Methuselah’s beard (Usnea longissima) so rare in British Columbia’s inland temperate rainforest?

Abstract

Coastal (CTR) and inland temperate rainforests (ITR) in western North America share a rich oceanic lichen flora. The distinctive Methuselah’s beard lichen (Usnea longissima Ach.) is an exception to this pattern of shared distributions, with very few ITR locations. Does this absence reflect dispersal limitations or climatic intolerance? To answer this question, we transplanted U. longissima thalli from the CTR to three ITR locations, assessing growth rates against reciprocal CTR transplants. Canopy microclimate measurements provided concurrent data on growth conditions. Growth rate responses (length, mass, and area) were evaluated after summer and full-year transplants. Notwithstanding extended drought conditions during the summer period, annual transplants at two of the three ITR locations supported growth rates comparable to those at the CTR source U. longissima population, with summer dewfall and autumn rains being major ITR hydration sources. Thalli transplanted to a third ITR site (summer measurements only) in a location transitional to the drier interior plateau lost both mass and length. Based on these findings, we suggest that the absence of U. longissima from much of the ITR reflects the combined influence of dispersal limitations and requirements for stands with long site continuity and topographically induced summer wetting of thalli by dewfall.