Stress tolerance of rare and common moss species in relation to their occupied environments and asexual dispersal potential

Summary1 Species rarity in mosses may be linked to narrow physiological tolerance and habitat specificity of established colonies and gametophyte fragments (i.e. fragments of stem apices). Stress tolerance and habitat light and moisture were compared for three rare and three common moss species. 2 Observed habitat specificity was more likely to correspond to physiology in the common species. In particular, one common species, Mnium spinulosum , was limited by intolerance of high light conditions and is characteristic of deeply shaded conifer stands, while another, Bryum pseudotriquetrum , lacked desiccation tolerance and is typical of moist areas such as stream banks. However, the rare species, Mielichhoferia macrocarpa , which occurred in the darkest and wettest sites was not intolerant of high light or desiccation. 3 The relationship between desiccation tolerance and habitat moisture (based on a mesic to xeric scale) was less clear than in prior bryophyte studies, suggesting that a more rigorous quantification of habitat moisture is needed. 4 Contrary to predictions, the rare species did not have narrower physiological tolerances than the common species. The match between environment and physiology for common species probably results from their occupation of more of the potentially suitable sites. 5 There may also be selection for broader physiological tolerance in rare species resulting from their greater reliance on asexual reproduction, which might explain their relatively high desiccation tolerance both as colonies and fragments.