Comparative ecology of a lowland and a subalpine species of Mnium in the northern Rocky Mountains

Factors that set the altitudinal limits of plants have been relatively well explored for many land plant groups, but not for bryophytes. Bryophytes typically represent a significant portion of alpine floras with many species specific to highland systems. Differences between highland and lowland bryophytes have been underinvestigated. In the present study spanning three growing seasons, a subalpine and a lowland moss were both reciprocally planted as apical fragments and transplanted as adults between sites at 1400 m and 2000 m in the Front Ranges of the Rocky Mountains, Alberta. The lowland species, Mnium spinulosum, was less tolerant of conditions at 2000 m than the subalpine species, M. arizonicum, was to conditions at 1400 m. In particular, M. spinulosum had lower establishment from both apical fragments and spores at higher elevation sites. Both species had significantly lower establishment during the abnormally cold growing season of 1999, but fragments of M. arizonicum were better able to adjust their investment in establishment. The effect of a dominant feather moss, Hylocomium splendens, on establishment and transplant health was tested for M. arizonicum. Establishment of M. arizonicum was lower in Hylocomium mats than on bare humus regardless of site elevation suggesting allelopathy; however, stem survival in adult transplants was higher in Hylocomium mats than in Mnium dominated microsites at the higher elevation suggesting facilitation. Competition, rather than a lack of physiological plasticity, probably determines the lower elevation limit of the subalpine moss, while poor establishment ability at low temperatures accounts for the upper elevation limit of the montane moss.