Juha M. Alatalo, Annika Jägerbrand, Mohammad Bagher Erfanian, Shengbin Chen, Shou-Qin Sun, Ulf Molau

Background and Aims: Climate change is expected to affect alpine and Arctic tundra communities. Most previous long-term studies have focused on impacts on vascular plants, but this study examined potential impacts of long-term warming on bryophyte communities.
Methods: Experimental warming with open-top chambers (OTCs) was applied for 18 years to a mesic meadow and a dry heath alpine plant community. Species abundance was measured in 1995, 1999, 2001 and 2013.
Key results: Species composition changed significantly from the original communities in the heath, but remained similar in the mesic meadow. Experimental warming increased beta diversity in the heath community. Bryophyte cover and species richness both declined with long-term warming, while Simpson diversity showed no significant responses. Over the 18-year period, bryophyte cover in warmed plots decreased from 43% to 11% in heath and from 68% to 35% in meadow (75% and 48% decline, respectively, in original cover), while richness declined by 39% and 26%, respectively. The decline in both cover and richness first emerged after seven years. Warming caused a significant increase in litter in both plant communities. Litter cover had a negative impact on bryophyte cover in both communities.
Conclusions: This study showed that bryophyte species do not all respond similarly to climate change. Total bryophyte cover declined in both dry heath and mesic meadow communities under experimental long-term warming (by 1.5-3°C), driven by general declines in many species. Principal response curve, cover and richness results suggested that bryophytes in alpine heath vegetation are more susceptible to warming than those in meadow vegetation, supporting the suggestion that bryophyte communities may be less resistant in drier environments than in wetter habitats. Species loss was slower than the general decline in bryophyte abundance, and diversity remained similar in both communities. Increased litter cover led to a steep decline in bryophyte cover.