Bryophytes are a diverse group of organisms with unique properties, yet they are severelyunderrepresented in plant trait databases. Building on the recently published European Red List ofbryophytes and previous trait compilations, we present the Bryophytes of Europe Traits (BET) dataset,including biological traits such as those related to life history, growth habit, sexual and vegetativereproduction, ecological traits such as indicator values, substrate and habitat, and bioclimatic variablesbased on the species’ European range. The dataset includes values for 65 traits and 25 bioclimaticvariables, containing more than 135 000 trait values with a completeness of 82.7% on average. Thedataset will enable future studies in bryophyte biology, ecology and conservation and may help toanswer fundamental questions in bryology.

Extinction risk is not randomly distributed among species but depends on species traits, their relationship to climate and land use, and corresponding threats by global change. While knowledge of which factors influence extinction risk is increasingly available for some taxonomic groups, this is still largely lacking for bryophytes. Here, we used random forest models to study which biological and ecological traits and bioclimatic variables are important predictors for extinction risk in European bryophytes. We hypothesized that species with a high extinction risk have a short life span, low dispersal capacities, and are more likely specialists than generalists in terms of ecological traits and bioclimate. Overall, we found bioclimatic variables to be the most important predictors for extinction risk, most notably precipitation seasonality, and related ecological traits such as continentality and elevational range. Important biological traits were plant size, life strategy and sporophyte production. In general, species living at climatic extremes and/or those with a narrow environmental range are more likely to be threatened. In addition, small-sized species and/or species with low reproductive effort and/or larger spore size are more likely to be threatened. Our findings imply that climate change may become an important driver of bryophytes extinction risk and that biological and ecological traits will be most relevant for species in coping with future threats.

Observed climate change in northern high latitudes is strongest in winter, but still relatively little is known about the effects of winter climate change on tundra ecosystems. Ongoing changes in winter climate and snow cover will change the intensity, duration and frequency of frost events. Bryophytes form a major component of northern ecosystems but their responses to winter climate changes are largely unknown. Here, we studied how changes in overall winter climate and snow regime affect frost damage in three common bryophyte taxa that differ in desiccation tolerance in a subarctic tundra ecosystem. We used a snow manipulation experiment where bryophyte cores were transplanted from just above the tree line to similar elevation (i.e. current cold climate) and lower elevation (i.e. near-future warmer climate scenario) in Abisko, Sweden. Here, we measured frost damage in shoots of Ptilidium ciliare, Hylocomium splendens and Sphagnum fuscum with the relative electrolyte leakage (REL) method, during late winter and spring in two consecutive years. We hypothesized that frost damage would be lower in a milder climate (low site) and higher under reduced snow cover and that taxa from moister habitats with assumed low desiccation tolerance would be more sensitive to lower temperature and thinner snow cover than those from drier and more exposed habitats. Contrary to our expectations, frost damage was highest at low elevation, while the effect of snow treatment differed across sites and taxa. At the high site, frost damage was reduced under snow addition in the taxon with the assumed lowest desiccation tolerance, S. fuscum. Surprisingly, frost damage increased with mean temperature in the bryophyte core of the preceding 14 days leading up to REL measurements and decreased with higher frost degree sums, that is, was highest in the milder climate at the low site.

Synthesis: Our results imply that climate warming in late winter and spring increases frost damage in bryophytes. Given the high abundance of bryophytes in tundra ecosystems, higher frost damage could alter the appearance and functioning of the tundra landscape, although the short and long-term effects on bryophyte fitness remain to be studied.