Peat mosses (genus Sphagnum) dominate most Northern mires and show distinct distributional limits in Europe despite having efficient dispersal and few dispersal barriers. This pattern indicates that Sphagnum species distributions are strongly linked to climate. Sphagnum-dominated mires have been the largest terrestrial carbon sinks in Europe over the last few millennia. Understanding the climatic drivers of Sphagnum species distributions is important for predicting the future Climatic drivers of Sphagnum species distributions 2 functionality of peatlands. We used MaxEnt, with biologically relevant climatic variables, to model and clarify the current distributions of 45 Sphagnum species in Europe. We used a dataset of 238 316 records from across Europe (30° to 90° N, -30° to 63° E; Sahara to the Arctic, Azores to Ural mountains). We used annual degree-days, annual water balance and their monthly standard deviations (i.e. seasonality) as climatic predictors over a range of spatial resolutions (from 10 to 200 km pixel size). With these climatic predictors, we produced reasonably accurate projections of the distribution of 45 species (overall AUC >0.8). Large pixels (100 and 200 km) resulted in loss of detail, but smaller pixels (10-50 km) performed well across fit measurements. Projected distributions at the 50 × 50 km resolution showed the largest resemblance to published distribution maps. Suitable climate for many Sphagnum species was associated with the northern, western and mountainous parts of Europe. We found that annual water balance was an important indicator of Sphagnum presence. Limits in relation to annual water balance were the same as reported by bioclimatic peatland models from North America. Most Sphagnum species were limited to annual degree-days between -5000 °C y -1 and 5000 °C y -1 . Seasonality in both climate variables separated species, with degree-day seasonality having a stronger influence than water balance seasonality.High degree-day seasonality as a consequence of cold temperature sets a northern distribution limit to some species. The results suggest that the future of Sphagnum diversity in Europe is most strongly dependent on changes in water availability and in seasonal temperature variation. Highlights• Peat mosses (Sphagnum) form northern peatlands and species have different distributions across Europe.• We model the climatic suitability for all European species using multiple databases and MaxEnt models.• The climatic suitability for most species can be accurately modelled with mean annual temperature and water balance and their variation over the year.• Sphagnum has its highest species richness in northwestern Europe.• The magnitude of temperature fluctuations over the year is an important climatic variable that separates current species distributions.

Introduction. Sphagnum L. forms much of the ground cover in northern peatlands. Different species show affinities for bioclimatic regions in Europe (oceanic/continental; northern/ southern) and species-specific tolerance of winter conditions can be a factor explaining their distribution. Methods. We focussed on low temperature in a series of experiments and tested (1) the innate ability of a selection of Sphagnum species to tolerate low temperature in relation to their microtopographic (wetness) and geographical (climate) distribution; (2) the rate of cold tolerance acquisition; and (3) the ability of species to survive a range of low temperature once cold hardened. Key results. Our experiments showed that maximal PSII efficiency (F v /F m , chlorophyll fluorescence), growth rates and survival were all negatively affected by sub-zero temperatures. Environmental conditions associated with the onset of winter (colder nights and shorter days) triggered the acquisition of cold tolerance in Sphagnum. Conclusions. The results were not unequivocal, but species associated with colder climates were generally more tolerant of sub-zero conditions. Species associated with the wettest and driest ends of the wetness gradient were more consistent in their responses than those in between, with wetter-dwelling species being less sensitive to sub-zero temperature than species found in drier microhabitats. Overall, our results suggest that adaptation to winter conditions contribute to the current distribution patterns of Sphagnum species.

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