The effects of winter stress on Sphagnum species with contrasting macro- and microdistributions

Abstract

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.

Sub‐arctic mosses and lichens show idiosyncratic responses to combinations of winter heatwaves, freezing and nitrogen deposition

“…Fen bryophytes have shown negative response in Fv/Fm to snow removal (Kuttim et al, 2019), which is in contrast to our results and indicative of habitat-specific responses to winter climate change as also observed among Sphagnum species (Campbell & Rydin, 2019).…”

“…The potential activity of summer photosystem II (Fv/Fm) was enhanced for many of the tested species to various extreme event treatments indicating that this part of the photosynthetic apparatus appears to benefit, and may influence cryptogam capability to cope with extreme winter climatic events. Fen bryophytes have shown negative response in Fv/Fm to snow removal (Kuttim et al, 2019), which is in contrast to our results and indicative of habitat‐specific responses to winter climate change as also observed among Sphagnum species (Campbell & Rydin, 2019). Hydration status greatly affects seasonal variation in moss CO 2 flux rates with largest variation among hydric mosses (Davey & Rothery, 1996) which is in line with the contrasting response between Hylocomium versus Polytrichum and Racomitrium .…”

“…This ability allows for great physiological flexibility and their survival in high polar deserts (Kappen, 2000; Perera‐Castro et al, 2021; Sancho et al, 2019). Moss vulnerability to freezing temperatures differs between species and, in part, depends on whether species grow in sites that are buffered to the most extreme low temperatures (Campbell & Rydin, 2019; Kuttim et al, 2019; Yin & Zhang, 2016), show physiological adaptations (Rütten & Santarius, 1992; Zhang & Zhang, 2020; Zuñiga‐González et al, 2016) and the response may depend on frost and snow combinations (Yin et al, 2021). Therefore, we can expect that mosses, typically growing in habitats with deep snow cover, will be more vulnerable to winter heatwaves and freezing temperatures than those growing on exposed ridges with shallow snow depth.…”

Climatic drivers of Sphagnum species distributions

“…In the north, seasonal variation is caused by low temperatures during winter and late autumn (before snow cover gives protection). Comparing species occupying similar micro-habitats, northern species in general tolerate such conditions better than their southern counterparts (Campbell and Rydin 2019).…”

“…Even though Sphagnum species have effective long-distance dispersal (Sundberg 2010(Sundberg , 2013, they vary in their distribution in Europe from south to north and from oceanic to continental areas. Experiments have indicated that northern species of Sphagnum tolerate cold autumns and winters better than species with more southern distribution (Campbell and Rydin 2019). There is also a large variation in drought resistance and growth response to temperature among Sphagnum species (Breeuwer et al 2008, Rydin andJeglum 2013).…”