Contrasting sensitivity to extreme winter warming events of dominant sub-Arctic heathland bryophyte and lichen species

1. Climate change in northern high latitudes is predicted to be greater in winter rather than summer, yet little is known about the effects of winter climate change on northern ecosystems. Among the unknowns are the effects of an increasing frequency of acute, short-lasting winter warming events. Such events can damage higher plants exposed to warm, then returning cold, temperatures after snow melt, and it is not known how bryophytes and lichens, which are of considerable ecological importance in high-latitude ecosystems, are affected by such warming events. However, even physiological adaptations of these cryptogams to winter environments in general are poorly understood. 2. Here we describe findings from a novel field experiment that uses heating from infrared lamps and soil warming cables to simulate acute mid-winter warming events in a sub-Arctic heath. In particular, we report the growing season responses of the dominant lichen, Peltigera aphthosa, and bryophyte, Hylocomium splendens, to warming events in three consecutive winters. 3. While summertime photosynthetic performance of P. aphthosa was unaffected by the winter warming treatments, H. splendens showed significant reductions in net photosynthetic rates and growth rates (of up to 48% and 52%, respectively). Negative effects were evident already during the summer following the first winter warming event. 4. While the lichen develops without going through critical phenological stages during which vulnerable organs are produced, the moss has a seasonal rhythm, which includes initiation of growth of young, freeze-susceptible shoot apices in the early growing season; these might be damaged by breaking of dormancy during warm winter events. 5. Synthesis. Different sensitivities of the bryophyte and lichen species were unexpected, and illustrate that very little is known about the winter ecology of bryophytes and lichens from cold biomes in general. In sharp contrast to summer warming experiments that show increased vascular plant biomass and reduced lichen biomass, these results demonstrate that acute climate events in midwinter may be readily tolerated by lichens, in contrast to previously observed sensitivity of co-occurring dwarf shrubs, suggesting winter climate change may compensate for (or even reverse) predicted lichen declines resulting from summer warming.