Rapid photosynthetic recovery of a snow-covered feather moss and Peltigera lichen during sub-Arctic midwinter warming

Background: Arctic lichens and mosses are covered by snow for more than half the year and are generally considered as being dormant for most of this period. However, enhanced frequency of winter warming events due to climate change can cause increased disturbance of their protective subnivean environment. Aim: To further understand cryptogamic responses to midwinter warming we compared the ecophysiological performance of one lichen and one moss species during a simulated warming event. Methods: We measured photosynthesis and dark respiration in samples of the moss Hylocomium splendens and the lichen Peltigera aphthosa removed from under snow, and on natural refreezing after the warming event, which was simulated by using infrared heaters suspended above the ground. Results: The moss exposed to light at +5 degrees C immediately after removal from their subnivean environment and from warmed plots showed positive net gas exchange within 332 s; the lichen required 1238 s. Photosynthesis and nitrogen fixation rates were equal to that, or higher than, during the preceding growing season. Upon refreezing after the event, moss photosynthesis declined considerably. Conclusions: The moss, and to a lesser extent the lichen, may contribute to subnivean midwinter ecosystem respiration, and both are opportunistic, and can take advantage of warmer winter phases for photosynthesis and growth. This ought to be taken into account in vegetation change projections of cryptogam-rich ecosystems.