Photosynthetic response of the Antarctic moss Polytrichum alpestre Hoppe to low temperatures and freeze-thaw stress

The effect of low temperatures and freeze-thaw stress on photosynthetic carbon exchange in an Antarctic population of the turf-forming moss species Polytrichum alpestre Hoppe was investigated using infra-red gas analysis. Photosynthetic recovery from freezing was found to depend on the absolute depth of low temperature experienced. Repeated freeze-thaw cycles caused a greater reduction in gross photosynthesis than constant freezing over the same period of time suggesting that the freeze-thaw event itself, and not just cold temperatures, causes damage. The frequency of freeze-thaw events was significant: freeze-thaw cycles every 12 h inflicted more damage than freezethaw cycles every 24 or 48 h. Most damage occurred during the first cycle; relatively little was recorded during subsequent cycles. At +10°C, gross CO2 flux was directly proportional to moss water content between 0.3 and 3.5 g·g−1 dry mass. Moss samples with a low water content withstood freeze-thaw cycles to -5, -10 and-20°C better than samples with a high water content indicating that desiccation in the field may improve survival at low temperatures. Microclimate data for field populations of Polytrichum alpestre at Signy Island suggest that sub-zero temperatures and freeze-thaw stress may act as limiting factors on the species' distribution and viability, particularly when the insulating effect of snow cover is small.