PLANT ACTIVITY UNDER SNOW AND ICE, WITH PARTICULAR REFERENCE TO LICHENS

In polar ecosystems primary producers have to cope with the very limited living conditions of the harsh climate. Vascular plants in the Northern Hemisphere extend to the northern-most edges of the continents, but only two taxa are present as far south as the Antarctic Peninsula region in the Southern Hemisphere. Lower plants, lichens in particular, become more important with increasing latitudes and form the dominant element of the Antarctic vegetation. Based on recent investigations and literature, this paper discusses to what extent lichens are better adapted to snow and ice than vascular plants. Vascular plants in high latitudes have high freezing tolerances but are photosynthetically inactive in winter (e.g., evergreen coniferous species), while lichens in a highly freezing-tolerant stage can be active and productive under winter conditions. Vascular plants can be active under snow but have no photosynthesis if the tissue is frozen. Recent in situ measurements indicate that lichens are able to photosynthesize at temperatures below -10-degrees-C, apparently in the frozen state. It was also found that photosynthetic CO2 exchange of dry thalli can be activated by snow during frost. Water uptake during winter was also recorded for coniferous trees at the arctic timberline. This uptake may reduce water stress in conifers but apparently has no relevance for metabolic activity. Water uptake from snow and metabolic activity at -10-degrees-C are possible for lichens because they are able to photosynthesize at water potentials lower than -20 MPa. Although lichens are adapted to be active in snow at low temperatures, strong light on clear days may inhibit the photosynthetic apparatus.