THE EFFECT OF WINTER FIELD CONDITIONS ON THE DISTRIBUTION OF TWO SPECIES OF UMBILICARIA

Abstract

SUMMARYUmbilicaria vellea (L.) Ach. and U. deusta (L.) Baum. have distribution patterns that are mutually exclusive. U. vellea occurs in vertically inclined microsites that are permanently snow-free. In contrast, U. deusta occurs on flat or gently sloping surfaces that receive a substantial snow cover. Earlier field work had suggested that this distribution pattern was maintained, at least in part, by the inability of U. vellea to store photosynthetic products and thereby maintain a positive carbon balance when growing in areas receiving a significant snow-cover. We report controlled laboratory experiments conducted in order to determine which aspect of the subnivean environment interfered with U. vellea, but not with U. deusta. The four experiments investigated effects of long-term continuous suhzero temperatures, alternating periods of freeze and thaw, exposure to slightly above-zero conditions (in the dark), and lastly, a combination of freezethaw and slightly-above-zero treatments.Only the slightly above-zero treatment in the dark interfered significantly with CO2 exchange in U. vellea. None of the treatments reduced significantly COj exchange in U. deusta. The decline in productivity of U. vetlea in the current experiments was identical to that found in material reciprocally transplanted in the field.We conclude that U. vetlea is excluded from snow-covered habitats because of an inability to store or mobilize stored respiratory substrate in the subnivean environment. In contrast, U. deusta which stores such products in great number, is able to survive long periods of time without a flux of photosynthates. It would appear that one of the costs of such an ability, is the allocation of very few reserves to growth.

Field Measurements of Photosynthesis of Umbilicarious Lichens in Winter

“…Acta 109(1996) 292 -298 © Georg Thieme Verlag Stuttgart· New York Lichens are able to withstand long-term continuous freezing and alternating periods of freezing and thawing (e.g. Scott and Larson, 1986). Lichens also have the capacity of photosynthesizing and respiring at temperatures below DOC (see Kappen et al, 1995).…”

Ice encapsulation protects rather than disturbs the freezing lichen

“…Previous studies have shown that many lichen species have an extremely high freezing tolerance when air-dry, being unaffected after exposure to temperatures as low as )196°C (Kappen & Lange 1972;Larson 1978). However, when hydrated, lichens are more susceptible to freezing, which is probably due to ice formation within cells (Kappen & Lange 1972;Larson 1982;Scott & Larson 1986;Nash et al 1987;Benedict 1990). Nevertheless, like plants, lichens do have mechanisms to avoid intracellular ice crystallisation; extracellular ice nucleation removes most liquid water before damaging intracellular ice formation processes are initiated (Burke et al 1976;Ashworth & Kieft 1992;Schroeter & Scheideg-Fig.…”

“…Previous studies have shown that many lichen species have an extremely high freezing tolerance when air‐dry, being unaffected after exposure to temperatures as low as −196 °C (Kappen & Lange 1972; Larson 1978). However, when hydrated, lichens are more susceptible to freezing, which is probably due to ice formation within cells (Kappen & Lange 1972; Larson 1982; Scott & Larson 1986; Nash et al. 1987; Benedict 1990).…”

“…Scott & Larson (1986) froze both air‐dry and moist thalli of two saxicolous species of Umbilicaria at −25 °C. The two species showed no negative effects to freezing.…”

“…Apparently, lichens should either be completely dry or very humid when freezing; otherwise, freezing can be damaging. Scott & Larson (1986) froze both air-dry and moist thalli of two saxicolous species of Umbilicaria at )25°C. The two species showed no negative effects to freezing.…”