Significance of bryophytes and lichens in arctic coastal plain


A mushroom living in a dream world of mosses
Photo: Matt Neuburg


Basis of food chain and energy source

The mosses and lichens are less freely consumed by the herbivores in the whole arctic region than the other angiosperms. Reason for low consumption on mosses and lichens may be that they are not very digestible due to a high crude fibre content resulting in part from lignin-like phenolic compounds in the cell walls. Species that would feed on mosses include many arthropods with sucking mouthparts, such as mites, and also tardigrades, dipteran and lepidopteran larvae. It is also suspected that animals such as geese may benefit from arachidonic acid which is present in mosses. This highly unsaturated fatty acid could increase limb mobility at low temperature and protect cell membranes against cold. (Sarah J. Woodin & Mick Marquiss, 1997)

Lichens are well known to be crucial for caribou and reindeer (Rangifer tarandus), and thus to indigenous human populations relying on these mammals. Graminoids, willow leaves and forbs are preferred by these animals in the summer, but lichens often represent some 60-70% of winter food. Generally, an adult reindeer requires up to 5kg dry weight of lichen daily in winter and grazing about 2000kg/m-2 in six months. Lichens are rich in carbohydrate, are readily digested by caribou though not by other mammals for the reasons stated above, and form an effective and available energy source in winter when animals need intensive metabolism to generate body heat. Yet, they are deficient in protein, lipids and several essential mineral elements. How then do the animals cope with this? The animals smartly utilize fat reserves and break down muscle, with replenishment through feeding on nutritious young angiosperm leaves in spring. (Sarah J. Woodin & Mick Marquiss, 1997)

Mosses form a minor part of the diet of several arctic rodents, and are more freely consumed by Lemmus spp.. In peak years, consumption by lemmings (L. sibericus) in Alaska may reach 25% of above-ground primary production, with mosses forming 5-20% of the diet in summer and 30-40% in winter. Though mosses are not readily disgestible by lemmings, they provide good source of unsaturated fatty acids and minerals such as calcium, magnesium and iron. (Sarah J. Woodin & Mick Marquiss, 1997)


General pattern of photosynthesis and respiration

“There is a great deal of similarity in the carbon dioxide exchange patterns of bryophytes and lichens (Longton, 1992). Both have simple structures and little control over water loss.  They are metabolically active when wet; short respiratory burst is sometimes experience  when they are demoistened after desiccation during which internal activity Is quickly resumed.  As they dry out, they first increase their net CO2 exchange rate, reach a maximum net cO2 exchange rate at intermediate water contents, and then with further drying they decrease their activity, which eventually ceases.  Any further change in moisture will affect their distribution patterns and hence the cryptogamic species composition of arctic vegetation.  Arctic bryophytes show lower rates of photosynthesis under ideal short-term conditions than do their subarctic, temperate, and tropical conspecifics (Oechel and Sveinbjornsson, 1978; Sveinbjornsson, 1980), whereas the present authors have found similar rates in congeneric arctic and tropical lichens (unpublished data) and Lechowicz (1982) found no latitudinal differences in maximum photosynthetic rates among lichen species. Arctic bryophytes and lichens exhibit distinct temperature optima, which are lower than those of temperate conspecifics and congenerics (Lechowicz, 1982; Sveinbjornsson and Oechel, 1980), and they are able to photosynthesize at lower temperatures than can temperate ones.” (Sarah J. Woodin & Mick Marquiss, 1997)


Decomposition - carbon sink that mitigates global warming

Slow decomposition of mosses allows the mosses to contribute significantly to the Arctic carbon sink. By photosynthesis, they "fix" carbon from the atmosphere to organic compounds and by slow decomposition they help trap the carbon instead quickly releasing them back to the atmosphere. It is estimated that northern peatlands contain 120,000x106 tonnes of carbon, equivalent to 24 years' emission from fossil fuels at the present rate, and more than 50% of emissions since 1860, with half derived from Sphagnum and a further component from other mosses. Annual carbon fixation by slowly decomposing mosses in peatlands and boreal forests was estimated as 6.5% of current emissions from fossil fuels. Therefore, mosses in the Arctic region serves as a buffer for alleviating global warming. Considering that the mosses in many regions of Alaska has already been disturbed by urban development and oil exploitation, the value of mosses in ANWR arouses much concern at the present. (Sarah J. Woodin & Mick Marquiss, 1997)


Reference:
1.    Sarah J. Woodin & Mick Marquiss. (1997). Ecology of Arctic Environment, under the article: Longton, Royce E. (1997). The role of bryophytes and lichens in polar ecosystems