Significance of bryophytes and lichens in arctic coastal plain

Nutrient cycling
The soil under bryophyte and lichen mats tends to be cold and moist.  Lichens and mosses affect nutrient cycling of the ecosystem by intercepting aerial deposition and leaching from dripping aboveground vascular plant parts(Cowles, 1984; Rosswall and Granhall, 1980).  Lichens with cyanobacterial symbionts and bryophytes with cyanobacterial associations provide the main input of nitrogen into the ecosystem(Alexander et al., 1978; Baselier et al., 1978,; Kallio, 1975)
Arctic ecosystems receive a higher proportion of nutrients input from precipitation and nitrogen fixation than do temperate systems, because chemical weathering is inhibited by low temperature and permafrost. Mosses and lichens have a major influence on nutrient cycling in tundra and other northern ecosystems through their role in nitrogen fixation, and the ability of mccosses to aumulate and retain elements from precipitation. Retention of precipitation by bryophytes is also likely to redice losses by leaching of nutrients already existing in the soil. The general role of mosses and lichens in nitrogen fixation bases on that the cyanobacteria growing on their stems and roots help transfer the nitrogen nutrients to the mosses and lichens themselves and also to the other plants, enriching the nitrogen content of the whole vegetation. (Sarah J. Woodin & Mick Marquiss, 1997)
Bryophytes act as efficient filters of nutrients arriving in precipitation, throughfall or litter and from the soil by absorbing them directly into their tissues, or retaining them externally in solution in capillary spaces. The annual growth increment of the moss layer at an Alaskan taiga site was found to contain nutrients in excess of inputs from throughfall. The mosses, and also the lichens, help increase the nirtogen concentration in the soil. Their absorption from the soil retains large amount of phosphorous and potassium in their cytoplasm. Mosses alone account for 75% of the annual accumulation of phosphorous in an Alaskan black spruce (Picea mariana) forest. Other nutrients such as calcium and magnesium are also intensively retained in the tissues of mosses and lichens. (Sarah J. Woodin & Mick Marquiss, 1997)
Nutrient immobilization in slowly decomposing bryophyte phytomass may thus have a major influence in restricting recycling, and therefore in controlling ecosystem development and productivity. In mires, absorption of nitrogen and other elements by Sphagnum reduces availability to other plants. Bryophytes therefore may increase the pools of nutrients in the Alaskan ecosystems, but reduce availability to other organisms. (Sarah J. Woodin & Mick Marquiss, 1997)

Arctic tundra domonated by mosses and lichens
Photo: http://www.r7.fws.gov/nwr/arctic/issues1.html

Maintenance of permafrost
Mosses and lichens are important in the structure and function of the ecosystems because of their effects as insulators and filters.  Their insulating properties is partly from increased reflectance and partly from the numerous air pore space when dry. They as an effective mulch, retaining moisture in the upper layers of the soil. Mosses and their undercomposed remains are particularly efficient in thermal insulation when dry, thus restricting heat penetration into arctic soils in summer. When wet and frozen in winter, their effect in reducing heat flux away from the soil is reduced. The net effect of mosses in decreasing soil temperatures in summer is generally greater than the converse effect in winter, and over much of the Arctic the distribution of permafrost is positively correlated with that of mire vegetation underlain by mosses. Thermocarst resulting from destruction of the vegetation by the summer use of tracked vehicles during early stages of arctic oil exploration demonstrated the importance of the moss layer in maintaining permafrost, which is an important habitat for many other species naturally occuring in Alaska as well as ANWR. Destruction of such vegetation can lead to extensive melting of permafrost, both directly and by accelerating the decomposition of organic matter. (Sarah J. Woodin & Mick Marquiss, 1997)
Apart from maintaining the natural permafrost habitat, mosses, and also lichens, provide microenvironments of vital importance for invertebrates, and in some communities for the establishment of vascular plants although the relationships may be complex. Lichens release compounds capable of supressing the growth of associated vascular plants and bryophytes. Sphagnum spp. control the environment of mires by lowering pH, by releasing H+ ions in exchange for other cations, and creating waterlogged, anaerobic conditions to which only a characteristic range of other organisms is adapted. (Sarah J. Woodin & Mick Marquiss, 1997)

Reference:
1.    Sarah J. Woodin & Mick Marquiss. (1997). Ecology of Arctic Environment