Significance of bryophytes and lichens in arctic Alaskan vegetation

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Brief Introduction

Seeing from the map in the previous page, the main distribution of vegetation in the Arctic National Wildlife Refuge is characterized as having mainly herbaceous classes of plants, which include sedges, mosses and few shrubs. In this region, as well as the whole Alaskan arctic region, bryophytes (commonly known as "mosses") and lichens are shown to be important in the tundra communities in terms of their cover, production and phytomass. They are generally not freely consumed by herbivores, though they are the main producers in the tundra region; many mosses are slow to decompose, resulting in high phytomass to production ratios indicative of slow turnover. The accumulating bryophyte phytomass has high thermal insulating, water-holding and cation exchange capacities. Thus, it exerts a powerful influence on soil temperature and water regimes and on nutrient cycling, and it forms a carbon sink of significance in terms of global warming.

The altitude that ANWR is located mroe or less belongs to the the region of "mild-polar" region, in which the summer is warmer than the cool-polar region with conditions less arid, but the winter is still severe. In this region, there is extensive grassheath, dwarf-shrub heath, mire and other closed phanerogamic vegetation. Mosses and lichens are extensively abundant in this region. Further south there are taller shrubs too co-existing with the bryophyte vegetation. A few examples are as follows:

Bryophytes:        Aulacomnium, Sphagnum,Tomenthypnum spp.
Lichens:             Cladonia, Peltigera, Cetraria spp.
Shrubs:               Alnus, Salix spp.

Mosses contribute significantly to above-ground production and phytomass in the region. In the extensive, relatively dry tussock tundra in Alaska, the production of mosses is around 70gm-2 but in the wetter areas, as near the coastal part of ANWR, the production can be as high as 164gm-2, because moss production generally increases with water availability and marine shore is usually more plentiful in nutrients. Lichens are generally less productive than mosses, but they contribute substantial phytomass in lichen heaths and woodlands.

Factors for their success - an analysis on their physiology


There are several factors responsibles for the success of mosses and lichens in Alaskan region. The two groups are remarkably similar in attributes beneficial in severe environments. Both tend to occupy and create relatively favourable microenvironments. Many species exhibit a broad response of net assimilation rate to temperature, with maxima at 10-15oC but with positive net assimilation and dark respiration continuing at or below 0oC. Light compensation and saturation intensities are typically lower in mosses and lichens than in vascular plants, and compensation levels decrease with temperature permitting positive net assimilation under cool, low-light conditions.

Moreover, most species have little access to soil moisture and lack an effective cuticle; this enables them to absorb water through much of their surface but results in rapid water loss under drying conditions. Yet, though the plants become inactive when dry, they resume normal metabolism rapidly on remoistening. Thus, mosses and lichens are thus adapted to switching rapidly between periods of metabolic activity and rest, utilizing favourable conditions whenever they occur. This may be facilitated by micromorphological features thought to facilitate simultaneous uptake by moss leaves of both water and carbon dioxide. They also resist frost very well by conferring tolerance of cytoplasmic dehydration resulting from extracellullar ice formation.

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.

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.

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.

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.

Nutrient cycling

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.

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.

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.

Maintenance of permafrost - in relation to oil exploration

A layer of moss or lichen acts 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.

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.


The above information is summarized and quoted mostly from Ecology of Arctimc Environent (Edited by Sarah J. Woodin & Mick Marquiss).