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).