Encyclopedia of the Arctic

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Sedge meadows

Sedge meadows are among the most distinctive and characteristic of tundra plant communities, appearing in imperfectly drained lowlands as dense green swards or lawns dominated by a variety of grass-like sedges and cottongrasses. Sedge meadows are wetlands related structurally, functionally, and botanically to the open peatlands or mires of the northern boreal forest. Although they occur across some 40 degrees of latitude, and are circumpolar in distribution, sedge meadows and their associated soils become considerably less extensive as one moves from the Low Arctic to the High Arctic. For example, sedge-dominated wetlands are restricted to less than about 3-5% of all the lands north of the treeline in Canada. They tend to be floristically impoverished habitats with only a few species of mosses, sedges, and dwarf shrubs providing the bulk of the primary production above and below ground. Total standing crop (1400 to 3700 g m¯²) and net production (100 to >300 g m¯²) is similar between High and Low Arctic meadows and tends to vary little among years.

The nomenclature for mires and wetlands differs somewhat between North America and Eurasia and can seem complex to the non-expert. However, mires are generally divided into homogeneous and polygonal types. Homogeneous mires are mostly situated in valleys, near lakes, pools, and along river banks. Ice-wedge polygons may have low-center or high-center morphologies, which correspond to different stages of development of the polygons. (Ice-wedge polygons formed from contraction and expansion of permafrost begin with ridges either side of the ice-filled crack containing a central pool- low center polygons- but as vegetation fills the central pool, the center becomes higher and open water becomes restricted to a narrow channel around each polygon.) Polygonal mires form only where the mean annual temperature is below -1°C. Frost action in the surface layer of these mires forces coarse-grained soil to the edge of the polygon while fine-grained soil remains in the center. Technically, low-center lowland polygons are considered fens whereas high-center lowland polygons are classified as bogs. In either case the majority of sedges, in terms of both species and individuals, are derived from two genera, Carex and Eriophorum. Each genus is rich in species, even though local diversity within a given sedge meadow is generally restricted to a half-dozen or less different taxa.

Arctic wetlands, by definition waterlogged throughout the year, are associated with peatlands, but can also occur on wet mineral soils. The so-called peat mosses, Sphagnum spp., are important components of Low Arctic sedge meadows but are replaced by other semiaquatic bryophytes (mosses and liverworts) in the High Arctic. Peat is defined as organic matter which originated as the result of incomplete decomposition of plant material under conditions of high moisture. Peat accumulation results from the abundant growth of sedges and mosses. Mineral matter is generally a minor component, and the soils are usually acid (pH of 4.5 to 6.0). Where peats are thin (15-40 cm) and overlie mineral soils, these are frequently termed half-bog soils. Where peats are thicker (>40 cm) bog soils occur. These two soil types are typical of the circumpolar Low Arctic. Soil classification schemes vary widely and there are three classification systems for Arctic soils in North America alone (see Soils).

It is common to divide mire or peatland plants into ecological groups according to trophic conditions (eutrophic, mesotrophic, and oligotrophic). However, the ecological characteristics of mire plants can be established only as a result of regional studies of ecological ranges in various zones. For example, for Russian scientists working in northwest Siberia, oligotrophic Sphagnum species are subdivided into 'wet' (e.g. Sphagnum balticum, S. cuspidatum, S. lindbergii, S. majus) and 'mesic' (e.g. S. angustifolium, S. capillifolium, S. fallax).

The zone of Arctic mires stretches in a wide belt across northern North America and northern Eurasia and includes both homogeneous and polygonal mires. The wet portions (hollows and cracks) are dominated by non tussock-forming sedges (Carex stans, C. rotundata, C. rariflora, Luzula), cottongrasses (especially Eriophorum angustifolium, but also E. brachyantherum, E. scheuchzeri, E. russoleum), grasses (Arctophila fulva, Dupontia fisheri), and both Sphagnumand hypnoid mosses (Drepanocladus, Mnium, etc.). The drier portions (ridges of low-center polygons and the centers of high-center polygons) are characterized by abundant dwarf shrubs (Betula, Dryas, Empetrum , Ledum, Salix, Vaccinium) and herbs (especially cloudberry, Rubus chamaemorus).

Tussock tundra is a special type of sedge meadow derived from caespitose clumps (dense clusters) of the cottongrass Eriophorum vaginatum. Tussocks are long-lived, ranging in age from 122 to 187 years. The individual tussocks are densely packed with dead culms (stems), leaf bases, and other detritus and are flammable enough to allow the spread of wildfire. In dry summers, thousands of acres can burn actively or smoulder for weeks. The fires generally kill many dwarf shrubs but burn only the outer layers of the tussocks, rejuvenating them by removing dead tissue and releasing nutrients into the soil for uptake into the plants. Tussock tundra thus has a very different ecology and surface hydrology from sedge meadows where E. vaginatum is absent or only a minor component.

Sedge meadows serve as important forage habitat for a number of terrestrial herbivores, most conspicuously caribou and reindeer (Rangifer tarandus) and muskoxen (Ovibos moschatus), but also a number of smaller animals such as microtine rodents (lemmings and voles) and Arctic hare. In addition to their significance for plants and terrestrial herbivores, mires constitute an excellent habitat for insects and, consequently, for nesting birds such as waders and geese. Grazing intensity is important in determining the structure of Arctic sedge meadows, mainly through increasing the cover of bryophytes and the availability of nitrogen. Of all the meadow plant species, rhizomatous graminoids are especially well adapted to moderate to heavy grazing (e.g. the sedges Carex stans, Eriophorum angustifolium, and E. scheuchzeri and the grasses Alopecurus alpinus and Dupontia fisheri). With the majority of their biomass held belowground, these species undergo compensatory regrowth after defoliation during the growing season. Evidence indicates that moderate grazing can thus stimulate shoot turnover and productivity relative to ungrazed sedge meadows. Under very heavy grazing, dwarf shrubs tend to disappear while ruderal mosses and nitrophilous grasses (i.e. A. alpinus) increase in abundance.

Human impact is an important factor in sedge meadows because they: (i) comprise critical habitat for sedentary and migratory herbivores within any given region, particularly in the High Arctic; and (ii) are easily degraded by a variety of anthropogenic disturbances. Substantial habitat loss within Arctic wetlands is well documented in North America and Russia in conjunction with petroleum development, mining, and atmospheric pollution. Significant impacts range from off-road vehicle traffic in spring and summer to airborne road dust and smelter pollutants year-round. Moist tundra generally resists mechanical disturbance better than wet tundra, but it is less resilient once disturbed.

Bruce Forbes

See also Peatlands and Bogs; Tussock Tundra; Wet Tundra

Further Reading

Botch, Marina S. and V.V. Masing, "Mire Ecosystems of the U.S.S.R.", in Ecosystems of the World 4B. Mires: Swamp, Bog, Fen and Moor, edited by A.J.P. Gore, Amsterdam: Elsevier Scientific, 1983: 95-152

Callaghan, Terry V., Alistair D. Headley and John A. Lee, "Root Function Related to the Morphology, Life History and Ecology of Tundra Plants", in Plant Root Growth: An Ecological Perspective, edited by D. Atkinson, Oxford: Blackwell, 311-340: 1991

Gorham, Eville, "Northern Peatlands: Role in the Carbon Cycle and Probable Responses to Global Warming", Ecological Applications, 1, (1991): 181-195

Henry, Gregory H.R., Ecology of Sedge Meadow Communities of a Polar Desert Oasis: Alexandra Fiord, Ellesmere Island, Canada, Ph.D. thesis, Toronto: University of Toronto, 1987

Henry, Gregory H.R. "Environmental Influences on the Structure of Sedge Meadows in the Canadian High Arctic", Plant Ecology, 134, (1998): 119-129

Kershaw, Kenneth A., "Studies on Lichen-Dominated Systems. X. The Sedge Meadows of the Coastal Raised Beaches", Canadian Journal of Botany, 52, (1974): 1947-1972

Muc, Michael, "Ecology and Primary Production of the Truelove Lowland Sedge-Moss Meadow Communities", in Truelove Lowland, Devon Island, Canada- A High Arctic Ecosystem, edited by Lawrence C. Bliss, Edmonton: University of Alberta Press, 157-184: 1977

Ovenden, Lynn and Guy R. Brassard, "Wetland Vegetation Near Old Crow, Northern Yukon," Canadian Journal of Botany, 67, (1989): 954-960

Roulet, Nigel and Ming-Ko Woo, "Hydrology of a Wetland in the Continuous Permafrost Region," Journal of Hydrology, 89, (1986): 73-91.

Tarnocai, Charles and Stephen C. Zoltai, "Wetlands of Arctic Canada", in Wetlands of Canada, edited by C.D.A. Rubec, Montreal: Polyscience, 27-53: 1988

Thannheiser, Dietbert, "Beach and Bog Vegetation of the Western Canadian Arctic Archipelago and Spitsbergen," Polarforschung, 46, (1976): 62-71