Defining The Arctic

The Arctic is a large and heterogeneous area (Figs. 6.1-6.2) and it cannot be expected that all regions will respond equally to global warming. It is therefore necessary to define the Arctic and its major ecological regions. These divisions can be geographical, climatic, or ecological. Climatically, the Arctic can be considered as that region of the Earth's surface that is underlain by permafrost, or permanently frozen ground. This is not entirely satisfactory as permafrost underlies approximately 20% of the Earth's terrestrial surface and occurs not only at high latitudes but also in some non-arctic locations at high elevations (Brown et al., 1995). Permafrost is, however, one of the main restricting factors limiting the northward expansion of trees; the absence of trees is the key ecological feature that distinguishes the tundra from the taiga or boreal forest. Geographically, the Arctic can be simply defined as the portion of the Earth's surface that lies north of the Arctic Circle (66° 330 N), but this contains a great diversity of habitats including portions of the boreal forest (taiga). In Chapter 5 the tundra-taiga interface was considered as the boundary

Taiga Tundra Satellite

Fig. 6.2 False-colour satellite infrared image of the circumpolar Arctic. Red areas represent greater amounts of green vegetation; light and dark green represent sparse vegetation; black areas represent fresh water, and white areas represent ice. (Walker et al., 2002, with permission of Taylor and Francis Ltd.)

High Arctic

Low Arctic

Taiga

Polar desert

(herb-cryptogam)

Polar semi-desert

(herb-cryptogam, graminoids, moss, cushion plants, mires)

Polar desert zone

Tundra

Tundra

(low shrub-sedge, tussock dwarf shrub, dwarf heath, mires)

Forest-tundra

Taiga

Taiga

Polar desert herb-cryptogam

Arctic tundra

(dwarf-shrub-herb, graminoids, moss)

Typical tundra

(sedge-dwarf shrub, dwarf heath, polygonal mires)

Lesotundra

Taiga

Fig. 6.2 False-colour satellite infrared image of the circumpolar Arctic. Red areas represent greater amounts of green vegetation; light and dark green represent sparse vegetation; black areas represent fresh water, and white areas represent ice. (Walker et al., 2002, with permission of Taylor and Francis Ltd.)

Fig. 6.3 Classification schemes for arctic vegetation in North America and Eurasia. (After Bliss & Matveyeva, 1992; see also Calow, 1998.)

zone between the arctic tundra and the boreal forest and therefore an ecological approach will also be used here and the southern extension of the Arctic will be defined by the northern limit to tree growth. Approximately 5.5% of the Earth's land surface comes under this heading. Many different types of plant communities exist within this zone and each has a northern limit. It follows therefore that there are many marginal areas within the Arctic.

A first approximation in distinguishing the various limits to plant distribution in the Arctic is the recognition of two major zones, namely the Low Arctic and the High Arctic. The former is dominated by woody shrubs (e.g. Alnus, Betula, Salix). The latter, the High Arctic, describes the region where much of the land is covered by permanent snow and ice and the vegetation is limited to a thin discontinuous cover of diminutive flowering plants, mosses and lichens. Even within the Low and High Arctic there are many recognizable margins which differ between North America and Eurasia and ecologists in these different regions have adopted terminologies that reflect these differences (Fig. 6.3).

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