Circumpolar Arctic Bioclimate Subzone D

 

Subzone D. Ambarchik, Kolyma River vicinity, Russia. Photo D.A. Walker.
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Circumpolar Arctic Bioclimate Subzone D
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Subzone D Description

Subzone D covers the southern parts of Banks Island and Victoria Island, much of Keewatin, southern Baffin Island, most of southern Greenland, and a broad band across Siberia and Chukotka. Climatically, Subzone D periodically receives relatively temperate air from the south during the summer while Subzone C receives predominately arctic air masses. The mean July temperatures at the southern boundary of Subzone D is about 9 °C. The boundary between Subzones C and D is considered of highest rank because it separates the northern drier tundras on mineral soils from the southern relatively moist tundras with moss carpets and peaty soils (Alexandrova 1980). This is approximately equivalent to the boundary between Bliss's High and Low Arctic (Bliss 1997). The major difference in pedology causes dramatic changes to the vegetation. The plants in Subzone D have strong hypoarctic (boreal forest) affinities (Yurtsev et al. 1978). Important hypoarctic (boreal forest and southern Arctic) species such as birch (e.g., Betula nana), alder (Alnus), willow (Salix), and heath plants (Ericaceae and Empetraceae) extend their ranges from the lower layer of subarctic woodlands, but are not dominant as they are in Subzone E. Low shrubs (>40 cm tall) occur along streams. Overall, the role of shrublands is much less prominent than in Subzone E. The plant canopy is usually interrupted by patches of bare soil caused by nonsorted circles, stripes, and a variety of other periglacial features ("spotty tundra"in the Russian literature). Vascular plants generally cover about 50-80% of the surface. Zonal vegetation on gently sloping upland surfaces consists of sedges (e.g. Carex bigelowii, Carex membranacea, Eriophorum triste, E. vaginatum, Kobresia myosuroides), prostrate and erect dwarf (<40 cm tall) shrubs (e.g., Salix planifolia, S. lanata ssp. richardsonii, S. reticulata, S. arctica, Betula exilis, Dryas integrifolia), and mosses. Prostrate dwarf-shrub communities, which are common on zonal surfaces in Subzone C, are confined mainly to wind-swept sites. The moss layer, consisting primarily of Tomentypnum, Hylocomium, Aulacomnium, and Sphagnum, contributes to the development of organic soil horizons on fine-grained soils. Soils in Subzone D have thin peaty horizons, and fine-grained soils are often nonacidic, whereas soils in Subzone E are usually acidic regardless of texture.

There is more regional variation in the zonal vegetation than in Subzones A, B, and C. Tussock tundra consisting of cottongrass tussocks (Eriophorum vaginatum) and dwarf shrubs is common on fine-grained acidic soils over much of northeastern Siberia and northern Alaska (Walker et al. 1994), particularly in areas that were unglaciated during the last part of the Pleistocene. In transitional areas to Subzone C and on nonacidic loess, Dryas spp. and Cassiope tetragona are important (Walker and Everett 1991). Some continental areas of Russia have dry steppe tundras that are relicts of cold dry Pleistocene vegetation (Yurtsev 1982). This subzone has also been called the "erect dwarf-shrub" subzone (Walker et al. 2002) and could be called the "Betula nana zone" because of the importance of Betula spp., and other erect dwarf-shrub species in this subzone.