1.3.1 Climate, topography and spatial heterogeneity
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Arctic biodiversity is structured by a range of environmental factors. To understand which environmental factors that are decisive to the species composition in an area, you need to combine information that vary at different spatial and temporal scales. Within the arctic tundra biome, for instance, the length of the growing season varies between one and four months among different regions and the mean July temperature varies between zero and twelve degrees Celsius. In addition to low air temperatures and short growing seasons the abiotic environmental conditions for life in the Arctic are characterized by low light intensity, which is partly compensated by with midnight sun, by low precipitation, which mainly comes as snow, by strong winds that cause abrasion and desiccation stress, low soil temperatures that affect plant nutrient availability and turnover rates.
This large climatic variation is reflected by the vegetation and, accordingly, it is practical to subdivide the Arctic into different bioclimatic sub-zones. Five sub-zones have been recognized, and are referred to as sub-zones A-E with A as the coldest and E the warmest sub-zone (Elvebakk et al. 1999, CAVM Team 2003, Walker et al. 2005) . The subzones A, B and C cover areas that are often referred to as the High Arctic and subzones D and E as the Low Arctic (i.e. sensu Bliss 1997)
The different bioclimatic subzones of the Arctic cover progressively larger areas and increasingly higher habitat and species diversity as one moves from A to E and the vegetation cover and its vertical complexity gradually increases as well. Sub-zone A covers 2% of the non-glacier Arctic with predominately polar deserts, while subzone E has almost continuous vegetation cover dominated by shrub tundra with more than 40 cm tall shrubs.
Arctic land is predominantly treeless permafrost with large areas of tundra. The Arctic environment is highly heterogenous, with for example dry stony ridges, periglacial features, areas of late snow lie, heath, or wet moss, all in close proximity (Thomas et al., 2008). Large areas have recently been reworked by glacial action and possess continuous underlying permafrost, influencing soil hydrology.
On a regional basis, northern areas consist largely of polar desert characterized by low precipitation and a short snow-free growing season. On a landscape scale, the habitat is comprised of a heterogeneous mosaic (Jónsdóttir, 2005). The ridge tops, blown free of winter snow, or areas kept clear of snow by wind eddies, may experience winter temperatures approaching -40°C, while organic soils protected under deeper snow do not exceed temperatures below -10°C (Coulson et al., 1995).
Topography, in combination with snow, strongly modifies the climatic and hydrological conditions experienced by the biota which creates both extensive and small scaled heterogeneity among biological communities within a single subzone. Use guide to explore and compare bioclimatic zones Melting snow and permafrost may provide a constant cold-water source throughout the summer resulting in cold, wet and boggy areas in direct proximity to drier polar desert vegetation. The shallow active layer in the permafrost exaggerates this effect by hindering drainage. Soils may also vary considerably in depth and form between short distances. Generally, the soils are thin, rarely more than a few centimetres deep, and cover moraine debris, patterned ground or bedrock. In wetter areas, moss may develop into thick carpets or turfs some tens of centimetres deep, efficiently insulating the ground beneath against insolation (Coulson et al., 1993).
The different bioclimatic subzones can be characterised by communities that develop in what have been termed “zonal habitats”. Zonal habitats are found on horizontal or slightly sloping, fine grained, well drained areas without the confounding effect of snow, site moisture, soil chemistry and texture or major disturbances and thus reflect the influences of the prevailing climatic conditions of the subzone (Chernov and Matveyeva 1997, Elvebakk 1999, Elvebakk et al. 1999, Razzhivin 1999).
In subzone A the vegetation cover in zonal habitats is sparse (<5% cover) mainly composed of herbs of compact growth foms (rosettes, cushions or matts) such as Papaver dahlianum, species of Draba and Cerastium. Mosses are largely absent in the zonal habitat, lichens are represented by saxicolous species (crustose lichens on rocks) and the soils skeletal, only contain small amounts of organic matter. The vegetation cover of zonal habitats in subzone B is much greater than in A, although still discontinuous, and usually dominated be prostrate dwarf shrubs (<5 cm tall), such as Salix arctica, S. polaris, and Saxifraga oppositifiolia, in addition to mosses and fruticose lichens. In zonal habitats of subzone C there is usually a closed vegetation cover, dominated by the Cassiope tetragona (not on very alkaline substrates), Dryas sp., graminoids, mosses and lichens. The vertical structure is still simple, i.e. of low canopy height and complexity.
In the two low arctic subzones, D and E, the undisturbed vegetation of zonal habitats is not only closed (100% cover) but also vertically more complex than in the high arctic subzones with 2-3 layered canopy. In subzone D, the zonal habitats are vegetated by either sedge-dwarf shrub-moss tundra or erect dwarf shrub tundra dominated by sedges, Empetrum nigrum, species of Vaccinium and Betula and more erect species of Salix than in the High Arctic. The canopy may reach up to 40 cm. In the warmest subzone E, the vegetation in zonal habitats is either tussock tundra dominated by Eriophorum vaginatum (on fine grained acidic soils) or shrub tundra dominated by >40 cm tall Salix, Betula and Alnus species.
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