Zonation is typified by the more or less latitudinal arrangement of Arctic terrestrial communities around the pole, as described above. It is also exemplified by the altitudinal distribution of biomes on mountains: even lofty peaks at the equator have treeless tracts of vegetation at high enough altitudes. Altitudinal zona-tion on Arctic mountains is less pronounced than on mountains of temperate and tropical regions. Nevertheless, as one ascends there is a trend for the vegetation to become more sparse and reminiscent of polar desert. Extreme cold, high winds, and lack of moisture curtail life. Within glaciers and ice fields, mountains may protrude as nunataks. These can support depauperate ecosystems, but even in bare mineral soils between cracks in the rocks, bacteria, microarthropods, and the occasional plant survive.
Zonation can also be found in bodies of fresh water and the sea. Although the scouring effects of ice make shallows inhospitable at sea and lake shores, and along river banks, vegetation and communities of animals thrive at depths below the ice. Light is the most limiting factor, and its penetration into the water column is influenced by the clarity (lack of suspended sediment and organic matter) of the water. Some Arctic lakes support vast beds of submerged mosses, whereas others are relatively clear of vegetation. Zonation associated with isostatic rebound is described above.
Succession may be thought of as zonation in time. Primary succession is typified by life first inhabiting the mineral environment as lichens encrust bare rocks. Lichens and mosses slowly extract nutrients from the mineral surface, and when they die they enrich the area with the carbon and nitrogen of their own remains. Soil begins to accumulate and provides a substrate for later stages of succession. More complex and diverse plant life colonizes the enriched places, and along with them come various invertebrates (mites, insects, etc.). Depending on the physical, chemical, and biotic conditions, ecological communities develop to various extents. The final assemblage in succession is referred to as the climax community. It is these communities that typify the biomes of polar desert, tundra, and forest-tundra noted above. Not all succession starts with such harsh substrates as rocks. On land, iso-static rebound brings marine silt, which is saline, above sea level, but like terrestrial soil salt-tolerant plants, such as lime grass and sea purslane, may colonize it. As the land rises and is washed free of salt by rain and groundwater flow, succession follows. In the region of Churchill, Manitoba, one may begin walking in the sea below low tide, and trudge inland past the strand, onto tundra, through the krummholz (a zone of stunted windblown trees), and into the boreal forest in a matter of only an hour or two.
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