The Alaska Vegetation Classification - Alaska Geobotany Center ...

Distribution and site characteristics-Mixed shrub-sedge tussock tundra is characteristicof polygonal ground and wet to mesic gentle slopes in northern and westernAlaska and in alpine areas of interior Alaska. It is one of the most extensive tundravegetation units in the State and occupies vast areas of the Arctic Foothills and theSeward Peninsula. It commonly occurs on Pergelic Cryaquepts and Histic PergelicCryaquepts (upland tundra and meadow tundra) soils. Permafrost generally is present30 to 40 centimeters (12 to 16 in) below the surface but may be as much as 90 centimeters(35 in) below the surface on the Seward Peninsula (Hopkins and Sigafoos1951, Racine and Anderson 1979).An organic mat of variable thickness in which the shrubs are rooted often is presentat the soil surface. The tussocks always are rooted in mineral soil so that the organicmat is never thicker than the active layer. Pockets of standing water usually arepresent at spring breakup and the soil remains moist to wet throughout the growingseason. In many areas, water will seep slowly into soil pits dug at any time in thegrowing season. The mineral soil is a fine-textured gley with gray or dark-graymottling, which indicates anaerobic conditions and periodic waterlogging. Soils aregenerally acid with pH values of 4.4 to 4.9 at the surface; these usually increasewith depth.Frost scars (unvegetated or slowly revegetating patches of mineral soil thrust tothe surface by frost action) may be absent or abundant. Carex bigelowiitends todominate on steeper, better drained, and less acid soils than Eriophorurn vaginaturn.Successional status-Shrub-tussock communities probably are the climax vegetationfor large areas of arctic Alaska. But if local climatic conditions lead to theaccumulation of organic matter, mosses and shrubs thrive at the expense of thetussocks (the tussocks must stay above the rising tide of peat but keep their roots inunfrozen mineral soil). Eventually, mosses and shrubs invade the tussocks, therebyleading to tussock senescence and death. This generally leads to the development ofwet shrub birch-ericaceous shrub communities. Disturbances such as fire favor thetussocks by burning back the shrubs and moss and by releasing nutrients from thepeat mat that the tussocks can use more effectively than the shrubs can use them(Fetcher and others 1984).Closely related types-These communities are very similar, if not identical, to mixedshrub-sedge tussock bog communities but occur in arctic and alpine areas (beyondthe trees) instead of in subarctic lowland areas (within the trees). They also aresimilar to tussock tundra (lll.A.2.d.) but have more than 25 percent shrub cover.They are similar to mesic shrub birch-ericaceous shrub communities and shrubbirch-ericaceous shrub bog communities, except that these two communities lacktussocks.Photographs-Drew and Shanks 1965, figure 11; Johnson and others 1966, figure13; Kessel and Schaller 1960, figures 2 and 3; Racine 1976, figure 33; Viereck 1966,figure 9.Primary references-Brock and Burke 1980, Holowaychuk and Smeck 1979,Hopkins and Sigafoos 1951, Johnson and others 1966, Racine and Anderson 1979,Viereck 1966.129