PHYSICAL PROPERTIES OF THE TUNDRA SOIL-WATER SYSTEM AT STORDALEN, ABISKO

The Stordalen mire at Abisko, North Sweden, is a 25 ha large peatland underlain by silt. Under neath an active layer of seasonal thaw the soils are perennially frozen. In general, the peat shows a low degree of humification. Differences in relative decomposition, as revealed by absorbency deter minations, are related to the micro topography. The organic material shows a very large pore volume, locally more than 96%. The pore shape tends to be vertically oriented in the upper 7-8 cm, whereas a transformation to horizontally oriented pores is evident below 20 cm depth. The peat layer averages 30 cm thickness over the mire and shows a transition layer of 3-20 cm above the mineral soil. Bulk densities of peat are 0.06-0.15 g cm-3 and average 1.3 g cm-3 in mineral soils. Soil water retention curves, showed differences between hummocks and small depressions, though, at the tension of wilting (15 atm), the average moisture content is 15% by volume at both micro sites. Water balance variables and the amount of plant available water were calculated. The soil moisture regime showed a yearly maximum close to saturation in connection with the melting of snow, followed by a decreasing trend throughout the summer. As regards water flow, the physical properties and the microtopographical characters lead to the conclusion that the hummocks act as inflow areas for precipitation. Lateral flow is likely to occur along the frost table whereas small depressions act as "ground water" outflow areas. Accordingly, the physical properties and water flow are strongly related to the microtopography and thus also to the vegetation.