AN OVERVIEW OF THE VEGETATION AND SOILS OF THE FLOODPLAIN ECOSYSTEMS OF THE TANANA RIVER, INTERIOR ALASKA

The soils and vegetation of 12 stages of forest succession on the floodplain of the Tanana River are described. Succession begins with the invasion of newly deposited alluvium by willows (Salix spp.) and develops through a willow-alder (Alnus tenuifolia Nutt.) stage to forest stands of balsam poplar (Populus balsamifera L.), followed by white spruce (Picea glauca (Moench) Voss), and finally black spruce (Picea mariana (Mill.) B.S.P.). The principal changes in substrate characteristics during the successional sequence are (i) change from sand to silt loam, (ii) increase in terrace height and distance from the water table, (iii) development of a forest floor, first of leaf litter and then live and dead feather mosses, (iv) burial of organic layers by flooding, and (v) the development of permafrost as soils are insulated by a thick organic layer. Soils and vegetation of six stands occurring in three successional stages used in the salt-affected soils study are described in detail: open willow stands (stage III), balsam poplar - alder stands (stage VI), and a mature white spruce stand (stage VIII). There is a general progression of plant species resulting from the modification of the environment by the developing vegetation and changes in soil characteristics. Life history and stochastic events are important in the early stages of succession, and biological controls such as facilitation and competition become more important in middle and late stages of succession.