VEGETATION SUCCESSION AND DISTURBANCE ON A BOREAL FOREST FLOODPLAIN, SUSITNA RIVER, ALASKA

Vegetation development along the Susitna River floodplain exhibits mul tiple successional pathways in response to disturbances such as floodi ng, ice scour, wind, browsing, and logging. These can rejuvenate sites or slow vegetation development through surface erosion, deposition, a nd stem bending. Youngest vegetation types were dominated by Variegate d Horsetail (Equisetum variegatum) on silty sites, Feltleaf Willow (Sa lix alaxensis) and Tall Blueberry Willow (S. novaeangliae) on medium-t extured sites, Balsam Poplar (Populus balsamifera) on sandy sites, and Yellow Dryas (Dryas drummondii) on cobbly sites. Although Thinleaf Al der (Alnus tenuifolia) was sparse in early succession, it grew more ra pidly than other species and created a closed overstory with poorly de veloped understory early in intermediate succession. Browsing disturba nces temporarily limited height growth of willows and Balsam Poplar. A fter Balsam Poplar exceeded the reach of Moose (Alces alces), it grew taller than Thinleaf Alder to form Young Poplar Forests. As trees matu red and natural thinning occurred, Old Poplar Forests formed with a wo ody understory including White Spruce (Picea glauca). Uprooted trees e xposed mineral microsites for germination of Paper Birch (Betula papyr ifera) seeds. These sites eventually developed into the Paper Birch -W hite Spruce Forests of late succession with a well-developed shrub und erstory.