| Summary: | Permafrost degradation leads to substantial changes in soil thermal and hydrologic characteristics. We investigated the effects of changes in active layer thickness and soil drainage on vegetation distribution near the arctic treeline on the Seward Peninsula, Alaska. We measured active layer thickness, soil moisture, density of tall shrub species, cover of low shrub species, and reconstructed white spruce establishment history along transects across the banks of a network of thaw ponds. We found that active layer thickness did not vary along our transects, but soils on thaw pond banks were significantly drier than those on level tundra or in thaw‐pond channels. Thaw‐pond banks were the only sites in which trees successfully established, and shrub communities on thaw‐pond banks were taller and more dominated by tall shrub species like willow and shrub birch. The data suggest that the establishment of tree and tall shrub species at the arctic treeline can be limited by the availability of well‐drained microsites, and the response of these species to regional climatic changes will be constrained by the availability of such microsites and thus contingent upon further degradation of the permafrost. Copyright © 2003 John Wiley & Sons, Ltd.
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