| Vegetation was analysed in each of eight white spruce, Picea glauca (Moench) Voss, treelimit ecotone sites in the Alatna River drainage of the Central Brooks Range in northern Alaska. Measurements were made of species composition and abundance, elevation, slope, aspect, and the percent ground that was covered by rock, by litter, or was bare. Soil measurements included horizon thicknesses, pH, and depth to permafrost. Spruce trees were cored and aged.; Vegetation associations were defined by a numerical clustering analysis program and by the grouping of releves in a Braun-Blanquet stand table. Based on composition and abundance data for one hundred and ten taxa and two hundred and forty-seven stands, eight major ecotonal associations were identified. Associations, averaging 40 species, were identified at the 47% similarity level number of the Sorensen's index.; Results demonstrate that the post-glacial vegetation has developed into different communities. Holocene climatic conditions and combinations of site-specific environmental factors have produced a mosaic of mesotopographical-scale vegetation associations. Multivariate community analysis suggested major abiotic constraints on vegetation patterns.; Stands and species are distributed along mesotopographical-scale complex gradients of elevation, temperature, precipitation, litter cover, snow depth and duration, rock cover, slope, and depth to permafrost. Primary sources of variation in floristic similarity among samples correlated best with surface drainage as interpreted from measurements of rock and litter cover. Rocky sites arer well-drained; sites with thick litter are poorly-drained. Therefore, the hypothesis that drainage conditions have major influence on the distribution of ecotonal vegetation associations is true.; Microsites with white spruce seedlings were found 5 km beyond, and at least 100 m above, the existing treelimit stands. This suggests local amelioration of temperature regimes or continued post-glacial migration of spruce up the valleys.; Understory ecotonal vegetation is a more finely-tuned indicator of climatic change than the observable treelimit. Therefore, a shift in species composition, distribution, or structure may be expected in the ecotonal vegetation with continued changes in climatic conditions. |
