Nine pollen profiles were obtained along a broad latitudinal transect extending from the Upper Nisling Valley to the Ittlemit Lake and Bear Lakes areas in the southwest Yukon Territory. Radiocarbon dated fossil pollen records from these profiles provided evidence for the reconstruction of postglacial environmental history of the area. Surficial samples combined to aid the interpretation of the fossil data. Records of these sequences suggest that the post-glacial vegetation in the area exhibited a successional development. Immediately after the deglaciation, the area was covered by a sparse herb dominated tundra assemblage. Betula glandulosa invaded the area at about 10,000 yr BP, which initiated the replacement of herbaceous tundra by a dwarf birch shrub-tundra. Picea glauca invaded the Upper Nisling Valley in the northern part of the study area and the Ittlemit Lake Basin in the south at least around 9,000 yr BP, while 8,600 yr BP in the central Aishihik Basin, marked the establishment of forest-tundra environment in the area. Significant rise of Picea pollen at 7,500-8,000 yr BP in the north-central part of the study area indicate the establishment of a modern boreal forest, which is approximately 1,500-2,000 years earlier than that indicated by the Antifreeze Pond pollen diagram from the Snag area of the southwest Yukon. It is notable that the pollen spectra of two lacustrine sediments suggest a phase change of lakes into marsh occurring around 5,000-6,000 yr BP in the area, and change of bog conditions has also been recorded at almost the same time in the area. These events occurred simultaneously in the area and probably reflect the change of climatic conditions, which might have led to the development of permafrost in the region. The development of early post-glacial vegetation in the area exhibited an unstable pattern. This instability was primarily related to the sequential invasion and migration of taxa from late-glacial refugia. Stable vegetation associations were established in the area as a result of migrators reaching their limit of climatic tolerance during the middle Holocene. Post-glacial vegetation history in the Ittlemit Lake Basin area seems to have exhibited a different pattern.
