Northwestern North America

Abstract

Pollen records from northwestern North America provide information on vegetation and climate changes during the last 16,000 years. Interpretations are supplemented in several regions by plant macrofossil and charcoal data that describe individual species occurrences and local fire history. The vegetation history on millennial timescales is attributed to climate changes resulting from slow variations in the seasonal cycle of insolation, including the period from 12,000 and 6,000 calendar years before present (cal yr BP), when summer insolation was greater than present. Tundra and sub-Alpine communities that were widespread in the late-glacial period were replaced by temperate forests in response to warmer and wetter conditions in the Holocene. During the early Holocene, the Pacific Northwest and summer-dry areas of the Northern Rocky Mountains featured more xerophytic and fire-adapted taxa, and conditions were warmer and drier than at present. These communities were supplanted by meosphytic forests in the middle and late Holocene as summer insolation decreased and the present climate was established. In the summer-wet areas of the Northern Rocky Mountains, mesophytic vegetation that characterized the early Holocene was replaced by dry- and fire-adapted communities after 6,000 cal yr BP as a result of weakened monsoonal circulation in the middle and late Holocene.