The appearance and disappearance of major vegetational assemblages: Long-term vegetational dynamics in eastern North America

Abstract

Contoured maps of pollen percentages from eastern North America illustrate that the vegetation changed continuously during the past 18000 years. The maps show that the geographic distribution for selected pollen types from 500 years ago parallels the major patterns in the vegetation, and that the correspondence is good enough to justify using maps of fossil pollen to interpret the patterns and composition of the broad-scale vegetation from earlier times. Data for Artemisia, Cyperaceae, Picea, Betula, Alnus, Abies, and Pinus pollen illustrate how the changing location and abundance of their plant taxa and populations altered the spatial pattern, local composition, and the overall structure of the vegetation and thus led to thedisappearance as well as appearance of major biomes and ecotones. For example, the Picea parkland biome that existed from 18000 to 12000 yr B.P. within a broad region south of the ice sheet disappeared within 2000 years after 12000 yr B.P., and the modern boreal forest biome began to develop across a large area of central Canada only after 6000 yr B.P. On a time scale of 104 to 106 yr, these types of vegetation changes involve a continual overlapping and separation of the abundance distributions for different taxa. Such changes are likely to have been a feature of vegetational dynamics for millions of years. © 1987 Dr W. Junk Publishers.