NOAA/WDS Paleoclimatology - Williams et al. 2004 Late Quaternary North American Vegetation Dynamics Data

This paper integrates the mapping of late-Quaternary biomes with palynological evidence for individualistic species responses to environmental change. We document vegetation history in boreal and eastern North America for the past 21,000 calendar years (21 ka), reconstructing past vegetation from fossil pollen evidence at ecological resolutions ranging from individual plant taxa to biomes. At these scales, climatic control of vegetation change is exerted at the level of individual species, from which higher-order properties of the vegetation emerge. Vegetation distribution and composition were relatively stable during full-glacial times (21-17 ka) and the mid- to late Holocene (7-0.5 ka), but changed rapidly during the late glacial and early Holocene (16-8 ka) and after 0.5 ka. Most plant distributions shifted northward, but taxa also moved east or west as, for example, the area of high abundances for spruce, pine, and other cold-tolerants expanded from eastern North America into central and western Canada. Modern associations such as beech-hemlock and spruce-alder-birch date to the early Holocene, whereas other associations common to the late-glacial (e.g. spruce- sedge-ash-hornbeam) no longer exist. Biomes are dynamic entities that have changed in distribution, composition, and structure since the last ice age and before. Distinct suites of biomes grew during the late Pleistocene and Holocene. The pollen-based biome reconstructions are able to capture the major features of late-Quaternary vegetation but downplay the magnitude and variety of vegetational responses to climate change by 1) limiting apparent land-cover change to ecotones, 2) masking internal variations in biome composition, and 3) obscuring the migrations and changes in abundance among individual taxa. The compositional differences between full-glacial and recent biomes of the same type are similar to or greater than the spatial heterogeneity in the composition of present-day biomes, both for individual plant taxa and for plant life forms (conifers, broad-leaved trees, and herbs). The spatial and temporal heterogeneity in biome composition allows maps of biomes to accommodate individualistic changes among taxa but masks climatically important variations in taxonomic composition as well as significant variations in vegetation structure.