Late-quaternary Vegetation Dynamics in North America: Scaling from Taxa to Biomes

This paper integrates recent efforts to map the distribution of biomes for the late Quaternary with the detailed evidence that plant species have responded individualistically to climate change at millennial timescales. Using a fossil-pollen data set of over 700 sites, we review late-Quaternary vegetation history in northern and eastern North America across levels of ecological organization from individual taxa to biomes, and apply the insights gained from this review to critically examine the biome maps generated from the pollen data. Higher-order features of the vegetation (e.g., plant associations, physiognomy) emerge from individualistic responses of plant taxa to climate change, and different representations of vegetation history reveal different aspects of vegetation dynamics. Vegetation distribution and composition were relatively stable during full-glacial times (21 000– 17 000 yr BP) [calendar years] and during the midto late Holocene (7000–500 yr BP), but changed rapidly during the late-glacial period and early Holocene (16 000–8 000 yr BP) and after 500 yr BP. Shifts in plant taxon distributions were characterized by individualistic changes in population abundances and ranges and included large east–west shifts in distribution in addition to the northward redistribution of most taxa. Modern associations such as Fagus–Tsuga and Picea–Alnus–Betula date to the early Holocene, whereas other associations common to the late-glacial period (e.g., Picea–Cyperaceae–Fraxinus–Ostrya/ Carpinus) no longer exist. Biomes are dynamic entities that have changed in distribution, composition, and structure over time. The late-Pleistocene suite of biomes is distinct from those that grew during the 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 range shifts and changes in abundance among individual taxa. The compositional and structural differences between full-glacial and recent biomes of the same type are similar to or greater than the spatial heterogeneity in the composition and structure of present-day biomes. This spatial and temporal heterogeneity allows biome maps to accommodate individualistic behavior among species but masks climatically important variations in taxonomic composition as well as structural differences between modern biomes and their ancient counterparts.