Abstract

1. Pollen data have been used to construct biome maps for today, 6000 yr. B.P., and 18,000 yr. B.P. for Canada and the eastern United States.
2. The inferred modern biome distributions agree well with independent reconstructions of North American vegetation prior to European settlement. Some discrepancies between the pollen data and the modern potential vegetation are caused by post-settlement clearing of the landscape and the consequent increase of herbaceous types in the recent pollen record.
3. Biome distributions at 6000 yr. B.P. reflected the warmer and drier conditions then prevalent in the continental interior, but the overall position of biomes was similar to that today. Boreal treeline in North America was not significantly north of its present position, in contrast to the 100-300 km shift reported for Siberia. At the last glacial maximum (18,000 yr. B.P.), steppe and tundra were prevalent in the Midwest and north-west Canada, and coniferous forests and woodlands grew in eastern North America. The open vegetation at 18,000 yr. B.P. was likely due to drier conditions and/or lower concentrations of atmospheric CO₂.
4. The composition and physical structure of biomes is not constant over time. Mid-Holocene biomes were similar in structure to those today, but shifts in the relative importance of individual plant functional types are large enough that the physical properties of biomes such as albedo, canopy conductance, and surface roughness likely varied even during the Holocene. Last glacial maximum biomes were structurally different from their modern counterparts. The biome maps therefore may obscure significant vegetational changes in space and time during the late Quaternary.
5. The difference between the highest and next highest affinity scores for each sample measures how strongly affinity scores discriminate among biomes. For many biomes, the difference is not large, and affinity score ties are not uncommon, highlighting the importance of tiebreak procedures when using the biomisation method.