Late Quaternary biomes of Canada and the eastern United States

Pollen data have been used to construct biome maps for today 6000 C-14 yr B P and 18,000 C-14 pr BY for Canada and the eastern United States. The inferred modern biome distributions agree well with independent reconst ructions of North American vegetation prior to European settlement. Some di screpancies between the pollen data and the modem potential vegetation are caused by post-settlement clearing of the landscape and the consequent incr ease of herbaceous types in the recent pollen record. Biome distributions at 6000 C-14 yr BP reflected the warmer and drier condi tions then prevalent in the continental interior, but the overall position of biomes was similar to that of today. The boreal treeline in North Americ a was not significantly north of its present position, in contrast to the 1 00-200 km shift reported for Siberia. At the last glacial maximum (18,000 C -14 yr BP), steppe and tundra were prevalent in the Midwest and northwester n Canada, and coniferous forests and woodlands grew in eastern North Americ a. The open vegetation at 18,000 C-14 yr BP was probably due to drier condi tions and/or lower concentrations of atmospheric CO2. The composition and physical structure of biomes is not constant over time. Mid-Holocene biomes were similar in structure to those of today, but shift s in the relative importance of individual plant functional types are large enough that the physical properties of biomes, such as albedo, canopy cond uctance and surface roughness, are likely to have varied even during the Ho locene. Last glacial maximum biomes were structurally different from their modern counterparts. The biome maps therefore may obscure significant veget ational changes in space and time during the late Quaternary. The difference between the highest and next highest affinity scores for eac h sample measures how strongly affinity scores discriminate among biomes. F or many biomes, the difference is not large, and affinity score ties are no t uncommon, highlighting the importance of tie-break procedures when using the biomization method.