NORTH AMERICAN VEGETATION AND CLIMATES FROM 21,000 YEARS AGO TO PRESENT: USEFUL DATA SETS FOR TESTING EARTH SYSTEM MODELS

By mapping pollen and lake-level data from 21,000 years ago to present, we present an overview of late-Quaternary vegetation and climate in eastern and northern North America. We use a series of high-quality datasets suitable for testing and refining the results of climate and earth-system models. Pollen and vegetation maps at 50 km resolution and 1,000 year time steps show the changing patterns of individual plant taxa, associations among taxa, plant functional types, and biomes. Each of these representations of the pollen data can be used for model testing. The mapped patterns illustrate how broad-scale properties of the vegetation critical for governing surface-atmosphere fluxes emerge from individualistic species-level responses to climate change. Many pollen taxa shift their distribution northward as the Laurentide Ice Sheet retreated and disappeared by 6000 years ago. Species associations have changed continuously, and biomes unlike any today were widespread from 16,000 to 12,000 years ago. Biomes are shown to be dynamic entities that have changed in distribution, composition, and structure since the last glacial maximum. Total tree cover has increased since the last glacial maximum and the proportion of deciduous broadleaved trees has increased in eastern forests. Climate reconstructions derived from pollen and lake-level data show the distribution of key climatic and bioclimatic variables. Moisture-balance trends during the Holocene varied regionally, with drier-than-present conditions in New England during the early Holocene followed by a mid-Holocene peak in aridity in the Midwest. Our data sets make available a wide variety of climatic and vegetational benchmarks suitable for testing the results of earth-system models at a variety of time scales.