Vegetation and environment in Eastern North America during the Last Glacial Maximum

Abstract

Knowledge of the vegetation and environment of eastern North America during the Last Glacial Maximum (LGM) is important to understanding postglacial vegetational and biogeographic dynamics, assessing climate sensitivity, and constraining and evaluating earth-system models. Our understanding of LGM conditions in the region has been hampered by low site density, problems of data quality (particularly dating), and the possibility that LGM vegetation and climate lacked modern analogs. In order to generate improved reconstructions of LGM vegetation and environment, we assembled pollen and plant macrofossil data from 21 and 17 well-dated LGM sites, respectively. All sites have assemblages within the LGM timespan of 21,000±1500 calendar yr BP. Based on these data, we prepared maps of isopolls, macrofossil presence/absence, pollen-analogs, biomes, inferred mean January and July temperatures and mean annual precipitation for the LGM. Tundra and open Picea-dominated forest grew along the Laurentide ice sheet, with tundra predominantly in the west. In the east, Pinus-dominated vegetation (mainly P. banksiana with local P. resinosa and P. strobus) occurred extensively to 34°N and possibly as far south as 30°N. Picea glauca and a now-extinct species, P. critchfieldii, occurred locally. Picea-dominated forest grew in the continental interior, with temperate hardwoods ( Quercus, Carya, Juglans, Liriodendron, Fagus, Ulmus) growing locally near the Lower Mississippi Valley at least as far north as 35°N. Picea critchfieldii was the dominant species in this region. The Florida peninsula was occupied by open vegetation with warm-temperate species of Pinus. Eastern Texas was occupied by open vegetation with at least local Quercus and Picea. Extensive areas of peninsular Florida and the continental interior had vegetation unmatched by any modern pollen samples. The paleovegetational data indicate more extensive cooling in eastern North America at the LGM than simulated by either the NCAR CCM0 or CCM1 climate models. The occurrence of cool-temperate conifers and hardwoods as far north as 34-35°N, however, indicates less severe cooling than some previous reconstructions. Paleoclimate inferences for the LGM are complicated by lowered atmospheric CO 2 concentrations, which may be responsible for the open nature and dominance of conifers in LGM vegetation.