VEGETATION AND CLIMATE CHANGE IN EASTERN NORTH-AMERICA SINCE THE LAST GLACIAL MAXIMUM

Response surfaces describing the empirical dependence of surface pollen percentages of 13 taxa on three standard climatic variables (mean July temperature, mean January temperature, and mean annual precipitation) in eastern North America were used to infer past climates from palynological data. Inferred climates at 3000-yr intervals from 18 000 years ago to the present, based on six taxa (spruce, birch, northern pines, oak, southern pines, and prairie forbs), were used to generate time series of simulated isopoll maps for these taxa and seven others (hickory, fir, beech, hemlock, elm, alder, and sedge). The simulations captured the essential features of the observed isopoll maps for both sets of taxa, including differences in migration patterns during the past 10000 yr that have previously been attributed to differential migration lag. These results establish that the continental-scale vegetation patterns have responded to continuous changes in climate from the last glacial maximum to the present, with lags less-than-or-equal-to 1500 yr. The inferred climatic changes include seasonality changes consistent with orbitally controlled changes in insolation, and shifts in temperature and moisture gradients that are consistent with modelled climatic interactions of the insolation changes with the shrinking Laurentide ice sheet. These results pose new ecological questions about the processes by which vegetated landscapes approach dynamic equilibrium with their changing environment.