VEGETATION AND CLIMATE-CHANGE IN NORTHWEST AMERICA DURING THE PAST 125 KYR

Vegetation records spanning the past 21 kyr in western North America d isplay spatial patterns of change that reflect the influence of variat ions in the large-scale controls of climate(1). Among these controls a re millennial-scale variations in the seasonal cycle of insolation and the size of the ice sheet, which affect regional climates directly th rough changes in temperature and net radiation, and indirectly by shif ting atmospheric circulation. Longer vegetation records provide an opp ortunity to examine the regional response to different combinations of these large-scale controls, and whether non-climatic controls are imp ortant. But most of the longer North American records(2,3) are of insu fficient quality to allow a robust test, and the Long European records (4-9) are in regions where the vegetation response to climate is often difficult to separate from the response to ecological and anthropogen ic controls. Here we present a 125-kyr record of vegetation and climat e change for the forest/steppe border of the eastern Cascade Range, no rthwest America. Pollen data disclose alternations of forest and stepp e that are consistent with variations in summer insolation and global ice-volume, and vegetational transitions correlate well with the marin e isotope-stage boundaries. The close relationship between vegetation and climate beyond the Last Glacial Maximum provides evidence that cli mate variations are the primary cause of regional vegetation change on millennial timescales, and that non-climatic controls are secondary.