REGIONAL SCALE CHANGES IN CLIMATES AND BIOMES SINCE THE LAST GLACIAL MAXIMUM IN WESTERN NORTH AMERICA

Western North America (WNA) is a generally arid region and moist environments are restricted to high mountains and to coastal northern California and the Pacific Northwest (PNW). Today, cool-season precipitation dominates the western and northern sectors of the region, whereas summer monsoonal rainfall is important in the southeastern and eastern sectors. Forest biomes are restricted to the wettest environments and woodland, steppe, and desert biomes dominate the lower elevations of the interior.

Fossil pollen and plant macrofossils provide evidence of changes in climates and biomes since the Last Glacial Maximum (LGM). At the LGM the PNW was dry and steppe biomes existed in what are now forests, whilst the currently arid Southwestern deserts were moist and supported woodlands. Relative to today, plant species' ranges shifted southward and to lower elevations. Forests were limited in extent, perhaps due in part to reduced water-use efficiency related to low levels of atmospheric carbon dioxide.

Following the LGM, plant species began to disperse upslope and northward. By 11,000 cal yr BP, forests were established in the PNW and in Southwestern mountain ranges, although woodlands persisted at lower-than-present elevations in the deserts. Desert biomes, almost absent at the LGM, spread northward from Mexico. Treelines were higher than today, suggesting warmer-than-present summer temperatures, and the persistence of Southwestern forests and woodlands below their current elevational limits indicates enhanced monsoonal precipitation.

By 6000 cal yr BP, conditions were more arid on the northeastern Great Plains and across the Great Basin and Colorado Plateau. In contrast, aridity decreased in the PNW and forests expanded into what had been steppe. Treelines remained above present elevations across much of WNA, and desert biomes approached their modern geographic coverage. Over the past 6000 years, aridity has decreased in all parts of WNA except for the Southwestern deserts. Forest biomes have expanded in the Sierra Nevada and parts of the PNW, and the forest/grassland boundary has moved westward on the northeastern Great Plains.

The conditions represented by the LGM through the Holocene exhibit nearly as large of a range of conditions as have occurred in this region through the past 800,000 years.