VEGETATION AND FIRE HISTORY IN THE GREATER YELLOWSTONE REGION, FROM THE LGM TO THE EARLY HOLOCENE

Fossil pollen and charcoal records from the Greater Yellowstone region provide information on the climate, vegetation, and fire history from the­ LGM to the early Holocene (~21 ka – 7 ka). During the LGM, nonglaciated valleys and exposed ridges supported tundra communities and isolated populations of conifers, including Engelmann spruce, subalpine fir, whitebark pine, lodgepole pine, and possibly Douglas-fir. After 17 ka, rapid warming triggered a sequence of plant migrations that followed ice recession. In most areas, tundra communities with birch, aspen, willow and juniper developed. Parklands of Engelmann spruce, whitebark pine, and subalpine fir replaced these communities ~13 ka as growing season temperatures increased; however, in central Yellowstone, tundra communities transitioned directly to lodgepole pine forest as nutrient-poor rhyolite substrate limited the colonization of other conifers. The Younger Dryas Cold Interval (~12.9 – 11.5 ka) is not evident as a distinct vegetation reversal in Yellowstone, but a slowing of reforestation at mid-elevations (~2000 m) suggests a protracted cool period that coincides with YD glacial advances in the Wind River Range. Between 12 and 11 ka, closed subalpine forests replaced parkland vegetation at most sites, and fire activity increased significantly. The early Holocene (10 – 7 ka) featured warmer-than-present summer conditions in Yellowstone, but effective moisture varied. In summer-wet (high summer: annual precip) areas of northern Yellowstone, growing season moisture was sufficient to support closed pine and pine/juniper forests and fire activity was low. Drought in summer-dry (low summer: annual precip) areas of the region featured high fire activity and open forests of Douglas-fir and steppe communities at low to mid-elevations and lodgepole pine at high elevations. Substrate continued to play an important role in the vegetation history of central Yellowstone, as nutrient-poor rhyolite limited the establishment of conifers other than lodgepole pine despite climatic changes. Thus, the paleoecologic record suggests that long-term variations in climate have been the large-scale driver of regional vegetation change, but nonclimatic factors, including substrate and elevation, were important in shaping subregional vegetation differences.