CLIMATE AND GEOLOGY HAVE SHAPED YELLOWSTONE’S ECOLOGICAL HISTORY

The late-Quaternary history of the Greater Yellowstone Ecosystem (GYE) is among the best studied in the world, thanks to interdisciplinary research led and inspired by Ken Pierce. Among his contributions is recognition that GYE is a geo-ecosystem in which both climate and geological events have shaped the long-term ecosystem dynamics of the region. Our understanding of the postglacial climatic and ecological history of GYE comes from pollen, charcoal, and geochemical data obtained from a network of small lakes across a range of elevations in GYE. These records show a history that is largely shaped by millennial-scale climate variations, beginning with establishment of tundra communities soon after glacial recession followed by the development of parkland and then forest in response to late-glacial warming. From 12.5-7.5 ka, warmer summers than present (present=pre-Industrial) and high fire activity led to a decline in late-glacial spruce dominance and an expansion of whitebark pine at all elevations. With continued mid-Holocene warming, whitebark pine communities were replaced by lodgepole pine forest and Douglas-fir parkland at middle elevations across the region. Cool, wet conditions in the last 4-5 kyr have resulted in an expansion of mixed-conifer forest and declining fire activity at many sites. The geologic controls on ecosystem dynamics are also evident in the vegetation history of GYE: Lodgepole pine forests have persisted on well-drained, infertile rhyolitic substrates throughout the Holocene despite changes in climate and fire activity. Low-elevation grasslands in northern Yellowstone are confined to fertile soils underlain by calcareous glacial sediments and shown little change in the last 8 kyr. Preliminary data from Yellowstone’s Lower Geyser Basin reveal that present-day thermal grasslands are relatively young having formed about 4 ka in response to changes in hydrothermal activity not climate. In summary, paleoecologic studies build on and support Pierce’s grand geo-ecosystem vision. The evidence also suggests that, while ecological change in the future will be governed by climate variations, specific responses will be strongly modulated by geology.