The paleoecologic record of the northern Rocky Mountains shows considerable spatial variability, which has been difficult to explain as a unified response to large-scale changes in effective moisture or temperature through time. Comparison of past and present climatic patterns suggests that the spatial heterogeneity in the fossil record arises from a local response to the effects of greater summer solar radiation during the early Holocene. Solar radiation directly determines evapotranspiration and indirectly controls the strength of the summer monsoon and the eastern Pacific subtropical high-pressure system. The Yellowstone region provides an example of an area where the indirect effects of solar radiation are manifest by the juxtaposition of two contrasting climatic regimes today and where these regimes were amplified during the early Holocene. Fossil-pollen data suggest that central and southern Yellowstone (the summer-dry area) became drier in the early Holocene, while northern Yellowstone (the summer-wet area) became wetter. The boundary between regimes, however, is controlled by topography and has not changed significantly in the Holocene. The spatial variations of Holocene vegetation and climatic changes in the northern Rocky Mountains may therefore be explained as a trade-off between strengthened monsoonal circulation and a stronger subtropical high in the early Holocene and the relative importance of these precipitation regimes on a topographically complex landscape.
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