HOLOCENE CLIMATES AND CHANGES IN BIOTIC COMMUNITIES IN THE INTERIOR OF THE WESTERN UNITED STATES

During the last glacial period westerly storm tracks shifted southward into the modern desert regions of the Interior western United States, bringing moisture from the North Pacific into this currently arid region. The strength and extent of the summer monsoon was considerably diminished during this time. Plant and animal species lived farther south and at lower elevations than they currently occur. Pinyon pines, junipers, and other woodland plants lived in areas that now host creosote bush, saguaros, and other hot desert species. By 11,000 years ago, pinyon pines disappeared from the lower elevation sites, followed over the next few thousand years by junipers as desert communities became established within their modern ranges. Although this pattern suggests that the westerlies were shifting northward, plant and animal species remained below their modern elevational limits into the early Holocene in many places, indicating the persistence of cooler-than-present summers. In a seemingly discordant pattern, upper treeline advanced rapidly upslope during the Pleistocene-Holocene transition and was as much as 80 m above modern limits by 10,000 years ago in Colorado, with the implication that growing season temperatures were higher than today at that time. This apparent contradiction persisted into the middle Holocene, as conditions became warmer and drier in the valleys of the Mojave Desert and Great Basin, whilst upper treeline declined in elevation in Colorado. The contradiction is not evident for the late Holocene, as cooler and moister conditions returned to these valleys in the late Holocene but there was no corresponding rise in the elevation of upper treeline.

The fossil record suggests that there were geographic and elevational gradients in the kinds and degrees of Holocene climatic changes and that there were disparate reactions to these changes among different species. It is likely that the source areas and trajectories of air masses changed through time, as did the seasonality of the associated precipitation. The Holocene record of change in the Interior suggests that there may be no simple northward and upward shift of biota in response to warming climates.