|Paleo Slide Set: Packrat Middens: Vegetation & Climate Variability in the Southwestern United States|
|Fossil packrat midden records of piñon pine along a north-south transect from Mexico to Colorado|
woodlands presently cover 20 million hectares above 1500-m elevation in
the western United States and comprise the third largest vegetation type
in the contiguous U.S. During the late Pleistocene, these woodlands covered
what are now the hot deserts of the southwestern U.S., mostly below 1500-m
elevation. This diagram depicts fossil packrat midden records of piñon pine
along a north-south transect from Mexico to Colorado. The tickmarks on each
vertical line represent over 350 radiocarbon-dated middens that show the
presence or absence of piñon pines along a 15 degrees latitude (ca.
1600 km) transect from Bermejillo, Mexico (Durango Province) to Ft. Collins,
Colorado. The diagram illustrates the local extinction of piñon populations
growing at desert elevations during the last deglaciation (ca. 11,000 14C
yr. BP) and the sequential migration to higher elevations and more northerly
latitudes during the Holocene (the last 11,000 years). Note that piñon's
distribution in the state of Colorado may be just a few hundred years old
and probably is not yet in equilibrium with modern climate.
In Colorado and probably northern New Mexico, this disequilibrium makes
it difficult to discriminate between the natural migration during the late
Holocene and the historical tree expansion due to fire suppression and overgrazing.
Diagram provided by Thomas R. Van Devender, and Julio L.Betancourt, U.S. Geological Survey
Arizona Sonora Desert Museum & U.S. Geological Survey
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