Paleo Slide Set: Packrat Middens: Vegetation & Climate Variability in the Southwestern United States
Fossil packrat midden records of piņon pine along a north-south transect from Mexico to Colorado

Piņon-juniper 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 piņon 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 piņon 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 piņon 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 piņon'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. 

Photo Credits:
Diagram provided by Thomas R. Van Devender, and Julio L.Betancourt, U.S. Geological Survey 
Arizona Sonora Desert Museum & U.S. Geological Survey