Paleo Slide Set: Packrat Middens: Vegetation & Climate Variability in the Southwestern United States
Comparison of the late Pleistocene and modern vegetation of the American Southwest.

Modern map (top) vs. Pleistocene map (bottom): Information about former plant distributions from packrat 
middens can be synthesized into regional vegetation maps through time. For example, compare the late 
Pleistocene epoch and modern vegetation of the American Southwest. From Pleistocene to modern, note the 
great contraction of alpine tundra, spruce-fir boreal forests in the southern Rocky Mountains, and the 
elimination of pluvial lakes that once filled large basins. Large expanses of pinon-juniper (pigmy) conifer 
woodlands were replaced by desert scrub. In the late Pleistocene, deserts comparable to modern, such as the 
Mojave Desert inhabited by the creosole bush and the Joshua tree, existed only in the lower reaches of the 
Colorado River Valley.

One of the more remarkable changes between the late Pleistocene and modern vegetation is the expansion of 
ponderosa pine (yellow pine) parklands and woodlands throughout the southern Rocky Mountains. During the 
Pleistocene, ponderosa pine was probably limited to latitudes south of the transect from Albuquerque to 
Flagstaff. During the last 10,000 years, ponderosa pine expanded its distribution to cover all of the 
Rocky Mountain States to the Canadian border. 

Photo Credits:
Cynthia Lunine 
University of Arizona