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 piņon-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.
University of Arizona
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