Paleobotany
and paleoclimate of the southern Colorado Plateau (page 1 of 2)
Adapted by R. Scott Anderson from R. Scott Anderson, Julio L. Betancourt, Jim I. Mead, Richard H. Hevly, David P. Adam. 2000. Middle- and late-Wisconsin paleobotanic and paleoclimatic records from the southern Colorado Plateau, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 158: 25-43.
Introduction
The biota of the Colorado Plateau during the middle (50,000-27,500 B.P.) and late (27,500-14,000 B.P.) Wisconsin time periods was dramatically different from that seen today. Evidence for these significant changes is found in packrat middens, alluvial and cave sites, and in ancient pollen samples collected from lake, bog, and wetland sites throughout the region. As an example, areas that are today forests of ponderosa pine were thickly-forested with a mixed assortment of different conifers, including subalpine species such as Engelmann spruce which today grow only at the highest elevations, thousands of feet above their former range. Differences are primarily a result of major climate changes associated with the last major glacial period. Studies indicate that during the middle-Wisconsin temperatures on the Colorado Plateau were approximately 3-4 degrees Celsius cooler than they are today, and perhaps 5 degrees cooler during the late-Wisconsin. There is also some evidence that these time periods were wetter as well, resulting in an environment on the plateau very different than that of today.
Regional Paleoenvironment Reconstructions
Middle Wisconsin (50,000-27,500 years before present)
During the Middle Wisconsin at middle elevations along the Mogollon Rim, forests comprised of Engelmann spruce, white fir, Douglas-fir, and sagebrush were common. Engelmann spruce today is a dominant species of subalpine forests on the plateau, while white fir and Douglas-fir are common in mixed-conifer forests. Sagebrush was a major understory species at the time, but today it is not common in the area or with the above-noted species. At higher elevations on the San Francisco Peaks and White Mountains, studies at Walker Lake and Hay Lake indicate that the subalpine forests of the Middle Wisconsin were similar to those we see today, but with sagebrush being a more common associate of the high-elevation conifers. The structure of the forest may have been more open than it is today as well. A lower elevation (1700m) packrat midden discovered in Canyonlands National Park yielded Rocky Mountain juniper, limber pine, and Douglas-fir macrofossils. Today these species are generally confined to areas above 2500 meters, particularly limber pine. The absence of ponderosa pine in this midden is notable. At the lowest elevations on the southern Colorado Plateau, a juniper-desert scrub community was widespread, and consisted mostly of Utah and one-seed juniper, sagebrush, prickly-pear cactus, agave, and saltbush.
Late Wisconsin (27,500-14,040 years before present)
Much more information is available for this time period than for the Middle Wisconsin. At the aforementioned Walker Lake site northwest of Flagstaff, this time period saw a decrease in pollen from subalpine pine species and a maximum of spruce and subalpine fir pollen recorded from 24,000 to 22,000 years BP. At Potato Lake on the Mogollon Rim, a mixed-conifer forest was converted to nearly pure Engelmann spruce after about 22,000 years BP. This period was likely the coldest during the last glaciation, with the elevational range of many species depressed 570 to perhaps 900 meters. Today Engelmann spruce is found generally above 3300 meters, but during the late Wisconsin it commonly grew at elevations as low as 2500 meters. At elevations from 1600 to 2100 meters, a number of sites have been studied, and thus scientists have a better picture of the paleoenvironment of the southern Colorado Plateau at these elevations during the late Wisconsin. In the Grand Canyon and at some sites on the central plateau, forests of limber pine, white fir, and Douglas-fir occupied this elevational belt. Common associates included Utah juniper, sagebrush, rosaceous shrubs, agave, and prickly-pear cactus. Below 1400 meters, Utah and one-seed junipers dominated plant communities, mixed with sagebrush and shadscale and blackbrush at the lowest elevations (450 meters).