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U.S. Forest Service

Climate Changes and Evolving Plants

About 20,000 years ago, North America was in the grip of an Ice Age. Northern regions were covered with ice and more southerly regions were wetter and cooler than today. In the Southwest, desert vegetation was restricted to elevations of less than 1,000 feet in Death Valley and at the mouth of the Colorado River. Extensive pinyon-juniper-oak woodlands covered elevations of 1,000-5,500 feet. Now, these regions are mostly desert. Spruce-fir, mixed-conifer, and subalpine forests covered areas similar to present-day pinyon-juniper woodlands. Surprisingly, ponderosa pine (Pinus ponderosa), a tree that today extends from central Mexico along the axis of the Rockies into Canada was virtually absent. Large mammals including mammoths, camels, horses, ground sloths, dire wolves, and saber-toothed tigers roamed the region.

About 11,000 years ago at the end of the Pleistocene, the Southwest grew hotter and drier. Most of the large mammals went extinct for reasons that are now hotly debated. Some say humans caused the extinctions, others blame climate change, and others propose a combination of causes. The plants and small animals migrated either northward or up mountainsides so now plant communities similar to those of the Ice Age occur at elevations about 2,500 feet higher than they did then.

Two maps of Arizona and New Mexico showing and comparing the vegetation distribution in the Pleistocene and the present. Maps adapted from Allen, Bettencourt, and Swetnam 1997.

Plants and animals that migrated up Sky Island mountainsides were trapped and isolated from other populations of the same species. Sometimes just a few individuals migrated up a particular mountainside (founder effect) and thus they lacked the full genetic variation of the species as a whole. Once isolated in a mountain range, the plants could no longer interbreed with individuals of that species from other mountain ranges. Finally, the plants began responding to local environmental influences that were different from those in other mountain ranges. These three conditions, founder effect, isolation, and local adaptation are the recipe for plant evolution. However, the length of time for this to happen, about 11,000 years, is relatively short in evolutionary terms. Nevertheless, botanists have found some examples of plant evolution taking place. They have assigned new scientific names to the new plants. Two examples are described in the following stories.

Examples of Sky Island Plant Evolution

Pack Rats Help Tell the Climate Change Story

Pack rats (Neotoma spp.) store plant and animal materials in their nests. The material becomes cemented into large crystallized urine masses, called “amberat”, that remains preserved for thousands of years. The preservation is excellent, allowing identification of species, and the deposits can be dated with the radiocarbon method. About 2,500 of these deposits have been analyzed and dated giving a clear picture of vegetation changes in the Southwest since the last ice age.

Pack rat. Pack rat (Neotoma spp.). Public domain photo.

Further Reading

Allen, C.D., J.L. Betancourt and T.L. Swetnam. 1997. Land Use History of North America (LUHNA): The Paleobotanical Record.

Next: Sky Island Plant Communities…