Impacts of Climate Change and Land Use  on the Southwestern United States

Impacts of climate change on life and ecosystems

Land Use History of North America (LUHNA):
The Paleobotanical Record


Craig D. Allen

U.S. Geological Survey, Midcontinent Ecological Science Center

Julio L. Betancourt

U.S. Geological Survey, Desert Laboratory

Thomas W. Swetnam

University of Arizona, Laboratory of Tree-Ring Research

LUHNA - The Paleobotanical Record An important backdrop for evaluating human impacts on southwestern landscapes is the long-term dynamics of vegetation change. Glacial climate and vegetation patterns have characterized most of the Pleistocene (the past 1.2 million years). Just 12,000 years ago, the Earth underwent major environmental changes in the transition to the current interglacial period, the Holocene. Dramatic swings in atmospheric chemistry and climate, as well as global ice volumes and sea level, coincided with massive shifts in biotic distributions. Vegetation change in humid areas has been reconstructed from analysis of pollen grains preserved in lake sediments, but pollen analysis has limited utility in arid regions. In the arid interior of North America, a novel way of reconstructing vegetation change has been the analysis of plant and animal remains preserved in fossil packrat (Neotoma spp.) middens, deposits that are ubiquitous in rocky environments. About the size of a laboratory rat, packrats gather plant materials at close range (ca. 100 m at most) and accumulate them in dry caves and crevices; there, the plant and other debris (including arthropod and vertebrate remains) are cemented into large masses of crystallized urine (referred to as amberat), which can persevere for tens of thousands of years. About 2,500 of these deposits have been dated within the limit of the radiocarbon method (the last 50,000 years) and analyzed for plant and animal remains (Betancourt and others 1990). The preservation of plant remains in packrat middens is excellent, allowing identification to species and empowering diverse morphological, geochemical, and genetic analyses (e.g., Van de Water et al. 1994, Smith et al. 1995, Hunter and others - in review). The extensive archive of sorted, identified and dated material represents the richest and best-documented source of plant remains in the world, with hundreds of species identified and available for corollary studies. A map of of Pleistocene vs. modern vegetation in the American Southwest shows remarkable changes during the last 12,000 years (Figs. 1, 2); some of these changes are ongoing and may confound cultural vs. natural explanations for modern changes in vegetation (Fig. 3) (Figs. 4, 5).

Figures 1 & 2 - As this comparison of modern and Pleistocene vegetation shows, southwestern landscapes have changed dramatically since the end of the last ice age, 12,000 years ago. During the last ice age, desert vegetation was restricted to the lower elevations (<300 m) in Death Valley and the mouth of the Colorado River. Hallmarks of the Sonoran Desert, such as the giant cactus, saguaro (Carnegiea gigantea) and the palo verde (Cercidium), with its photosynthesizing bark, were displaced far south into Mexico. Creosotebush (Larrea tridentata), the dominant shrub of the Chihuahuan, Sonoran and Mojave deserts, had its northernmost populations along the Arizona-Sonora border. Extensive pinyon-juniper-oak woodlands, now restricted to the highlands, covered what are now desert elevations (300-1700 m). The extensiveness of spruce-fir, mixed-conifer or subalpine forests and woodlands during glacial times is evidenced by the fact that they covered much the same territory as modern pinyon-juniper woodlands, currently the third largest vegetation type in the U.S. (20 million hectares). The biggest surprise from the packrat midden record is the virtual absence of ponderosa pine (Pinus ponderosa), a tree that today extends from central Mexico along the axis of the Rockies into Canada. In the U.S., much of that range developed through migration during the last 10,000 years. Populations of this commercial species in the northern Rockies and western High Plains may represent arrivals within the last few millennia and perhaps the last few centuries. In these areas, increasing stand densities may be the consequence of natural expansion rather than fire suppression, or some combination of both.

Figures 4 & 5 - An isolated stand of pinyon pine (Pinus edulis) at Owl Canyon, north of Ft. Collins, Colorado represents the endpoint of its northward migration since the end of the last ice age (Betancourt et al. 1991). This 5 sq. km stand was colonized by pinyon pine less than 500 years ago, possibly from accidental plantings by Cheyenne and Arapaho, who carried pinyon nuts in their "trail mix" on their treks along the Front Range. The nearest potential source populations are 250 km to the south near Colorado Springs. It is unclear what role humans played in propagation of seeds and plant migration during the Holocene. The older photograph, looking north into Wyoming, was taken ca. 1950 by J. D. Wright, the more recent image by R. M. Turner in 1989. Note the increase in canopy cover during only 40 years, which is characteristic of an expanding population. This expansion of pinyon pine could be mistakenly ascribed to more recent human impacts such as fire suppression and overgrazing.


Betancourt, J.L., Van Devender, T.R., and P.S. Martin. 1990. Packrat middens: The last 40,000 years of biotic change. University of Arizona Press, Tucson, AZ. 467 pp.

Betancourt, J.L., Schuster, W.S., Mitton, J.B., and R.S. Anderson. 1991. Fossil and genetic history of a pinyon pine (Pinus edulis) isolate: Ecology 72:1685-1697.

Hunter, K.L., Riddle, B.R., Betancourt, J.L., Cole, K.L., Van Devender, T.R., and W.G. Spaulding. In review. Polyploidy changes during the past 21,000 14C years in the North American desert shrub Larrea tridentata. Science.

Smith, F.A., Betancourt, J.L. and Brown, J.H., 1995. Effects of global warming on woodrat (Neotoma cinerea) body size during the last deglaciation. Science 270: 2012-2014.

Van de Water, P.K., Leavitt, S.W., and J.L. Betancourt. 1994. Trends in stomatal density and 13C/12C ratios of Pinus flexilis needles during the last Glacial-Interglacial cycle. Science 264:239-243.

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