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.
References
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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