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,
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:
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.