Landscape History - Mountains and Plains: The Ecology of Wyoming Landscapes - page 22

also existed in relatively cool, moist valley bottoms at
lower elevations. Droughts during this time killed some
of the grasses that stabilized sand dunes in the region,
resulting in more blowing sand and shifting dunes.
there is evidence that the Platte and other large rivers
were sometimes dry during this period. 37
the work of paleoecologists also has been facilitated
by the middens of bushy-tailed woodrats, often referred
to as packrats, because they collect leaves, seeds, cones,
bones, and other material and deposit them in their
nests (fig. 2.9). not particularly tidy, the woodrats uri-
nate and defecate on their middens time and time again,
creating hard masses that often persist for thousands of
years. By identifying the plants represented in differ-
ent layers of a midden, and studying middens of dif-
ferent ages, scientists draw conclusions about how the
vegetation has changed where the woodrats have for-
aged. Radiocarbon dating provides a rather precise time
frame, with one midden in northeastern Utah having
deposits that are 40,000 years old. 38 Studying vegeta-
tion change using pollen or middens found at treelines
is especially instructive, because trees are sensitive to
water stress. thus, climate change is easily detected as
the tree fragments appear and disappear from top to
bottom in the middens.
Middens have provided some surprising informa-
tion. First, although it is commonly thought that most
of the same plant species have been in Wyoming for

the past 2 million years or so, now it is clear that Utah
juniper arrived in southwestern Wyoming about 9,000
years ago, having migrated roughly 200 miles north-
ward as the climate warmed following glaciation. 39 even
more surprising, ponderosa pine did not reach the Big-
horn Mountains of Wyoming until about 3,000 years
ago. 40 Pinyon pine did not arrive in the state until about
700 years ago and is still found naturally only where
the Green River flows into Utah. 41 this small tree also
occurs in northern colorado, just south of owl canyon
on the east side of U.S. highway 287.
University of Wyoming paleoecologist Stephen t.
Jackson and his colleagues studied middens in the
foothills of the Pryor Mountains, at an elevation of
about 5,000 feet on the north side of the Bighorn Basin
(fig. 2.10).42 42 they observed the following changes. About
10,000 years ago, when glaciers were retreating up the
mountains, the climate in the foothills was cool and
humid, as indicated by the presence of ground juni-
per, Rocky Mountain juniper, chokecherry, wood rose,
and ferns—all of which today are found primarily in
relatively moist woodlands and shrublands. in more
shallow layers of the middens, deposited about 8,000
years ago, fragments of drought-tolerant plants were
found—namely, pricklypear cactus, saltbush, winterfat,
and later, Utah juniper. the plants represented in older
midden deposits were absent. From about 4,400 to 2,700
years ago, limber pine expanded into the area, for rea-
sons that are not yet clear. 43 About 2,700 years ago, more
arid conditions returned, as indicated by the absence of
limber pine, presumably because it could not survive the
droughts. Utah juniper and curlleaf mountain-mahog-
any became more common, both of which are abundant
around the middens now.
this Bighorn Basin research hinges to a large degree
on the presence or absence of the various species of
shrubs and trees at lower elevations. Another study,
conducted in the Wind River Mountains by Patricia Fall
and her associates from Arizona State University, stud-
ied changes in alpine treeline using pollen data from
a lake at about 10,200 feet. 44 they found that, prior to
about 11,300 years ago, with glaciers still nearby, the
climate was relatively cold, and the vegetation around
the lake was largely alpine tundra. However, by about
10,600 years before present, forests of whitebark pine,
engelmann spruce, and subalpine fir surrounded the

Fig. 2.9. Bushy-tailed woodrats are 11-18 inches long. About
half their length is tail. the rodents gather identifiable plant
materials for their nests, which may persist for more than
10,000 years. the nests are known as middens. Photo by Phil
Dotson / Science Source.