Landscape History - Mountains and Plains: The Ecology of Wyoming Landscapes - page 21

PINE SAGEBRUSH

SAGEBRUSH PINE

wetter drier

8 7 65432123456789101112

14 C (ybp)

yr B.P.

3,520

HOLOCENE

5,850

8,460
10,060
10,200

TRANSITION

25,540
22,300

20,880

PLEISTOCENE

21,890

21,190

26,640

Fig. 2.8. Fossil ice wedge exposed at the Rawlins landfill,
which suggests that a tundra-like environment existed during
the Pleistocene (Mears 1981, 1987). the rod is 3 feet long.
elevation 6,900 feet. Photo courtesy of Brainerd Mears, Jr.

>29,000

TEPHRA
61,000 ybp

and Douglas-fir forest at middle elevations. Forest fires
became more common. in the past 6,000 years, the cli-
mate has generally cooled, allowing spruce, subalpine
fir, and whitebark pine to expand their range. this trend
was reversed during brief, warm, dry periods, such as
the Roman and Medieval warm periods, about 2,000
and 1,000 years ago, respectively. these intervals of past
drought provide important insights on how ecosystems
will respond to the warmer, drier conditions projected
for the future (see chapter 3).
A comparatively warm, dry period spanning 3,000
years in the early Holocene caused an expansion of
big sagebrush, greasewood, juniper, and grasses in
the lowlands. During this time, grasses covered some
north-facing slopes up to 7,900 feet elevation in the
mountains. 36 Most mountain tree species, such as
engelmann spruce, subalpine fir, and lodgepole pine,
survived at higher elevations, though they probably

Fig. 2.7. the ratio of pine and sagebrush pollen has changed
during the past 60,000 years. A higher peak on the left indi-
cates that pine pollen was more abundant than sagebrush, an
indication that the climate was wetter; a higher peak on the
right indicates that sagebrush pollen was more abundant. Based
on data from Grays Lake, southeastern idaho. 14 c yr B.P. = years
before present as determined by carbon-14 dating. Adapted
from Beiswenger (1991).