Geoscience Reference
In-Depth Information
the summers. Edlund and Byrne concluded that the Sierras must have lacked
the pronounced summer drought that is so characteristic of the region today.
h e pollen preserved in Lake Moran sediments also shows that the early
Holocene was a time of rapid change. Over the course of several thousand
years, the pine forests thinned, transforming the region into a more open
landscape occupied by drought-adapted oaks as the climate continued to
warm and dry. h is warming reached a maximum about 7,000 years ago.
Pollen evidence from other lakes throughout the West also suggests a
period of warm and dry climate during the early Holocene. Closed-basin
lakes grew smaller as the climate grew drier. h ese lakes include Pyramid
Lake in western Nevada and Owens Lake in eastern California, both of
which decreased in size between 8,000 and 6,500 years ago.
charcoal and wildfires
h e climatic trends indicated by vegetation change are also rel ected in the
frequency and intensity of wildi res in the West. For instance, the abundance
and size of charcoal fragments that reached Lake Moran increased during
times of large forest i res. h ese charcoal fragments settled to the bottom
of the lake and became part of the bottom sediments. Charcoal can range
in size from i ne dust particles to large chunks up to an inch or more across.
h e i ner particles are carried longer distances in the wind. Furthermore,
larger, more intense i res produce higher plumes of air that rise above them,
carrying charcoal farther from the site of a i re. Because large forest i res ot en
occur during periods of warmer and drier climate (particularly if they follow
a wetter period, which increases the amount of vegetation that later becomes
fuel), the size and amount of charcoal fragments in buried sediments provide
additional evidence for past climate conditions.
At Lake Moran, the charcoal record suggests that large i res were uncom-
mon during the late Pleistocene. h is comes as no surprise, given the wetter
and cooler climate that prevailed. But during the transition into the early
Holocene, charcoal abundances increased as summers became warmer and
drier. h is increased amount of charcoal from forest i res remained high until
about 7,000 years ago. In fact, the early Holocene appears to have been a
period of more extensive and intense forest i res than any time since. Byrne
and Edlund speculated that the large number of forest i res may have sig-
nii cantly shaped forest communities, and therefore i re would have been
