Geoscience Reference
In-Depth Information
figure 25. An ancient tree stump
submerged in the West Walker River,
eastern Sierra Nevada. (Photo cour-
tesy of D. J. DePaolo, University of
California, Berkeley.)
By counting the annual growth rings of these submerged trees, Stine
showed that the trees had lived upward of 160 years. h e droughts must have
lasted at least that long to allow the trees to survive in that location. h e
trees were i nally killed when the droughts ended abruptly, followed by dra-
matically high rainfall that rapidly raised the lake levels to drown them. Stine
dated the outer growth layers of the tree stumps in Mono Lake and in other
lakes and rivers in the central and eastern Sierra with radiocarbon methods.
He found that the ages of all these trees clustered around two distinct peri-
ods: AD 900-1100 and AD 1200-1350.
Tree-ring studies from a broad region of North America indicate that
climate conditions over the past 2,000 years became steadily more variable,
with especially devastating droughts during this period as compared with
earlier times. h ese records also suggest that the driest period in the West
occurred between AD 900 and 1400. For instance, researcher Edward Cook
and his colleagues at the Lamont Doherty Earth Observatory Tree Ring
Laboratory in Palisades, New York, have studied tree-ring records that rel ect
the summer values of the Palmer Drought Severity Index—a measure of the
moisture content of soil in the root zone. h ey compared tree-ring records
from throughout North America and found that over half of the American
