By
David W. Stahle,University
of Arkansas, Fayetteville
Dr. Stahle cores trees with Alan Alda in the segment,"What
Happened at Jamestown" |
Thanks
to a number of natural phenomena, our knowledge of prehistoric
climatic conditions is surprisingly detailed. Scientists
have devised several ingenious methods to measure environmental
conditions from the centuries- and even millennia- before
modern meteorological instruments. Some of these methods
are so accurate they reveal seasonal climate conditions
with exact calendar year dating and can provide the environmental
context for understanding human history.
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Forest
Archives
The
annual growth rings of many tree species provide an excellent
record of climatic variability. The rate of growth among certain
trees is directly linked to moisture availability, so fat
growth rings indicate wet years and skinny growth rings document
drought years.
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Conservationist
Jamie Sayen (left) and dendrochronologist Ed Cook pose
with a Sycamore.
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Dendrochronologists, scientists who study tree-rings, take
narrow core samples from living trees. We know the exact calendar
date of the outside ring, so we can synchronize the patterns
of wide and narrow rings back in time among hundreds of individual
trees in a given climatic zone. By matching the rings among
many trees, patterns emerge that reflect the unique history
of favorable and unfavorable climate, giving dendrochronologists
an accurate chronology of growth that corresponds exactly
to each and every calendar year.
Some
trees can live to an extraordinary age, allowing us to paint
a clear picture of the climate over several hundred years.
By examining moisture-sensitive baldcypress trees along the
Blackwater River in southeastern Virginia, my colleagues and
I were able to determine the environmental conditions during
the early English colonization of North America.
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Growth
rings from a baldcypress show the droughts that devastated
the Jamestown and Roanoke settlements.
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Core
samples from these very old trees revealed that the first
colonists at Jamestown Island had the misfortune to arrive
during one of the worst extended droughts in centuries (lasting
from 1606-1612). The three-year drought from 1587-1589 was
in fact the most extreme drought in 800 years, and may have
been an important factor in the disappearance of the Lost
colony of Roanoke Island, one of the great mysteries of American
history. Though the Jamestown and Roanoke colonists have been
criticized for poor planning, the tree-ring data show that
even well prepared settlers would have been seriously threatened
by the climatic conditions they faced upon arrival in the
New World.
Dendrochronologists
have also used tree-ring data to reconstruct climate and crop
yields of the Anazazi on the Colorado Plateau some 1000 years
ago. The
roof timbers found in these ancient ruins have been exactly
dated, indicating the year the timbers were cut with stone
axes for construction. By matching the construction dates
with the climate and crop yield chronologies, we find that
the remote cliff dwellings and other villages of the Anazazi
were usually constructed during times of favorable climate
and crop surplus. Unsurprisingly, village abandonment often
appears to have occurred during drought and crop
shortfall.
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Photos:
University of Arkansas
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