Tree rings tell drought story
By Cherry Sokoloski
North Forty News
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Tree ring science is not exactly one of the core courses in a college
curriculum. Tree ring scientists aren't usually in the headlines, either,
except in times of extreme drought such as Colorado is now experiencing.
This year, they're getting lots of attention.
Connie Woodhouse, a tree ring scientist for the National Oceanic and Atmospheric
Administration in Boulder, recently spent a day in the Poudre River drainage
to sample cores from aged ponderosa pines, estimated to be 300 to 400 years
old. She and fellow scientists were looking for climate patterns in the
trees, hoping to compare this year's growth with that of other drought
years over the past several centuries. Scientists such as Woodhouse are
called paleoclimatologists, meaning those who study ancient climates.
Woodhouse studies climate change by looking at tree rings. Each ring reflects
the amount of growth in a given year, with wide rings denoting years of
extensive growth and narrow rings just the opposite. The factors affecting
growth vary depending upon location and the species of the tree. For instance,
trees growing high in the mountains near tree line are limited more by
temperature than by moisture. The ponderosa pines studied in October near
the Pingree Park Road are affected mostly by moisture.
The samples taken from the Poudre won't be analyzed for a few months, but
Jeff Lukas, a scientist at the University of Colorado, provided results
from two other samples collected by Woodhouse in Wyoming. Taken in August,
they're from near Encampment, south of Saratoga and the Vedauwoo area east
of Laramie. The Encampment rings are the narrowest since 1902, he said,
while the Vedauwoo rings are similar to those of 1880.
Rings vs. streamflow
Water watchers are particularly interested in drought patterns this year,
especially the length and pattern of former droughts, so they can make
some guesses about how long this drought is likely to last. They are also
interested in approximate streamflows during previous droughts.
Scientists have developed statistical models that use tree rings to estimate
streamflow in centuries past. To construct the model, they use actual streamflow
data for a given year and compare that to tree ring measurements of the
same year. Streamflow has been measured for about 100 years along the Front
Woodhouse said the rings of ponderosa pine and Douglas fir correlate pretty
well with streamflow, since both species are drought-sensitive and their
rings tend to reflect moisture in a given year. Along the Front Range,
studies linking tree rings to streamflow have been made on Boulder Creek,
Clear Creek near Golden and the Cache la Poudre.
Using data from these studies, scientists have discovered patterns to previous
droughts, and many have concluded that the current drought is a one-in-100-years
event. They have also found other years that seem to match this year in
terms of low streamflow: 1723, 1842 and 1880. Tree rings provide only an
estimate, however, of conditions in past centuries.
Along the Poudre, actual gauge measurements show 2002 to have the lowest
streamflow since 1881, when records on the river were first kept.
The first job in tree ring sampling is selecting a tree. Trees used for
climate studies need to be old, and they also need to be in an ecosystem
that reflects climate as purely as possible, without being influenced by
other factors. An optimal location is on a south slope with rocky ground,
so there is no place where water can be collected and stored. The tree
should be in the open, by itself, and should have no visible insect damage.
Once a tree is selected, the process of sampling begins. The technique
is strictly low-tech, a method that has remained virtually unchanged for
more than 100 years.
First, a quarter-inch increment borer is used to get to the center of the
tree. The tool cuts out a pencil-sized core of wood, often producing a
bird-like squawk from the tree. Once the center is reached, an extractor
or "spoon" is inserted into the increment borer. Then, the scientist gently
pulls the extractor out, bringing with it the tree ring sample.
The rings can be seen fairly well as soon as the sample is taken, but the
samples must be returned to the lab for mounting and studying. To protect
the long core, it's inserted into a paper straw - the kind people used
to use for sipping sodas.
The story of the straws, made by Sweetheart, brings scientific laboratories
a bit closer to everyday reality. Woodhouse, along with many other scientists,
actually gets her straws from an establishment in Tucson, Ariz., called
"Uncle Bob's Popcorn." Much of this country's tree ring research began
at the University of Arizona in Tucson, and scientists found that Uncle
Bob was a great source for the paper straws they needed. He helped the
scientists, and they fluffed up his business quite a bit, too.
Sweetheart no longer makes the paper straws, so Woodhouse will likely have
to start using plastic ones instead.
The core samples, in their Sweetheart wrappings, are dried on the way back
to the lab, by sitting in the sun on a research vehicle's dashboard. At
the lab, the samples are glued onto mounts and sanded, making individual
rings much easier to see.
Dendrochronology--the study of time through tree rings--has its American
roots in the beginning of the 20th century, with the work of A.E. Douglas.
Tree rings contribute to three main areas of knowledge: climate study,
forest ecology and archaeology. Tree rings can tell the history of forests,
including what species were dominant in given periods. They can also tell
about insect infestations and forest fires.
Tree ring patterns have been used to date Anasazi ruins of the Southwest.
To accomplish this, scientists use both living trees and dead stumps, looking
for overlapping patterns which indicate overlapping years. With this method,
they can extend the tree ring log back several centuries, and stumps found
at Anasazi sites can be dated. Rings can also date ancient avalanches,
and they can chronicle the effects of air pollution.
So, is walking around in the mountains and boring trees, well, boring?
Not according to Woodhouse. On that lovely October morning up the Poudre,
someone commented that coring trees was a great way to spend the day. She
responded, "It's a great way to spend a life!"