Trees help unlock climatic history
Aspen, CO Colorado
EAGLE ” On a steep hillside near Eagle there is a library, of sorts.
The slope is 45 degrees and has a crumbly gypsum exterior overlying a hard bed. The steepness and loose material makes it hard to walk, and the slope has little vegetation.
But a few smallish Douglas fir trees grow there, and to scientists expert in studying tree rings, they provide a long record of the climate of the Eagle River Valley.
The oldest living tree on the slope has been dated to the year 1107.
Dead trees now lying on the slope are even older. One such remnant has been traced to 203 A.D., making it the oldest date found in Colorado in a tree studied for the variations in moisture.
This antiquity is entertaining, even startling. Christians were still being periodically slaughtered in the Roman Empire when the older piece of wood began growing. The still-living tree came to life soon after the first Christian crusade to the Middle East.
But there’s greater purpose for examining the tree rings than sheer novelty. Viewed along with other rings from across the Colorado River Basin, the trees document that the early part of the 20th century was, relative to other times, particularly wet.
This simple fact has enormous repercussions. Our system of reservoirs, dams and canals, and the legal apparatus set up to administer the water, is premised on a volume of water that ” viewed against the longer history ” has been rare. It’s like basing your annual household budget on the income you receive when the Christmas bonus arrives.
The trees above Eagle also show a more distant past when droughts were more prolonged than those of recent history, something scientists call “megadroughts.”
Scientists are unsure of what caused these extended droughts. As such, they could return.
If so, says Richard Seager, a scientist at Columbia University’s Lamont-Doherty Earth Observatory, “the water resources in the West would be severely stretched to the point that social conflict, of some sort, would be inevitable as states and cities and different users argue over diminishing resources.”
The trees at Eagle are well known among tree-ring scientists ” called dendrochronologists. The first recorded visit was by a dendrochronologist named Edmund Schulman in 1945. Touring the West on one of his many journeys, he saw the trees standing alone and, following a well-educated hunch, went exploring.
What he found ” a tree that dated to the year 1250 ” has drawn dendrochronologists several times since, most recently Connie Woodhouse. A petite, dark-haired woman, she is both a dendrochronologist and a paleoclimatologist, or one who reconstructs climatic conditions of the past.
Based in Boulder with the National Ocean and Atmospheric Administration during her visits in 2004 and 2006, she is now an associate professor with the University of Arizona’s Department of Geography and Regional Development.
The Eagle site draws tree-ring experts, says Woodhouse, because the trees on this particular north-facing slope “are very sensitive to variations in moisture. It’s a very stressed site.”
Wet years are documented in these trees by wide rings, while narrow rings reflect drought years. The most parched of years, such as 2002, yield nearly non-existent rings.
The steepness of the Eagle site explains the age of the trees. It’s not a place where people would get firewood. Also, the ground is bare, whereas a nearby slope with a more moderate gradient has vegetation. That vegetation helps spread fires, and as a result the trees there are much younger.
What has preserved these Douglas fir trees also makes for exciting times when scientists visit.
“We didn’t use ropes, but we were thinking about it,” says Woodhouse. “You can walk on it if you’re careful, but if you’re carrying a chainsaw, your footing can be pretty precarious. It made me sort of nervous.”
Woodhouse is among several dozen dendrochronologists in the West. Most are associated with universities. The tree rings help tell the stories not only of climate, but of ecology, fire history, and even human settlement.
Eagle isn’t the only site for old trees. A tree near Green Mountain Reservoir in Summit County, for example, has been dated to 300 A.D. Grand Mesa west of Eagle County also has old Douglas fir trees.
Bristlecone pines are even older than Douglas fir, but their growth rings best reflect temperatures, not moisture. Among the sites in Colorado is one called Windy Ridge, located at timberline above Hoosier Pass. Another stand is on Mt. Evans.
Other species of trees are occasionally used. Lodgepole pine are usually skinny and begin dying after about 120 years. Even in that short life, their rings generally yield little useful information.
Spruce and fir trees can live 300 or more years, occasionally longer, and tend to reflect variations in summer temperature in their rings. An Englemann spruce near the Fraser Experimental Forest was 800 years old.
“But old spruce are really hard to find,” says Woodhouse.
Pinon pine are also useful. One stand of pinon trees consulted by dendrochronologist is found north of Gypsum, along Trail Creek. “Those trees are sensitive to moisture, but not quite as old,” says Woodhouse.
Elsewhere, Douglas fir in Utah have been dated to the third century B.C.
At Eagle, the researchers generally collect two cores from each tree. These cores are a little narrower than a standard pencil, or about the same diameter as the round chopsticks you get with takeout Chinese food.
Each of the 7.5 inches of the bore extracted from the oldest living tree at Eagle yielded more than 100 rings.
Tree-ring researchers work backwards from the present. The first task is to establish a chronology using the living trees, first in one area, then in other areas, comparing the rings and how they vary. Severe drought tends to be widespread, in some cases broadly across the Southwest. So the same patterns of narrow rings show up in trees across a larger region.
Once a chronology for living trees has been established, those rings can be compared or cross-referenced against those of dead or remnant trees. The process is much like matching up carpet patterns.
This chronology of both live and dead trees can then be compared with the annual streamflow records. The oldest gage records in Colorado River Basin date to 1894.
With tree rings thus calibrated against known water levels, streamflow can then also be modeled for centuries past.
This peek into the past may be fascinating, but it’s also somewhat troubling. Although 2002 had extremely low stream levels, it was not unique, says Woodhouse.
And while the drought of the 1950s was a hallmark of the 20th century in the Southwest, it wasn’t unusual at all when viewed against previous centuries.
“We have had a couple of those droughts every century in the past,” she says.
Further in the past are records of sustained and sometimes severe droughts. Most notable was what paleoclimatologists called the Medieval Period, from A.D. 800 to 1300.
“We had 13 years in a row below long-term averages embedded in a period of about 60 or 70 years when there really weren’t many big wet years, such as those that tend to refill the reservoirs,” says Woodhouse.
In contrast, the existing drought in the Southwest ” which began in 1999 and has left the Powell and Mead reservoirs half empty ” has had two above-average years, as measured by Colorado River flow.
This clearer picture of the past matters in several ways. It tells water officials that longer droughts may be possible even as more and more people arrive in the Southwest, straining the hydraulic system that delivers water to homes, ski-area snowmaking reservoirs, and low-lying farm fields.
Waters from the Colorado River and its tributaries currently serve upwards of 35 million people in the area between Cheyenne and Denver to San Diego and Los Angeles. Led by Nevada and Arizona, the Southwest continues to lead the nation in growth, with another 16 million people expected.
If the past was sometimes much drier than the 20th century, global warming theory also suggests a consistently drier future. Computer models are unclear whether global warming will result in less, or more, moisture in the Southwest. But 16 of the 19 computer models agree that the Southwest will be sharply warmer, meaning earlier runoff of snow.
Hotter temperatures also means more evaporation. Because reservoirs lose so much water to evaporation ” six feet off the surface of Lake Powell every year ” hotter temperatures make them less effective.
The climate of the Eagle Valley has been constantly shifting since the last glaciers melted about 12,000 years ago. Even aside from human-caused global warming, it’s unreasonable to expect that this century’s climate will be the spitting image of the 20th century’s climate.
But it’s also useful to have a “normal” defined, or at least a broad range of what constitutes “normal,” to help measure the man-caused changes in the climate.
That ultimately is the value of the old trees near Eagle, and many more like them in the Colorado River Basin. By understanding the past, says Woodhouse, we have a better shot at making sense of the future.
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