Climatologist's Toolbox


A close-up of a cross section of a sugar pine from Sequoia National Park reveals a series of fire scars (Photo © A.C. Caprio).
Talking Trees: A living diary of climate

By translating the story told by tree rings, we have pushed back the horizons of history in the United States for nearly eight centuries before Columbus reached the shores of the New World, and we have established in our Southwest a chronology for that period more accurate than if human hands had written down the major events as they occurred. -- A. E. Douglas, 1929

When Andrew Ellicott Douglas, director of the University of Arizona's Steward Observatory, published his fascinating chronicle of discovery in National Geographic nearly seven decades ago, it was clear to him that he had found a long-overlooked "key to prehistoric chronology." Perhaps even more startling, was his sense that one day the concentric rings of wood laid down by trees as they grew would be a barometer for measuring future climate:

"When a real theory of climate has been developed and we can predict drought and flood over a period of years, this Arizona story in tree rings will have played a creditable part in developing that climatic foresight which is perhaps the most valuable economic advantage yet lying beyond our reach."

While we are yet unable to predict future climate with certainty, the information locked within trees has become a standard scientific gauge, one used to date mysterious ruins, recall times of insect plagues and centuries-long drought, and chronicle distant volcanic eruptions. And, as if to fulfill Douglass' prophecy, scientists are now using their ability to decode the information in trees to successfully forecast the seasonal conditions that lead to severe episodes of wildfire.

Rosetta trees?
The legacy of A. E. Douglass now resides in "a warren of dim rooms" tucked beneath the football stadium at the University of Arizona. There, in the Laboratory of Tree Ring Research (LTRR), nearly a million specimens of wood -- from pen-sized cores to cross sections the diameter of small silos -- are used to ferret out the secrets of the past, including those of climate.

Known as dendrochronology (defined), the science of tree rings has matured and perhaps a thousand people worldwide are now reading the stories that reside in living trees or in ancient timbers found at archeological sites. While Douglass became famous for his studies of Pueblo ruins, he knew that an understanding of the effects of changing weather on trees was critical if the technique was to be at all viable, says Martin Munro, a researcher at the LTRR.

The dendrochronologist, says Munro, reads the distinctive patterns of wide and narrow rings which appear in pieces of wood from different trees. It is more than simply a matter of counting rings. Patterns from different trees must be matched to pinpoint the year a tree produced a particular ring, a process known as crossdating.

Year-to-year changes in the weather create the patterns, says Munro. Trees respond to temperature, rainfall and other environmental conditions by altering growth. In boom years, the tree adds extra tissue and the ring will be correspondingly thick. During lean years, growth is slowed and rings are thinner. While that sounds like a straightforward way to get at the issue of climate, Munro notes that it is not all that simple. Trees grow better or worse in response to a combination of weather conditions, depending on their physiology (defined) and where they are growing.

"Some trees might respond to changes in overall rainfall, others to the amount of rain during the late summer, and others to the seasonal temperatures that limit the length of the growing season." Other things, like the scars inflicted by fire and plagues of insects, also leave characteristic but indirect clues to climate since widespread forest fires and insect infestations are favored by certain conditions of weather. Human activities such as smelting and the burning of fossil fuels etch their signatures in wood as well. Dendrochronologists interested in global climate change have paid special attention to very long-lived trees like the bristlecone pines in the White Mountains of California. These trees are the oldest living organisms on Earth, providing in some cases 5,000-year-long chronologies of growth.

In short-lived trees, it is hard to disentangle slight changes in growth caused by gradual changes in climate. But once the bristlecone pines and other Methuselahs of the tree world settle into a quiet old age, they are unperturbed by these influences and can be used to document the effects of global warming, Munro says. Scientists speculate the gnarled and ancient trees may be the first and most sensitive biotic indicator of changes to come.

Get Shorty...
Dendrochronologists also have sought evidence of global change in shorter-lived trees in those parts of the world where climatic change would be most pronounced. For example, many people think global warming would be more of an issue nearer the poles. That idea has made trees from Alaska, Scandinavia and Siberia prime candidates for study. The advantage of using trees to study climate is that they are living records of past climate and weather. Those records are available in parts of the world where there are few weather stations and where consistent and accurate records of weather rarely go back more than 100 years.

Importantly, tree ring data can be replicated by taking and comparing multiple samples to ensure that a signal is not peculiar to a single tree or stand of trees. "Dendrochronology is not simply a matter of reading single climatic variables" from a single tree, says Munro. There are lots of things there to measure, count and compare.

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