[asa] Western Trees Dying Due to Warming

In today's Science:
http://www.sciencemag.org/cgi/content/full/323/5913/447

An insidious problem has taken hold in the forests of the American West,
> quietly thinning their ranks. Mortality rates in seemingly healthy conifer
> stands have doubled in the past several decades. Often, new trees aren't
> replacing dying ones, setting the stage for a potentially dramatic change in
> forest structure, says Phillip J. van Mantgem, a forest ecologist at the
> U.S. Geological Survey (USGS) in Arcata, California. Warmer temperatures and
> subsequent water shortfalls are the likely cause of the trees' increased
> death rate, he and his colleagues report on page 521. (
> http://www.sciencemag.org/cgi/content/abstract/323/5913/521)
>

> "This is a stunningly important paper," says David Breshears, an ecologist
> at the University of Arizona, Tucson. For years, he and others have lamented
> massive diebacks that occur when fungal and insect pests ravage stands of
> trees. "What's harder to detect," he explains, is any subtle but significant
> shift in the trees' background death rate. "They have done a very thorough
> job" of documenting it.
>

Warming is the clear culprit but is there a smoking gun for global warming?
It depends who you talk to. Note that this study does not include the
indirect causes of tree death from global warming because of the Pine Bark
Beetle. Nor does it discuss the positive feedback for global warming caused
by tree death both through less CO2 intake and greater wildfire risk.

Next, the researchers evaluated the possible causes of the increased
> mortality rate. No matter how they sliced and analyzed the tree data--by
> size, type, elevation, and location--they still detected the increase in
> mortality. Air pollution couldn't be blamed because the increase occurred in
> pristine as well as polluted areas.

> Ultimately, "the finger seems to be pointed to warming." says Breshears.
> Temperatures in the United States have risen about 0.4°C per decade in the
> past 40 years. Snowpack of the regions examined diminished over the time
> period they studied and is melting earlier, effectively lengthening the
> summer drought. Warmer air also leads to more evaporative loss, exacerbating
> the effect.

> Michael Goulden, an ecosystem ecologist at the University of California,
> Irvine, thinks the data fall short of pinning the problem on global warming,
> as regional warming related to natural climatic variation could be to blame.
> But Julio Betancourt of USGS in Tucson, Arizona, disagrees. "Models suggest
> that most of this change was due to the buildup of greenhouse gases," he
> says. Moreover, local Pacific Northwest and Southwest climates tend to
> fluctuate in opposite directions.
>

Rich Blinne
Member ASA

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Received on Fri Jan 23 19:11:25 2009