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TREES THEN AND NOWA 1938 photograph shows ponderosa pines in Arizona's Gus Pearson Natural Area (left). Recent monitoring of trees, such as that done from a gondola over a research plot in the Gifford Pinchot National Forest in Washington state (right), shows that the rate at which trees in U.S. and Canadian forests have been dying has doubled over the past 50 years; that rate is independent of tree deaths from catastrophic events.IMAGE CREDITS: Courtesy of P. van Mantgem; Univ. of Washington

Those trees falling in the forest with no one listening — in the changing climate of the West, they’re falling about twice as fast as they were 50 years ago, says a new study.

These background, or noncatastrophic, mortalities aren’t the result of wildfires or the huge outbreaks of pine beetles. The recent increase in temperature is likely to blame, the researchers suggest.

Records from 76 plots of apparently healthy, old-growth temperate forest in the western United States and Canada show that the small number of routine, noncatastrophic tree deaths in a year has doubled since 1955, reports a team of researchers from eight institutions.

Rates of routine tree deaths in these locations, now one or two percent annually, aren’t keeping up with the tree birth rate, says the study, published in the Jan. 23 Science.

The numbers may sound tiny, but consider how they compound over time, says coauthor Mark E. Harmon of Oregon State University in Corvallis. “A lot of little numbers can add up to a big number.”

If the trend continues, forests could dwindle to sparser spreads of younger, skinnier trees, said Phillip J. van Mantgem, now of the U.S. Geological Survey’s Western Ecological Research Center field station in Arcata, Calif., during a January 21 teleconference. Punier forests might store less carbon than they do now, van Mantgem said. Biologists count on these forests as helpful absorbers of carbon dioxide, but van Mantgem cautions that the new finding “raises the possibility that western forests could become sources of carbon dioxide to the atmosphere, further speeding up the pace of global warming.”

In terms of forests slowing the effects of greenhouse gas output, “we’ve actually had a subsidy from nature,” says Oliver Phillips of the University of Leeds in England. For a variety of reasons, “we shouldn’t expect the biosphere to continue to provide it.”

He welcomes the study as “the first report that I know of at this scale in the temperate zone.” Work he and others have done in the tropics shows a different dynamic there, with the growth of new trees keeping pace with the death of old ones.

Tree data for the study came from various mortality surveys conducted between 1955 and 2007. From the 1970s to 2006, the mean annual temperature of the western United States increased at a rate of about 0.3 to 0.4 degrees Celsius per decade, the researchers say..

Sites in the temperate zone studied stretch from British Columbia to Arizona and include half-hectare spots as well as spreads covering nearly 16 hectares. The studies have monitored the health of 58,736 trees, of which 11,095 died.

The researchers found that mortality has edged upward in a wide range of trees: dwellers at high and low altitudes, shade-tolerant species and shade-hating ones, slim youths and broad-trunked lords of the forest. With such variety, scientists look to some widespread environmental effect.

Recent warming has altered water dynamics in the West, the researchers say. Studies have revealed more precipitation falling as rain rather than snow, along with earlier snow melts and longer droughts. A wide range of forests might be drying out more than they used to, stressing trees more.

Also, warmer temperatures could be boosting populations of diseases and pathogens. Warming has played a role in the massive beetle outbreaks chewing through swaths of forest, so perhaps smaller, everyday infestations are getting more serious too.

Such widespread changes probably didn’t come from air pollution such as ozone, the researchers conclude. Ozone does harm trees, but plots in national parks with relatively clean air showed rising mortality too. That the declines occur over broad swaths of intact park forest likewise makes it unlikely that fragmenting of forests would explain the results, the team argues.

The researchers make “a good case” for climate change as the driver for such broad effects, Andrew Sugden, an editor at Science who previously studied tropical rain forest ecology, said during the teleconference.

This new study “certainly shows how systematic long-term monitoring of forests is essential as a warning system to potentially more dramatic changes,” says Simon Lewis, a researcher at Leeds who has studied tropical forests.

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