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Spruce growth accelerates on Alaska's treeline as temperatures rise

Doug O'Harra
Photo courtesy U.S. Fish and Wildlife Service

Those tough, gnarled white spruce that eke out an existence at the far northeastern frontier of Alaska's boreal forest have been growing faster in recent decades, apparently boosted by warming temperatures.

The findings, based on an extensive analysis of growth rings and wood density going back almost 1,000 years in trees at the tundra edge along Alaska's Firth River are in dramatic contrast to the ailing Interior forests, where warmer temperatures and drought have cut growth rates, increased forest fires and triggered insect outbreaks.

"I was expecting to see trees stressed from the warmer temperatures," said lead author Laia Andreu-Hayles, a tree ring scientist at Columbia University’s Lamont-Doherty Earth Observatory, in this story. "What we found was a surprise."

The research also used new techniques that bolstered the use of tree ring data in climate studies, the scientists said, addressing what had become knotty and sometimes controversial issue. (Some skeptics have argued that tree ring data can't be used to reconstruct pre-modern temperatures. More on this below.)

The study, published last month in Environmental Research Letters, is part of a broader effort to track the ecological impact of climate change through dendrochronology -- the study of the growth rings inside trees.

As anyone who has ever counted the concentric lines etched into an old stump knows, the quality and intensity of each growing season in a tree’s life gets recorded year by year.

"In warm years, trees tend to produce wider, denser rings and in cool years, the rings are typically narrower and less dense," explains this story about the project. "Using this basic idea and samples from a 2002 trip to the refuge, Andreu-Hayles and her colleagues assembled a climate timeline … going back to the year 1067."

They examined the tree ring width in 232 samples from 111 trees (30 alive, 81 preserved in the cold climate) that spanned 935 years through 2002. They also looked at the maximum density of new growth late in the season within the rings, based on 246 samples from most of the same trees, covering about the same period of time.

"Trees were growing in scattered groves over hilly slopes in an open valley at the Firth River watershed," the scientists wrote in the paper. "The subfossil wood was collected from both standing and toppled dead trees. Collectively, these samples have yielded one of the very few millennial-length tree-ring records available for northern Alaska."

What they found was startling. An analysis of tree-ring width and ring density showed increased growth beginning about 100 years ago. After 1950, the trees grew even faster.

"For the moment, warmer temperatures are helping the trees along this part of the forest-tundra border," said study coauthor Kevin Anchukaitis, a tree-ring scientist at Lamont, here. "It's a fairly wet, fairly cool, site overall, so those longer growing seasons allow the trees to grow more."

The results might help settle one of climate science's ugliest controversies. In the quest to sort out whether the Earth's climate has been warming, scientists have long used tree ring widths as a "proxy" for temperature. It's one of the sources for the much-discussed "hockey stick graph" showing a sudden upswing in temperatures after 1,000 years of stability.

But there's a problem. Since 1950 or so, tree ring widths in some northern forests have stopped varying with temperatures. Warmer summers didn’t produce wider rings -- and sometimes even coincided with severe declines in growth. Alaska’s interior spruce forests have become a prime example.

"As scientists looked for ways to get around the problem, critics of modern climate science dismissed the tree ring data as unreliable and accused scientists of cooking up tricks to support the theory of global warming," this story explains. "The accusations came to a head when stolen mails discussing the discrepancy between tree-ring records and actual temperatures came to light during the so-called 'Climategate' episode of 2009-10."

This issue over tree ring widths no longer tracking rising temperatures is nicknamed the "divergence problem" by scientists. But it might be explained by the complex ecology of northern forests and their need for additional rainfall in the face of hotter weather.

"Beyond a certain threshold level of temperature the trees may become more stressed physiologically, especially if moisture availability does not increase at the same time," Rosanne D'Arrigo, senior research scientist at Lamont-Doherty Earth Observatory and one of the study’s co-authors, told the Christian Science Monitor in 2009 during the height of the controversy.

The new study found evidence of this same "divergence" -- tree ring width of these Far North spruce did not track rising temperatures very well after 1950s. But the wood density measurements paid off -- indicating rising temperature even when the tree ring widths did not.

"Trees tend to produce cells with thicker walls at the end of the growing season, forming a dark band of dense wood," this story explains. "While tree-ring width in some places stops correlating with temperature after 1950, possibly due to moisture stress or changes in seasonality due to warming, tree ring density at the site studied continues to track temperature."

The study demonstrated that scientists can still use growth rings as a way to gauge earlier climate, the scientists said.

(In a recent Dot Earth post, New York Times reporter Andrew Revkin blogged about how this new development might impact the controversy over tree rings, including reactions from other tree ring scientists.)

"This is methodologically a big leap forward that will allow scientists to go back to sites sampled in the past and fill in the gaps," Glenn Juday, a leading forest ecologist at the University of Alaska Fairbanks, who was not part of the study, said in this story.

The scientists said they plan to come back to Alaska and other northern forests to take more samples.

Contact Doug O'Harra at doug(at)alaskadispatch.com