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HOMER - Forest scientists who first blamed the beetle epidemic on Alaska's aging forests are giving more and more credit to rising temperatures as an explanation for the unstoppable rampage.
"I am personally convinced," said Glenn Juday, a professor of forest ecology at the University of Alaska Fairbanks.
Juday's study of past warming cycles, based in part on examination of carbon isotopes in wood samples, shows the period of 1977 to 1996 to be Alaska's warmest in two centuries, according to the Anchorage Daily News.
Back around 1990, when bark beetles began to swarm across the Kenai Peninsula, scientists warned that the epidemic would be hard to stop until Alaska got a few cool, wet summers.
Unfortunately for the spruce trees, Alaska no longer seems to have such a thing as a cool, wet summer.
A 14-year string of warmer than average summers, coupled with a long-running drought, resulted in the biggest insect infestation of trees in recorded North American history. Alaska now has 3.1 million acres of gray dead spruce, half of that on the Kenai Peninsula.
With last week's annual flight of new beetles taking place in hot, sunny weather, the dying, red-needled trees will continue to spread in places such as the Kenai River corridor and west of Cook Inlet. They're spreading more slowly, however, with so many trees already dead.
"If we had an infinite amount of trees, the red-needle index would remain high every year," said Ed Berg, an ecologist at the Kenai National Wildlife Refuge who first linked the outbreak to rising temperatures.
The old forests of Southcentral Alaska were a necessary precondition for such a widespread outbreak, scientists say. Many of the spruce trees were several centuries old, said Jerry Boughton, forest health protection program leader for the U.S. Forest Service in Anchorage.
"That made them more susceptible to insects," Boughton said. "That's not to say this warming did not exacerbate the problem."
Beetles are part of the forest's natural cycle of death and rebirth. But Berg's studies of tree-ring density and lake deposits have found that past beetle outbreaks remained localized.
The greater warmth probably increased the damage by accelerating the beetles' life cycle, Berg said, while also putting trees under stress from drought.
Scientists say some warming resulted from a 20-year ocean-temperature cycle. Some also came from an underlying increase in global temperatures due to emission of greenhouse gases.
Juday said in Anchorage the 1977 to 1996 period averaged 3.2 degrees Fahrenheit warmer in winter than in the rest of the century, and 1.7 degrees warmer in summer.
In Fairbanks, that 20-year cycle was 4.5 degrees warmer in winter than the preceding 20-year cycle. It was 2.6 degrees warmer in summer than the coolest 20-year cycle of the century, he said.
Foresters are beginning to look at regeneration of the forests. On the northern Kenai Peninsula, a ready seed source for new spruce and hardwoods remained after beetles swept through, said Michael Fastabend, who runs the beetle mitigation program for the Kenai Peninsula Borough. But south of Deep Creek, he said, the depletion of mature trees and the fast growth of tall grass appear to cut off regeneration.
He said the borough plans to scrape the ground after logging borough-owned tracts and replant immediately to beat the grass.
In the future, the southern Peninsula around Homer probably will include more open grassland interspersed with slowly expanding islands of spruce, he said.