Scientists link warmer weather to beetle outbreak
Scientists link warmer weather to beetle outbreak
11 September 2005
published by www.adn.com
The bugs are a normal part of the forest cycle, but recent high temperatures encouraged growth
HOMER — When Scott Brandt-Ericksen read of Sen. Hillary Clinton’s recent trip to Alaska and her alarm over bark beetles migrating north due to global warming, he burst out laughing. Then he got annoyed.
“While global warming may be an issue, the spruce bark beetle kill problem has not been tied to global warming,” he wrote in a letter to the Daily News. “It is insulting and a disservice to the public to have the AP and the Daily News peddling such malarkey as if it were scientific fact.”
The Ketchikan Gateway Borough attorney elaborated in a later interview: “Is it an urban legend, or is there any scientific basis? That’s one of my pet peeves, when people have repeated speculation enough for it to be considered fact.”
Indeed, in the decade that it took for the spruce bark beetle epidemic here to reach unprecedented proportions, killing four million acres of forest in Southcentral Alaska, the notion that higher temperatures were to blame evolved from a tentative hypothesis to an article of accepted wisdom. By 2002, The New York Times said the dead forests may be “one of the world’s most visible monuments to climate change.”
Brandt-Ericksen was not alone in wondering whether circumstantial evidence was being misused for political purposes. The forests had been allowed to grow old and vulnerable, many people said. Have scientists really been able to prove a link between warmer temperatures and the miles of gray, lifeless forests?
In fact, they say, they have.
Through a combination of field sampling in buggy forests, computer analysis of tree growth rings and historical fire data, and careful examination of competing theories, forest scientists have reached a consensus that the woods of the Kenai Peninsula would not have been so thoroughly wiped out without an increase in the local summer average temperatures of some 2.5 degrees Fahrenheit since 1987.
“The difficulty was you had a natural coexistent system between trees and bugs, and outbreaks had happened many times in the past,” said Glenn Juday, a professor of forest ecology at the University of Alaska Fairbanks. “So what made this different?”
Finding the answer involved a classic piece of scientific sleuthing. But the quiet background research was obscured for a while by noisier foreground political battles fought over mankind’s supposed role in bringing on the epidemic, either through mismanagement of the forests or pollution of the earth’s atmosphere with greenhouse gases.
For residents of the Kenai Peninsula, the loss of their forests came with little warning. When the spruce bark beetles first hit in the late 1980s, entomologists predicted the outbreaks would be relatively short-lived, killing off the oldest trees. Homeowners were instructed in defensive techniques such as stripping bark off red-needled spruce and felling trap trees. These efforts proved useless in the face of the coming beetle hordes, which attacked healthy young trees and flew so thick they left a brown tide line on the shores of Cook Inlet.
Today, beetle activity has abated because there are few mature trees left to attack. The last red-needle outbreaks on the Kenai could be seen this summer in the high country south of Turnagain Pass. The area is traditionally one of the last to lose snow in spring, but earlier melting in recent years may have made them vulnerable, entomologists say. With most of the Kenai’s old forests now dead and rotting, the real beetle action lately has been to the west, in the Iliamna Lake and Kuskokwim River areas. New outbreaks have also been seen closer to Anchorage, in the Indian and Bird Creek valleys.
One thing was clear from the start: the beetles hadn’t “migrated north, drawn by higher temperatures,” as The Associated Press account of Sen. Clinton’s visit put it. The beetles have always been part of the Kenai Peninsula forests, serving an ecological role in killing off small stands of ripe, older trees.
A taciturn ecologist for the Kenai National Wildlife Refuge who calls himself a “forest detective” was the first scientist to begin probing the links between the spreading beetle outbreak and warmer temperatures. Ed Berg, a former philosophy student and carpenter, started with circumstantial evidence a string of warmer-than-average summers beginning in 1987.
“We had summer temperature records in Homer going back to 1932,” he said. “I perceived this was an unusually long run of warm summers.”
Studies had shown that spruce beetle hot spots usually stop spreading because of downturns in temperature. Two consecutive cold winters freeze the beetle larvae buried under the spruce bark, or a wet, cool summer discourages the emergence and flight to new trees. But this time there was no cooling-off period to stop the beetles.
Berg found that past drought periods had triggered short beetle outbreaks, presumably by stressing the spruce. He assembled evidence of current drought. What’s more, in unusually warm weather like the Kenai was getting, the two-year life cycle of the insect could be cut to one year, effectively doubling the population of beetles.
One such hot year was 1993. That was the summer Berg started going into the woods to take core samples of trees. In areas where beetles had killed trees in the past, he found evidence in tree rings of a burst of growth in young spruce once the older canopy thinned. He was then able to look elsewhere to find and date earlier outbreaks.
His conclusion: Beetles were a natural recycling agent on the Kenai Peninsula, where the coastal climate meant few lightning-started wildfires. Earlier outbreaks, some of them extensive, had always eventually shut down. This kind of wholesale forest destruction was unprecedented.
“That was a key that earlier events had been low-frequency and spatially separated,” said UAF professor Juday, who carried Berg’s work into the broader conversation about changes in the Arctic.
Some skeptics remained. Jerry Boughton, a forest health specialist with the U.S. Forest Service, stressed that the woods were filled with aging trees susceptible to bug attacks. For a time, this became the preferred explanation for those who wanted to see more active management, including logging, of the forests.
As a Voice of the Times editorial put it in 2001: “Though a New York Times reporter once claimed that the Chugach National Forest beetle explosion was caused by global warming, the experts seem to feel zealous fire suppression and logging bans caused the trees to grow old, densely spaced and susceptible to insect attack.”
The problem with that hypothesis, said Juday, is that the data don’t back it up. The aging forest contributed to the problem, he said. But there was no evidence that the forests hadn’t been just as old and vulnerable during outbreaks 200 years ago.
Logging was never a factor in keeping Kenai Peninsula forests thinned, and the policy of suppressing all fires lasted only two decades, beginning around 1960, not long enough to have a major effect on the age of the trees, Juday said. Human management is a useful explanation for forest insect problems in the Rocky Mountain West, he said. But it didn’t apply here.
Others suggested sloppy clearing practices by more recent loggers were to blame. Scientists responded that the epidemic may have gotten a boost from trees left on the ground after clearing of transmission lines but still wouldn’t have grown to such proportions without the higher temperatures.
The final word on the beetles and climate change may have been heard last year, in the Arctic Climate Impact Assessment published by an inter-governmental forum, the Arctic Council. It was the work of 300 scientists from 18 countries. Juday, who wrote the chapter on forest health, said it was one of the most thoroughly peer-reviewed pieces he’s written. He said he sifted through other explanations the “decadent forest,” the genetic vulnerability of the hybrid Lutz spruce in the area before concluding that warming temperatures were the major cause.
“We’ve got a pretty good tentative conclusion now. But science can only do so much,” said Juday. “You develop a hypothesis, you make observations, you try all the reasonable explanations and knock some of them down and then see what’s left standing.”
Boughton, who now works for the federal forest service in Pennsylvania, said he has always believed that rising temperatures helped push the beetles’ spread in Alaska. But he said he draws the line when anyone tries to attribute those dead trees to excessive burning of fossil fuels rather than to natural climate cycles. Others, including members of Alaska’s congressional delegation, have expressed similar reservations.
That’s where the debate is heading now, said Juday. It’s beyond the reach of forest ecology. Juday, who thinks human emissions are partly to blame for climate change, is impatient with misuse of the scientific work from any political direction.
“Look, both factors are involved. You can’t deny that natural changes are part of it. Nobody flipped a switch and turned that off,” he said. “But nobody is trying to argue that case.”
He said he has cautioned advocacy groups about oversimplifying the forest health issue for dramatic purposes. “We have to put the brakes on some of these enthusiasts who want to use one forest fire to stop global warming,” he said. On the other hand, he said, the evidence of change in the Arctic from forest fires, receding glaciers, coastal erosion is consistent.
Berg has gone on to do other work on the Kenai refuge, investigating treeline changes, for example, including the straightening up of the stunted alpine hemlock known as krummholz. Lately he has been taking core samples of drying muskegs, finding that the surface spread of woody plants such as blueberry and dwarf birch is a brand new phenomenon. Peat deposits dating back 10,000 years show no evidence of woody roots, he said.
“The shrub invasion seems to be going full bore,” Berg said.
Ed Holsten, a retired forest service entomologist gradually won over to the global warming theory, said he expects Alaska will face more insect problems in the near future, even as the bark beetle’s day fades. He mentioned such spreading pests as the birch leaf miner, which mottles birch leaves, and the larch sawfly.
Not that the spruce bark beetle is going away. Scientists expect the bug will remain part of the warmer ecosystem, ready to nip off the young surviving spruce once they reach maturity. A handful of older trees seem resistant to attack, for unknown reasons. But Juday’s chapter concludes that efforts to replant spruce and bring back the old forest on a new cycle may be doomed.
For forest scientists, the interesting question no longer is what caused all the trees to die, but what kind of landscape will replace them.
