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Trees may be finding way to fight back against beetle

Posted: Tuesday, March 02, 2004

Alone, standing green and tall, a healthy white spruce seems almost out of place amid the stand of graying deadwood killed by bark beetles.

Around Homer, perhaps 5 percent of mature spruce trees exhibit an apparent ability to survive the annual attacks of the current cycle of beetle infestation plaguing the Kenai Peninsula. And that has scientists intrigued.

Indeed, those survivors may well hold a key to ensuring that forests 50 or 100 years from now have a better chance of surviving the next time nature sends waves of those tiny harbingers of death across the landscape, according to a U.S. Forest Service researcher.

Trish Wurtz has a degree in forest ecology from the University of Oregon and works at the Pacific Northwest Research Station's Boreal Ecology Cooperative Research Unit in Fairbanks. She's been studying trees in Alaska since 1987 and currently is experimenting with some 3,200 peninsula white spruce seedlings planted in 2000 near Clam Gulch.

Using seeds representing 53 genetic families collected from around the peninsula some pre-infestation, some taken in 1998 from surviving parents after the beetles had attacked those stands, Wurtz is looking for the reasons those trees avoided death.

Some number almost certainly survived merely by chance, having never been hit by the critical mass of beetles needed to overwhelm a spruce tree's natural defenses. In areas of the peninsula where the devastation has not been so complete, these so-called "escapees" may make up a significant number of those still alive.

But on the lower peninsula, where the death has been nearly absolute, the "survivors" are much more likely to have resisted the beetle attack due to some favorable genetic condition. Wurtz hopes her research will find that key.

"Spruce trees fight back," she said. "They produce pitch. They change the chemistry of their resin and make 'traumatic resin.' It literally pushes them (the beetles) out of the tree."

At low infestation levels, this reaction works fairly well, Wurtz explained. But when the numbers of beetles gets very high, as happened in the late 1990s, the resistant systems are overwhelmed.

"Very few trees can survive that," she said.

South of Kasilof, the forest exhibits about 95 percent mortality in trees six inches in diameter or larger.

"I'm interested in those 5 percent that make it," Wurtz said, and specifically those that make it for some genetic reason, rather than just blind luck.

"The questions are whether this super-capacity is genetically based and can we exploit it over the long term to make the trees of the Kenai Peninsula and Southcentral Alaska more resistant in the future?" Wurtz said.

Ongoing Canadian research projects using insects that attack saplings of Sitka and white spruce appear to confirm some genetic component to resistance, she said.

But doing the same science with bark beetles is considered "a career-buster" among tree scientists because bark beetles attack mature trees. Wurtz said there aren't many scientists willing to wait decades for trees to mature to see if their hypotheses stand up.

But long-term study is worth doing, and in the meantime, Wurtz said there should be an effort to find and protect the survivors. Current logging operations on the peninsula, even those that may be done merely to increase fire protection, often involve cutting out the live trees with the dead. Wurtz said that in the near future, the service would begin trying to compile a database of survivor trees, perhaps using high definition photo analysis and information provided by landowners.

Meanwhile, there are experiments that may help peg whether surviving members of the predominant species in the Homer area, the white spruce, indeed have a genetic resistance.

"The ultimate test will be to wait 30 years, but we can look at seedlings to see if they have resistance characteristics by using other insects as proxies, such as aphids and weevils," she said. "We can look and see if they are damaged by those insects."

Another experiment involves making an extract juice from the phloem the tree tissue that conducts nutrients and that the beetles attack. A cotton ball impregnated with the juice and placed in a petri dish with a beetle will cause the beetle to try to escape the dish if it contains a chemical noxious to the beetle.

Those tests have yet to be conducted on the 3,200 test trees being grown by Wurtz. They are, as yet, still too young.

Rene Alfaro with the Pacific Forestry Center in Victoria, British Columbia, has developed a method of counting resin ducts that conduct noxious resins. Wurtz said Alfaro has discovered that trees with more ducts tend to resist the white pine weevil better. Whether duct density would translate to beetle resistance in white spruce has yet to be determined, but Wurtz said she expects to collect branches this summer and have them studied by Alfaro next year.

Eventually, Wurtz said, she would like to see survivor trees found and protected, and their seeds collected and stored. White spruce only produce cones every few years, and their seeds are becoming increasingly valuable for scientific research.

"It's like the beetles highlighted resistant trees, but we aren't paying attention," she said.

The ultimate goal, she said, is to make the whole peninsula less susceptible to the effects of beetle outbreaks. That doesn't mean trying to grow only beetle-resistant stands. That, she said, likely would result in an evolution of the beetles to overcome the resistance. What it could mean, however, is scattering resistant seeds throughout the forest, so that when the beetles come again and they will the devastation won't be so widespread and complete.



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