Spruce Trees Toxic to Salmon
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INTRO: Trees and plants are famous for their chemical defenses against insects. But it would seem the chemical defense used by spruce trees in Alaska hasn't worked so well. Over the last ten years, some three million acres of Alaska forest have been destroyed by the spruce bark beetle. As Doug Schneider reports in this week's Arctic Science Journeys Radio, scientists are now worried about the collateral damage the tree's chemical warfare may inflict on salmon.
STORY: It may take years for spruce bark beetles to kill a spruce tree. Beetle larvae feed on the woody capillary tissue the tree uses to transport nutrients to its branches and needles. Eventually the capillaries are cut and the tree dies. But before it dies, it attempts a counterattack. The tree produces chemicals, called terpenes, that are supposed to drive the beetles off. Sometimes it works. And sometimes it doesn't, according to Dr. Bob Wheeler, forestry extension specialist at the University of Alaska Cooperative Extension Service in Fairbanks.
WHEELER: "Well, the spruce bark beetle has co-evolved over thousands of years with the spruce tree and their defensive mechanisms. In fact, the bark beetles have developed not just a resistance to terpenes, but in some cases some terpenes are an attractant to the bark beetle. The bark beetle has developed a means of sensing some of the terpenes and use it as a homing beacon to find trees that are under attack or under stress."
Over the last decade or so, spruce bark beetles have killed more than 38 million spruce trees on Alaska's Kenai Peninsula, south of Anchorage. Before these trees died, they produced a lot of terpenes. Dr. Wheeler says that as these trees decay, the terpenes wash into rivers, lakes and coastal waters, where they may be toxic to salmon and other fish.
WHEELER: "If we have an abnormally large outbreak of spruce bark beetlesand we know that in response to the bark beetle attack the spruce trees produce copious quantities of resin that are highly concentrated in terpenesthen the spruce trees could be contributing large quantities of these compounds into the river systems. That was the basis of our research project that we did."
Wheeler didn't have enough funding to conduct a major study of the issue, but he did collect ten water samples from rivers and nearshore areas on the Kenai Peninsula where spruce trees were hit hard by beetles. While he found little in the way of terpenes, he did find other chemicals that may be related.
WHEELER: "Part of the concern is that the terpenes themselves we believe may be short lived, being bioderived into other compounds through metabolism within bacteria in the water. Bacteria actually convert these compounds into other compounds. There is evidence that these other compounds may be more toxic than the terpenes they were derived from. The bacteria are to some extent contributing to the potential for toxicity."
Wheeler says research is needed to understand how the terpenes make their way into the marine ecosystem, and how their chemical structure changes. He also says more needs to be learned about the effect of terpenes and related chemicals on salmon. Wheeler hopes to use improved techniques to collect and test more water samples this summer.
OUTRO: This is Arctic Science Journeys Radio, a production of the Alaska Sea Grant Program and the University of Alaska Fairbanks. I'm Doug Schneider.
Thanks to the following individual for help in preparing this script:
Dr. Bob Wheeler
For more information about Bob Wheeler, spruce bark beetles and the Alaska Cooperative Extension Service, check out these web sites.
Another Arctic Science Journeys Radio story about spruce bark beetles,
Alaska Cooperative Extension Service
Spruce Bark Beetles
Arctic Science Journeys is a radio service highlighting science, culture, and the environment of the circumpolar north. Produced by the Alaska Sea Grant College Program and the University of Alaska Fairbanks.
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