Refuge Notebook: Birch protected from hares, sauna bathers, but not moose

Posted: Friday, July 23, 2010

When visiting friends of Finnish descent in northern Wisconsin, I often experience the custom in sauna baths of whisking my bare skin with leafy birch branches to get a pleasant tingling sensation. The branches have fresh leaves in the summer, and dried leaves in the winter that are rehydrated by dipping the whisk (called "vihta" in Finnish) into hot water. The Russians have a similar custom in their baths (banyas), with birch switches called "venniks."

Photo By Ed Berg
Photo By Ed Berg
The small blisters on this birch branch contain papyriferic acid, a chemical that the plant produces to discourage browsing by animals such as snowshoe hares and moose.

When I came to Alaska I only tried this birch whisking technique once, with disastrous results. I quickly found my skin covered with a white, sandy material instead of the sweet smell of birch leaves. There was something different about the birch in Alaska.

It turns out that birch branches in Alaska are constantly being browsed by moose and snowshoe hares, and have evolved a potent chemical defense to protect themselves. Birch twigs secrete small blisters of a sticky resin called papyriferic acid which in time dry to form white scales. These scales were the white, sandy material that I found on my skin. Try grabbing a birch twig and running it through your fingers, and you will see what I mean.

Papyriferic acid is not poisonous to moose and snowshoe hares, but hares will usually starve rather than eat twigs that are heavily encrusted with these blisters or scales. Moose however are more tolerant, and seem to be able to munch down the encrusted twigs without batting an eye.

Birch twigs only produce papyriferic acid scales in response to mechanical damage, such as browsing. Birch trees grown in fenced areas (exclosures) show little scaling, and few scales are found high up in a birch tree, beyond the reach of moose. (Check this last point out the next time you see a downed birch tree.)

From an evolutionary perspective, chemical warfare is constantly shifting arms race. The plants produce a protective chemical, but in time the animal digestive system evolves the necessary enzymes to break down the chemical and render it ineffective as a plant defense. The birch is presently ahead of the arms race with snowshoe hares, but is losing to moose. It is now the birch's turn to evolve a more potent chemical to defend itself against moose.

In the Interior, where there are generally many more snowshoe hares than moose, the papyriferic acid scales are probably more effective in defending birch than on the Kenai where moose are the dominant browser.

John Bryant, an emeritus UAF researcher, has studied plant chemical defenses for several decades. He has assayed papyriferic acid in birch in many areas of the boreal forest, and found that birch in Connecticut contained no papyriferic acid, and that the birch twigs in northern Alaska contained the most, as much as 50 percent. He found an especially strong relation between forest fires, snowshoes hares and papyriferic acid scaling. Forest fires often produce a lot of young birch plants, which can feed a lot of hares, which in turn stimulates a lot of papyriferic acid scaling on the birch twigs.

Bryant and UAF chemist Tom Clausen have proposed that birch with its papyriferic acid and other compounds may have potential medical applications. The papyriferic acid in birch for example causes the hares to excrete more sodium in their urine, which suggests that it might be useful for treating high blood pressure. There appear to be a variety of these "anti-herbivory" compounds in many plant species that discourage animals from feeding on the plants, and many of these biologically active compounds probably have as yet unknown human applications of some form or another.

Ed Berg has been the ecologist at the Kenai National Wildlife Refuge since 1993. Ed will be teaching his weekly 1-credit "Geology of Kachemak Bay" course at the Kenai Peninsula College in Soldotna and Homer, starting September 1 and 2, respectively.

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Previous Refuge Notebook articles can be viewed on the refuge website You can check on local birds or report your bird sighting on the Kenai National Wildlife Refuge Birding Hotline at 907-262-2300.

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