The Smell of Christmas
One of many Christmas pleasures is the aroma of a freshly cut tree. Both spruce and pine have it. So do cranberry fields in the morning, orange peelings, and some paint thinners. The substances known as essential oils are responsible for these odors. They are now called terpenes, compounds produced by plants from isoprene, a hydrocarbon that has its five carbon atoms arranged in a branched chain. Isoprenes combine to give many terpenes. Camphor and pinene are among the simple two-isoprene terpenes that help form the odor of pine. Larger terpenes can be found in rosin, steroids, and chlorophyll. Thousands of terpenes are known and new ones are regularly found.
Most terpenes found in conifers are volatile and evaporate during warm days. When it is cool the needles accumulate large amounts, some of which escape if the needles are damaged. Simply brushing against a green bough on a ski slope is enough to bring forth the rich smell of pine.
In southeast Alaska it is quite cool in the summer, so that many terpenes do not evaporate. But terpenes are water-soluble, and the heavy rains dissolve them and wash them into the sea. A northwestward current runs along the coast, carrying the terpenes from the Pacific rain forests along the coast as far as the fjords of Prince William Sound. During their journey the terpenes are slowly converted into biomass by marine bacteria, which starts a food chain that contributes to our fisheries.
In central Alaska during summertime the terpenes evaporate in large quantity. They come not only from spruce, but from many plants in the boggy areas. Blueberry and Russian tea are plants in a group called Ericads that are particularly rich in terpenes. After evaporation, these terpenes oxidize in the sunlight and form a blue haze. Even with no fires, the flat areas look a little smoky unless there is a breeze to flow these compounds away. As temperatures drop in the fall, evaporation slows and you may be treated to a clear view of mountains over a hundred and fifty miles away.
Not only evergreens have terpenes. Paul Reichardt and John Bryant of UAF discovered a new compound in birch trees, a thirty-carbon terpene which is part of a molecule distasteful to arctic hare. Willow is the normal food for these browsers, but painting a little birch extract containing this complex terpene on fresh willow shoots turns them away. The terpene is produced by birch trees only when very young. (You may have noticed the white sandpapery substance on the twigs of birch saplings. It is the anti rabbit terpene in almost pure form.) Mature trees are not eaten by rabbits and refrain from making the unneeded compound. But were it not for the defense provided by the compound in young trees, paper birch would be far less common in Alaska. This is one of many examples where plants (and animals) form terpenes to discourage their enemies.
It is the small terpenes that prevent most insects and animals from eating conifer foliage in the same way as the birch terpene discourages hare. Our trusting spruce grouse may be called "fool hens", but they have learned to live on conifer needles and thus have abundant forage all year long. Apparently they crush the needles, let the terpenes evaporate through their mouths, and swallow the nutritious remainder. And a favored meal it must be because, even during fall when the birds are heavy with cranberries, they still find room for a spruce-needle salad.
Plants under stress commonly produce extra defense compounds. When conifers suffer from air pollution their terpenes change in both composition and amount. We found five times more beta-pinene than normal in yellowed needles on the road-side spruce discussed last week . This is one of the shifts in biochemical composition that indicate damage is related to air pollution rather than some other cause.
Terpenes may protect conifers from most animal damage, but they also account for the flammability of a green forest. Crown fires, balls of burning terpenes, can jump from tree to tree and spread rapidly. You can see liquid terpenes on needles of a spruce branch held over a campfire. The terpenes cannot be retained by needles that have been heated so they leak out, giving a wet look to the bough just before it burns. The needles contain isoprene, too. This pre-terpene has the volatility of lighter fluid and may well be the first ignited compound in the spread of a forest fire along the tops of conifers. Warmed by burning volatiles, the terpenes evaporate and fuel an expanding fireball that ignites the next tree downwind.
So when you smell your Christmas tree, think of terpenes, hazy summer days, and contented spruce grouse, and remember to keep lighted candles at a safe distance.
