Skinny Trees and Paleoforests

Sometimes I put things together and think I've answered a scientific question. It's great fun and dangerous only if I start to believe my own theory. This week's answer to an idle question: spruce trees growing on permafrost are skinny because their cold feet make them think they're growing farther north.

That's not as silly as it sounds. Honest.

The background for my hypothesis comes from a research paper given to me by geologist Don Triplehorn: "Climatic Indications from Growth Rings in Fossil Woods," by British scientists G.T. Creber and W.G. Chaloner. It explains why forests once grew at far higher latitudes than they now do.

Real forests, with big trees, did grow at very high latitudes indeed. An abundance of fossil wood, including rooted stumps, lies in the high Arctic and even in Antarctica to prove it. Creber and Chaloner showed that warmth, not light, limits the growth of ring-forming trees. In June, struggling plants on Cornwallis Island, near 75 degrees north in the Canadian Arctic, receive nearly the same amount of light as thriving trees in the parks of Washington, D.C, near 40 degrees north latitude. But today, a tree in the high Arctic wouldn't be sufficiently warm long enough for its metabolic processes to function properly.

Growth rings in fossilized trees establish the presence of changing seasons, including warm summers, in high latitudes. However, a tree growing in the long-gone warm Arctic wouldn't look like a tree growing in the ancient temperate zone. Even then, a high-latitude tree would probably be skinny, like the pipecleaner spruce of Alaska's interior.

The tendency for a tree to have a narrow span in comparison to its height is energy-efficient at high latitudes. The British report included an elegant diagram showing sun angle compared to tree shape, using a stylized conifer to make the image clear. Lower latitudes mean the sun rises higher in the sky; the best shape for a spruce-type tree there is fairly fat, so the horizontal reach of its branches gathers the most sunlight. In the diagram, the temperate-zone conifer was a chunky pyramid--nice Christmas tree material--with a height-radius ratio of about 2 to 1.

Near the northern tree line, the low-angle sunlight promotes a more vertical form. There, the optimal proportions for a conical tree crown are 8 units in height for 1 in radius.

This part of the hypothesis isn't mine, nor even really a hypothesis; it's well established that sun angle dictates tree shape through the processes of natural selection operating over time. Northern spruce stand tall and skinny because that's their most efficient shape.

The permafrost part is utter speculation. It leapt to mind after a recent family weekend drive--1100 miles from Fairbanks to Valdez, over to Anchorage, and back home past Denali Park.

You see a lot of trees when you drive that far through Alaska, and some barely earn the name. Outside Glennallen, the highway passes through what we called Wimp Forest--acres of paltry black spruce where the height to radius ratio must be about 12 to 1 (and where a tree twelve feet tall is uncommonly high). Wow, bad permafrost here, we said.

The interesting thing is that nearby trees on better soil look fine, generally a bit broader than their kin growing near Fairbanks. That's to be expected, given the slightly higher sun elevation south of the Alaska Range.

Okay, I admit there are lots of causes for stunted trees. But I suspect the specific form of stunting shown by black spruce living on near-surface permafrost comes about because the soil temperature in the root zone mimics that found much farther north, where the skinny shape would be an advantage in gathering light. The cold soil would affect the speed with which chemical interactions could take place at root level, and that might trigger genetic signals encouraging a tree to assume its far northern form.

Further, the mistake might confer an evolutionary advantage. Scrawny trees offer less wind resistance, and a tree growing on permafrost couldn't develop much of a root system to prevent its being blown over. Thus the higher-latitude form would be more likely to survive gusts and produce offspring.

Well, this sounds logical. Just remember, please, that the preceding two paragraphs are completely unfounded in fact. I leave it to forest ecologists and tree physiologists to explore the idea--and expect I'll hear from them if they already have!