Tracks and twisted trees

Posted: Sunday, February 22, 2009

The lower cross-country ski loop at Eaglecrest can be a great, gentle ski. There seem to be some new routes through the meadows this year, and I must get a map so I can navigate properly up there.

But the lower loop offers more than nice skiing. I've been there a few times in the past couple weeks (all that wonderful weather!), and the animal-tracking was pretty lively. There were the tiny, delicate stitch marks of a mouse or vole dashing from one sheltering hemlock to another. A snowshoe hare had fossicked about under some hemlocks where the snow was not so deep, and lollopped downslope to another thicket. We found day-old furrows plowed by wallowing porcupines and two-toed plunge marks of a passing deer. A weasel had passed through, perhaps last night.

The best tracks were those of ptarmigan, who had been very busy in several spots. We found 'splash-down' marks where they landed in the snow, subsequent walking tracks zigzagging here and there, and then a take-off marked by shallow wing traces. Some birds had clearly alternated a walk with a short flight, all out in the open away from shrubs or trees. We did not see any signs of foraging in these areas.

Another source of interest was the numerous dead trees standing barkless and often showing a twisted grain in the wood. Some of the twisted trunks were very pretty, in subtle tones of gray and tan. My fellow skiers were curious about why some trunks were twisted and others were not. I decided I should find out, and here is what I've learned so far (there is probably more to the story!).

By looking around in various places in town, I soon found out that trees of the same species can be twisty in one location but straight-grained in another. A little library work showed that many kinds of trees have the ability to twist, and that, up to a point, twisting makes the tree stronger and better able to resist breakage, but too much twist weakens the tree. Twisted grain is a response to wind that exerts more force one side of an asymmetrical tree crown than on the other. So trees growing, with lots of close neighbors, in a forest seem to have less twist than trees in the open.

If the crown is asymmetrical for some reason, wind can torque the tree, forcing the wood-making cells to grow in a slanted direction. A right-handed spiral, for instance, occurs if the tree crown is asymmetrical to the south and exposed to a westerly wind. Almost all of the twisted trees in the ski meadow have a right-hand twist, some with a strong twist and others with just a slight one. By inspecting asymmetry of the crowns of the living trees, one could presumably deduce the direction of winds in that meadow, or alternatively, if one knew the direction of prevailing winds in the meadow, one could deduce the direction of crown asymmetry. Doing this might be a test of the hypothesis about the effect of wind on lopsided tree crowns.

Why would a tree crown be asymmetrical? Perhaps it gets more sun on one side than another, or one side has been injured. Or perhaps snow or lichens build up on one side and offer more resistance to wind. Wind itself can be a cause of crown asymmetry-strong, recurring winds can prune trees such that most of the branches are on one side. However, such pruning leaves the branches on the side away from the wind, so that wind-torque would be small.

Branches on a twisted trunk may also twist. On the trees we inspected, we found that branches twisted in the same direction (from base to tip) as the trunk from which they sprang.

A given trunk can change the direction of twist as it lays down layers of new wood. So a trunk might be straight in the middle (wood produced when the tree was young) and strongly twisted in the newer, outside layers. There's an example of this in an old snag along the lower loop; see if you can find it!

• Mary F. Willson is a retired professor of ecology.

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