Seasonal Contraction and Elongation of Tree Trunks

The following is an article from The Annals of Improbable Research.

by Linda A. Kowalski, PhD. Viratest Carcinogen Monitoring Ltd., Vancouver, British Columbia
Geoffrey F. Auchinleck, P. Eng. Automed Corporation, Richmond, British Columbia

Estimating the total yield of standing timber in British Columbia is essential both to the forest industry and to environmentalists. Right now there is no adequate survey of the total softwood standing crop in BC, mainly because of inaccuracies in measuring the height of the trees.l We suggest that the reason for this inaccuracy is the seasonal change in height due to contraction of tree trunks during winter and elongation during summer.

Figure 1. Average measured heights (from ground to crown) of softwood trees.

During summer, the apical meristems and cambia of mature tree trunks grow both in height and in girth. In winter, freezing of water in xylem vessels, combined with cessation of growth, results in a decrease of volume and a concomitant reduction in height. This leads to the hypothesis that measurements of tree height and assessments of yields will vary seasonally. Our measurements show this to he true.

For years now, the BC Parks Board has been affixing index markers to many of the trees in their parks. (See Figures 2a and 2b.) We measured several specimens of Douglas Fir (Pseudotsuga menziesii), Western hemlock (Tsuga heterophylla), and Western red cedar (Thuja plicata) from top to ground level (A), from the index marker to the top (8), and from the index marker to ground level (C) (see Figure 1).

The Seasonal Variation in Height
On average, the total height of each Douglas fir tree, measured from the ground to the top, was 30 percent greater in summer than in winter. The average total height, both of Western hemlock and of Western red cedar, was also 30 percent greater in summer (see Figure 1). The BC Parks Board index markers were significantly closer to the ground in January, 1994 than in August, 1994 (see Figure 2, which shows the same tree and Parks Board Tree Estimation Marker).

The decrease in height was directly proportional to the measured snow depth (see Figure 3). This was true for all three species of trees. (We note, however, that all changes in height occurred below level of the index markers. Conversely, there was no seasonal variation in the distance measured from the markers to the tree tops.

Figure 2. Western hemlock showing position of Parks Board Tree Height index markers in (a-above) January, 1994, and (b-below) August 1994.

What Causes This?
There is a significant seasonal change in the heights of Douglas fir, Western hemlock, and Western red cedar. The trees are 30 percent taller in summer than in winter.

The direct relationship between snow depth and total height loss leads to two hypotheses about how and why this occurs.

Increased snow depth may lead to freezing and contraction of water within the xylem, tending to pull the trees into the ground. However, the observation that all the change in height occurs below the index marker argues against this hypothesis.

Figure 3. Snow depth vs. differences in height (August-January) of 100 softwood trees.

Alternatively, it could be that the index markers themselves are causing the phenomenon. Because all of the odd shrinkage and growth occurs below the level of the markers, we suspect this to be the case. Many metals, such as chromium2 and vanadium3 are growth promoters or mitogens. It is possible that trace amounts of metal leaching from the nail in the markers have biological effects that result in excessive summer growth.

In either case, the rapid growth during the spring may result in excessive elongation of the trunk creating an elastic tension. This would cause the trunk to snap back into the ground during cold periods, resulting in the audible cracking sound that is observed when trees freeze.

We suggest that seasonal variation in growth is a source of great inaccuracy in estimating total timber yields. In the future, all estimates of standing timber should be done only in winter, from ground level, to avoid measurement of stretched trees in summer.

1. "Variation in length of rulers composed of pine harvested before 1923," J. Collins, Antique Pine and Softwood, vol. 34, 1994, p. 129.

2. "DNA polymerase arrest by adducted trivalent chromium," LC. Bridgewater, F.Re. Manning, E.S.
Woo, and S.R. Patierno, Molecular Carcinogens, vol. 9,1994, pp. 122-133.

3. "Vanadate enhances ornithine decarboxylase expression in C3H/l0Tl/2 cells," A.1. Davison, A. Stern, D.J. Fatur, and S.S. Tsang, Biochemistry International,vol. 24, 1990, pp. 461-6.


This article is republished with permission from the May-June 1998 issue of the Annals of Improbable Research. You can purchase back issues of the magazine or subscribe to receive future issues, in printed or in ebook form. Or get a subscription for someone as a gift!

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