Measuring Tree Growth

By Eve Pranis

Puzzling Out Tree Growth
Challenge your students to figure out just where tree growth takes place. (Trees add length from the tips of twigs and rootlets, and width when the cambium adds new xylem and phloem cells.) They might mark different spots on twigs, then measure the distances between markers over time. If they observe twigs from the tips down, they'll notice fine lines encircling the twig where the bark color changes. These mark the position of the last terminal bud -- the end of last year's growth. (On conifers, a shift in needle color typically signifies new growth.) Students could also explore whether twigs on different trees grow at the same rate, or what conditions (temperatures, pests, diseases, water) seem to affect the amount of annual growth.

Students might suggest measuring the circumference of the trunk or branches. (This type of growth may be difficult to quantify in a short period!) If you have access to tree slices or "cookies," students can count and examine annual growth rings formed as the cambium adds new cells. They might even try to compare weather records with the appearance of unusually large or small rings. You can also use these slices to examine the inner workings of the trunk.

Measuring Up
How high is up? Challenge students to figure out how to measure the height of a tree, then test their theories. Here is one sunny-day option. Drive a stick straight into the ground in a sunny spot. Measure the length of stick from the ground to the top. Then measure the length of the stick's shadow and the length of the tree's shadow. The tree height equals the length of its shadow multiplied by the stick height divided by the stick's shadow (shadow of tree/ height of tree = shadow of stick/ height of stick).

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