BOTH in natural and in planted forest stands trees vary greatly in diameter, form, and height, even when the trees are all of the same age. For example, in a natural forest stand of Douglas fir, aged seventy years and located on a poor growing site, the trees may range from 5 to 18 inches in diameter at breast height (4.5 feet above the ground). The numbers of trees in the range of diameter .classes are not equal, but are small in classes at the extremes of the range and large in the central classes. If these numbers were plotted upon the diameter class, a typical bell-shaped curve would be formed, ordinarily denoted a frequency curve.
Meyer, Walter H. 1930. Diameter Distribution Series in Evenaged Forest Stands. Yale School of Forestry Bulletin 28. 105 pp.
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