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Broadleaved Trees of Yosemite National Park (1947) by C. Frank Brockman


BLACK COTTONWOOD

Populus trichocarpa Torr. & Gray — Willow Family (Salicaceae)

Black cottonwood in Yosemite Valley
[click to enlarge]
Photo by Anderson

Black cottonwood in Yosemite Valley

Although not a tree of particular beauty, the black cottonwood is one of the most easily recognized of our broadleaved trees. It is rarely found above 4,500 feet and is common in the Yosemite Valley, Wawona, and Hetch Hetchy areas where it grows along streams or in moist meadows. Numbers of fine specimens can be readily found along the Merced River in Yosemite Valley. Several are growing on the bank of Yosemite Creek which borders the cabin area of Yosemite Lodge.

Young trees, as well as the branches and upper trunk of mature specimens, are characterized by smooth, pale gray bark which assumes a dark gray, heavily ridged and furrowed character on old trunks. In the latter case the bark is occasionally as much as two inches thick. The larger trees, which may attain an age of from 60 to 90 years, are 80 to 90 feet tall and from two to three feet in diameter. In most instances the trunks are free of branches for a distance of from one-half to two-thirds of their height, with a short, ragged, open crown characterized by heavy, upright branches. These branches are quite brittle, or “brash,” and often snap off during periods of high wind or heavy snow.

The thick, leathery leaves, finely toothed along the edges, are from two to seven inches long, broad at the base and tapering to an acute point. They are shiny green on the upper side, pale to silvery-white beneath, and are further characterized by large, conspicuous veins. Mid-veins, and often the slender round leaf stems, are sometimes slightly hairy. In the fall before dropping from the tree the leaves assume a dull, yellowish-brown color which, although suffering by comparison with the fall color of the California black oak and Pacific dogwood, nevertheless adds a note of interest to our fall color display.

Foliage and seeds of black cottonwood (Inch squares on background)
[click to enlarge]
Photo by Brockman

Foliage and seeds of black cottonwood (Inch squares on background)

Perhaps its most interesting characters are the flowers and seeds. Staminate (male) and pistillate (female) flowers are borne on different trees. The former, which bear the pollen, are produced in great numbers in dense, pendent, tassel-like clusters (aments) one to two inches long. The latter, also borne in pen. dent aments, are more loosely clustered. At maturity they are four or five inches long and appear as grape-like clusters of round, green globules, each one containing a multitude of tiny, brown seeds to which are attached a number of soft, cottony filaments by means of which the seeds are wind dispersed. It is to this character that the tree owes its name. In midsummer when the green seed-laden capsules open, the air is filled with these wind-borne seeds and the ground in the vicinity of these trees is often covered with a thick, downy carpet of “cotton.”

The twigs are slender to moderately stout, sometimes slightly angular, and vary in color from orange-brown to greenish-brown. The buds, similarly colored, are as much as three-quarters of an inch long, narrowly conical with six or seven overlapping scales, but their most distinctive characteristic is the presence of gummy, fragrant resin. Leaf scars which, with the twigs and buds, assist in winter identification of this tree are large and conspicuous, being slightly raised, broadly crescent-shaped to triangular in outline (sometimes three-lobed), and alternate, with three large vascular bundle scars appearing thereon.

The natural range of the black cottonwood includes an extensive area of the Pacific west — from southern Alaska and the Yukon south through British Columbia, Washington, and Oregon to southern California.



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