A shrub up to 15 or 20 ft high, occasionally a tree twice as high, of elegant form; bark almost black, not peeling; young shoots resinous, warted. Leaves glandular, broadly ovate, rounded or slightly heart-shaped at the base, pointed, double-toothed; 1 to 2 in. long, 3⁄4 to 11⁄2 in wide; dark dull green, slightly hairy above; paler and soon almost glabrous beneath; veins in three to five pairs; stalks 1⁄4 to 1⁄2 in. long, at first somewhat hairy, then glabrous. Male catkins up to 2 in. long. Fruiting catkins 1 to 11⁄4 in. long, the lobes of the scales about equal in size, slightly downy or glabrous.
Native of western N. America; introduced in 1897 to Kew, where it thrives very well and makes a graceful small tree. It is allied to B. papyrifera, but from the smaller-growing varieties of that species it is distinguished by the bark not separating into layers, and in being almost black. The very resinous young twigs and glandular young leaves also mark it.
The B. occidentalis of Sargent is B. papyrifera var. commutata (q.v.).
From the Supplement (Vol. V)
The description on page 425 was made from a cultivated tree and does not cover the whole range of variation of this species. The leaves are commonly simply toothed, less frequently double-toothed. The leaf-apex may be rounded rather than acute (but never acuminate, as often in B. papyrifera). The leaves lack the axillary tufts which normally characterise B. papyrifera.
† var. inopina (Jeps.) C. L. Hitchc. B. occidentalis f. inopina Jeps. – Twigs downy as well as glandular. Leaves usually hairy on both surfaces. Native mainly of California but extending north as far as southern British Columbia; introduced to Kew in 1980.
B. occidentalis hybridises freely with B. papyrifera in the wild, and the result is a confusing series of intergrading forms.