3. [Alnus tenuifolia] Nutt. Alder.
Leaves ovate-oblong, acute or acuminate, broad and rounded or cordate or occasionally abruptly narrowed and cuneate at base, usually acutely laciniately lobed and doubly serrate, when they unfold light green often tinged with red, pilose on the upper surface and coated on the lower with pale tomentum, at maturity thin and firm, dark green and glabrous above, pale yellow-green and glabrous or puberulous below, 2′—4′ long, 1½′—2½′ wide, with a stout orange-colored midrib impressed on the upper side, and slender primary veins running to the points of the lobes; petioles stout, slightly grooved, orange-colored, ½′—1′ in length; stipules ovate, acute, thin, and scarious, ½′ long, about ⅛′ wide, covered with pale pubescence. Flowers: staminate aments 3 or 4 in number in slender-stemmed racemes, nearly sessile or raised on stout peduncles often ½′ long, during the winter light purple, ¾′—1′ long and ¼′ thick, becoming 1½′—2′ in length; calyx-lobes rounded, shorter than the 4 stamens; pistillate aments naked during the winter, dark red-brown, nearly ¼′ long, with acute apiculate loosely imbricated scales, only slightly enlarged when the flowers open. Fruit: strobiles obovoid-oblong, ⅓′—½′ long, their scales much thickened, truncate and 3-lobed at apex; nut nearly circular to slightly obovoid, surrounded by a thin membranaceous border.
A tree, occasionally 30° tall, with a trunk 6′—8′ in diameter, small spreading slightly pendulous branches forming a narrow round-topped head, and slender branchlets marked at first by a few large orange-colored lenticels and coated with fine pale or rusty caducous pubescence, becoming light brown or ashy gray more or less deeply flushed with red in their first winter and ultimately paler; more often shrubby, with several spreading stems, and at the north and at high altitudes frequently only 4°—5° tall. Winter-buds ¼′—⅓′ long, bright red, and puberulous. Bark rarely more than ¼′ thick, bright red-brown and broken on the surface into small closely appressed scales.
Distribution. Banks of streams and mountain cañons from Francis Lake in latitude 61° north to the valley of the lower Fraser River, British Columbia, eastward along the Saskatchewan to Prince Albert, and southward through the Rocky Mountains to northern New Mexico; on the Sierra Nevada of southern California, and in Lower California; the common Alder of mountain streams in the northern interior region of the continent; very abundant on the eastern slopes of the Cascade Mountains, and on the southern California Sierras; forming great thickets at 6000°—7000° above the sea along the headwaters of the rivers of southern California flowing to the Pacific Ocean; the common Alder of eastern Washington and Oregon, and of Idaho and Montana; very abundant and of its largest size in Colorado and northern New Mexico.
4. [Alnus rhombifolia] Nutt. White Alder. Alder.
Leaves ovate or oval or sometimes nearly orbicular, rounded or acute at apex, especially on vigorous shoots, gradually or abruptly narrowed and cuneate at base, finely or sometimes coarsely and occasionally doubly serrate, slightly thickened and reflexed on the somewhat undulate margins, when they unfold pale green and covered with deciduous matted white hairs, at maturity dark green and lustrous on the upper surface, frequently marked, especially on the midrib, with minute glandular dots, light yellow-green and slightly puberulous below, 2′—3′ long, 1½′—2′ wide, with a stout yellow midrib and primary veins; petioles slender, yellow, hairy, flattened and grooved on the upper side, ½′—¾′ long; stipules ovate, acute, scarious, puberulous, about ¼′ in length. Flowers: staminate aments in slender-stemmed pubescent clusters, usually short-stalked, during the summer dark olive-brown and lustrous, ¾′—1′ long and about 1/16′ thick, beginning to lengthen late in the autumn before the leaves fall, fully grown and 4′—6′ long and ¼′ thick in January, with dark orange-brown scales, and deciduous in February before the appearance of the new leaves; calyx yellow, 4-lobed, rather shorter than the 2 or occasionally 3 or rarely single stamen; pistillate aments in short pubescent racemes emerging from the bud in December, their scales broadly ovate and rounded. Fruit: strobiles oblong, ⅓′—½′ long, with thin scales slightly thickened and lobed at apex, fully grown at midsummer, remaining closed until the trees flower the following year; nut broadly ovoid, with a thin margin.
A tree, frequently 70°—80° high, with a tall straight trunk 2°—3° in diameter, long slender branches pendulous at the ends, forming a wide round-topped open head, and slender branchlets marked by small scattered lenticels, at first light green and coated with pale caducous pubescence, soon becoming dark orange-red and glabrous, and darker during the winter and following summer. Winter-buds nearly ½′ long, very slender, dark red, and covered with pale scurfy pubescence. Bark on old trunks 1′ thick, dark brown, irregularly divided into flat often connected ridges broken into oblong plates covered with small closely appressed scales. Wood light, soft, not strong, brittle, close-grained, light brown, with thick lighter colored often nearly white sapwood.
Distribution. Banks of streams from northern Idaho to the eastern slope of the Cascade Mountains of Washington and southeastern Oregon, and southward from the valley of the Willamette River, Oregon (near Salem, Marion County, J. C. Nelson) over the coast ranges and along the western slopes of the Sierra Nevada to the mountains of southern California (San Bernardino, San Jacinto, and Cuyamaca Ranges); the common Alder of the valleys of central California, occasionally ascending on the southern Sierra Nevada to altitudes of 8000°, and the only species at low altitudes in the southern part of the state.