2. Ulmus americana L. (American elm, white elm)

Pl. 570 k, l; Map 2664

Plants trees to 35 m tall. Twigs tan to reddish brown, usually hairy when young, often becoming nearly glabrous with age, rarely completely glabrous, never with corky outgrowths or wings. Winter buds 3–8 mm long, narrowly ovoid to conic, sharply pointed, brown to reddish or purplish brown, minutely pubescent with white to red hairs. Petioles (3–)4–9 mm long. Leaf blades 6.5–12.5 cm long, 3.3–7.5 cm wide, elliptic to oblong-elliptic or occasionally oblong, the base strongly asymmetric, short-tapered to a sharply pointed tip, the major marginal teeth 2–4 mm deep, sharp, all or most with 1–3 smaller secondary teeth, the upper surface smooth or somewhat roughened, the undersurface sparsely hairy along and between the main veins, usually also tufted in the vein axils, the secondary veins 11–21 on each side of the midvein, the lateral veins seldom (occasionally 1 or 2 per side) forked toward their tips; juvenile leaves never lobed. Inflorescences drooping umbellate clusters appearing before the leaves develop in the spring on second-year twigs. Flowers with the stalks 10–20 mm long, the calyces shallowly (5–)7–9-lobed, the tube glabrous, the lobes broadly rounded, glabrous or the margins sparsely hairy. Fruits 0.9–1.3 cm long, 0.6–0.8 cm wide, elliptic, tan, not appearing inflated, the body and wings glabrous on the surfaces, but the wing margins densely hairy. 2n=28, 56. March–April.

Scattered nearly throughout the state, but apparently absent from the northwestern corner (eastern U.S. west to Montana and Texas; Canada). Bottomland forests, mesic upland forests, banks of streams and rivers, margins of ponds, sinkhole ponds, and oxbows, and tops and ledges of bluffs; also roadsides and disturbed areas.

Plants vary in the relative roughness of the leaves and the pubescence of the twigs, and these have been named as follows: Plants with roughened leaves and pubescent twigs have been called f. alba (Aiton) Fernald; plants with roughened leaves and glabrous twigs have been called f. intercedens Fernald; plants with smooth leaves and pubescent twigs have been called f. pendula (Aiton) Fernald; and plants with smooth leaves and glabrous twigs have been called f. laevior Fernald. Elias (1970) noted that smooth and roughened leaves may be found on the same tree; thus these minor variants are unworthy of formal taxonomic recognition.

Most references list U. americana as a tetraploid, with 2n=56, but actually both diploid and tetraploid races are found in Missouri, sometimes coexisting in the same stand; triploids are known from elsewhere (Whittemore and Olsen, 2011). Further study is needed to determine the relationship between these genetic forms.

Ulmus americana formerly was grown widely as a street tree and shade tree, but the use of the species has been limited by Dutch elm disease, which has killed millions of elms since its introduction into North America (for further discussion, see the paragraph under the generic treatment of Ulmus).