3. Ulmus L. (elm)

Plants shrubs or more commonly trees, 2–35 m tall. Bark longitudinally fissured into more or less parallel ridges (in our species; see the note below on U. parvifolia), not breaking into loose plates or strips (forming thin, elongate plates on younger trunks of U. crassifolia, but these remaining relatively tight), lacking warty protuberances, medium to dark gray. Twigs often somewhat zigzag, sometimes developing corky wings or outgrowths, the winter buds of various sizes and shapes, with several overlapping scales. Leaves 2-ranked. Leaf blades elliptic, oblong, or ovate to obovate or broadly oblanceolate, widest slightly below to slightly above the midpoint, rounded to more or less bluntly angled at the base, variously symmetrically to strongly asymmetrically so, sometimes with a minute notch at the attachment point, tapered to a sharply pointed tip (except in U. crassifolia and sometimes U. pumila), the margins bluntly or sharply and doubly toothed (often more or less singly toothed in U. pumila), the upper surface smooth to strongly roughened, the undersurface much paler than the upper surface, glabrous or minutely hairy along the veins, the hairs occasionally occurring as dense tufts in the vein axils, the venation pinnate, with secondary veins not forming loops or a network, extending to the margins, each ending in a tooth, equal in development and spacing, unbranched or occasionally forked, straight and more or less parallel, the basal pair shorter than median veins. Flowers all perfect (but see the note below), appearing in clusters (these sometimes umbellate or short-racemose) either before the leaves develop in the spring from buds on previous year’s growth or in autumn in the axils of leaves on current-year’s growth. Calyces 1.5–3.0 mm long, shallowly to deeply 4–9-lobed, green to greenish yellow or reddish brown, turning tan to brown after flowering. Stamens with the anthers red to brownish red or purple to nearly black. Fruits samaras, appearing flattened, with 2 wings spreading in the same plane on either side of the seed and an apical notch, papery, the surface glabrous or finely hairy, lacking outgrowths. About 40 species, North America, Central America, Europe, Asia.

Vegetative specimens of Ulmus cannot be identified with any high degree of confidence, and should be avoided by collectors. Critical identification requires flowers or fruits. Leaves of juvenile plants, or of fast-growing shoots from plants that have been cut back or damaged, tend to be large, strongly toothed, strongly roughened on the upper surface, and uniformly pubescent on the underside. They are similar in all species of elm, and are very difficult to identify. The keys and descriptions below all refer to leaves of fertile adult trees. Some species may develop corky wings or outgrowths on their twigs, vs. others that never do so. However, trees normally develop cork only on some twigs, and some trees may not show cork formation on any twigs even if it is usually characteristic of the species. The presence of corky twigs on a specimen can be very useful for identification, but it is important not to read too much into the absence of cork on twigs of a particular branch.

Flowers of Ulmus have both staminate and pistillate organs well-developed and are generally described as having perfect flowers. It has recently been shown that many individuals of U. minor are functionally staminate and set no fruit (López-Almansa et al., 2003), and the reproductive biology of other elm species should be checked.

The seeds and buds of elm are eaten by deer, rodents, and birds. Elm wood is hard and strong, and it is used for furniture, fenceposts, flooring, and general construction, and as a veneer for other woods. In the past, it was widely used for manufacturing wheel hubs, saddle trees, ship’s hulls, and agricultural and kitchen implements, uses where its hardness and flexibility are especially valuable. It has interlocked grain and is difficult to split, making it especially suitable for items that require bending, such as hockey sticks or curved pieces in furniture. Elm bark is used for tanning leather. Elms were once widely used for construction of bark canoes, and fiber from elm bark has been used in the past to make ropes, cords, and roofing felt. Elm foliage was long used as fodder for livestock in Eurasia, and is still important for this purpose in northern India. Many bird, insect, and mammal species feed on the buds, twigs, and seeds of various elm species.

Elms are very widely grown as street trees and shade trees, but their use has been limited by an exotic disease, known as Dutch elm disease, which was first noted in Europe in 1919 and subsequently spread across North America in the mid-twentieth century (Strohel and Lanier, 1981; Wolkomir, 1998). Dutch elm disease is caused by exotic wilt fungi (Ophiostoma spp.); the native range of these fungi is still unknown. All of our native elm species are impacted by the fungus. In the best-studied species, U. americana, there is great variation in disease resistance from tree to tree, and several disease-tolerant trees of U. americana have been identified (Wolkomir, 1998) and propagated, and are now available commercially (Townsend et al., 2005).

Ulmus serotina Sarg. (September elm or red elm) occurs in northern Arkansas and southern Illinois. It resembles U. thomasii in most of its characters, but it flowers in the fall rather than the spring, and also has glabrous twigs and buds and usually smaller leaves. Old reports of U. serotina from Missouri are based on a mislabeled specimen that was actually collected in Arkansas (Steyermark, 1963). Another fall-flowering (but non-native) elm that is commonly cultivated in Missouri is U. parvifolia Jacq, (Chinese elm). For further discussion of this species, see the treatment of U. pumila.