Elms are eastern deciduous trees well valued for shade and ornamental appearance, as well as valued commercially for their heavy, exceedingly hard wood. There are about 45 species of elms, with six native to the North American continent east of the Rocky Mountains.
American elm (Ulmus americana) is a native tree that has had a wide range from the entire eastern states through the Central Plains. It has, however, over the past 60 years, been decimated by Dutch elm disease, a fungal blight caused by the ascomycete, Ophiostoma (Ceratocystis) ulmi, and transmitted through a vector, the native elm bark beetle, Hylurgopinus rufipes. The fungus sporulates within the bark tunnels of the beetles, which brush against the sticky fruiting bodies as they make their way out to fly to other trees, carrying the spores with them. Mature trees are more susceptible to the blight than younger trees, the infected trees developing black weeping lesions at bough angles, with gradual death of the tree.
The fungus was introduced in the United States in Ohio in 193O, reaching Tennessee in 1946, and also appearing in Quebec that year. By 1975, the fungus had spread to California and Manitoba. Within 15 years in Canada, the fungus had spread over 25,000 square miles, killing 600,000 trees. Innumerable stands of mature trees have been destroyed, and American elm has now been severely restricted throughout much of its previous range. However, it is not in risk of extinction because younger trees are relatively resistant to the disease.
Several other species, such as Chinese elm (U. parvifolia) Siberian elm (U. pumila), and red or slippery elm (U. rubra), are resistant, or relatively so, to the fungal blight. Siberian elm is rapidly growing with either tree-like or shrubby appearance, well suited to urban locales, sprouting up along fence-rows and alleyways. After being introduced in the 1860s, it is now quite common, with a range similar to U. americana. Elms are characterized by elliptical leaves with serrated edges and an asymmetric leaf takeoff on either side of the leaf stem, with one side a wedge and the other side rounded. American elm is large-leafed, while Siberian, Chinese, and other elms have much smaller leaves.
Elm trees are all wind-pollinated, and prolific pollen producers. Anthesis begins early in the year, from tiny greenish flowers, with pollen appearing from late January into early March, depending on locale and meteorological factors. Milder winters appear to speed pollen maturation with an earlier onset, although ambient temperatures seem to have less impact on elm pollen counts than with other tree species. Pollination is intense over a short period of time. Individual trees may mature and release their pollen load over a 2- to 3-day period. Not all elms pollinate in the spring: Chinese elm, red elm, September elm (U. serotina), and cedar elm (U. crassifolia) pollinate in late summer or early
Adapted from: Annals of Allergy, Richard Weber, MD,
February 2001.