Ulmus americana, generally known as the American elm or, less commonly, as the white elm or water elm, is a species of elm native to eastern North America. The trees can live for several hundred years. It is a very hardy species that can withstand low winter temperatures. The American elm was once exceptionally common as a street tree, its tolerance of urban conditions making it a popular choice to line American streets in the 19th and early 20th centuries. The coming of beetles bearing Dutch elm disease devastated the American elm population, starting in the 1920s. The species still holds out in some locations, but is also no longer nearly as common as during its height. The wood was seldom utilized until the advent of mechanical sawing. It is the state tree of Massachusetts and North Dakota.
Description The American elm is a deciduous tree which, under ideal conditions, can grow to heights of 21 to 35 meters (69 to 115 feet). The trunk may have a diameter at breast height (dbh) of more than 1.2 m (4 ft), supporting a high, spreading umbrella-like canopy. The leaves are alternate, 7–20 centimeters (3–8 inches) long, with double-serrate margins and an oblique base. The leaves turn yellow in the fall. The perfect flowers are small, purple-brown and, being wind-pollinated, apetalous. The flowers are also protogynous, the female parts maturing before the male, thus reducing, but not eliminating, self-fertilization, and emerge in early spring before the leaves. The fruit is a flat samara 2 cm (3⁄4 in) long by 1.5 cm (1⁄2 in) broad, with a circular papery wing surrounding the single 4.5 millimeters (1⁄8 inch) seed. As in the closely related Ulmus laevis (European white elm), the flowers and seeds are borne on 1–3 cm (0.39–1.18 in) long stems. American elm is wholly insensitive to daylight length (photoperiod), and will continue to grow well into autumn until injured by frost. Ploidy is 2n = 56, or more rarely, 2n = 28. For over 80 years, U. americana had been identified as a tetraploid, i.e. having double the usual number of chromosomes, making it unique within the genus. However, a study published in 2011 by the Agricultural Research Service of the United States Department of Agriculture revealed that about 20% of wild American elms are diploid and may even constitute another species. Moreover, several triploid trees known only in cultivation, such as 'Jefferson', are possessed of a high degree of resistance to DED, which ravaged American elms in the 20th century. This suggests that the diploid parent trees, which have markedly smaller cells than the tetraploid, may too be highly resistant to the disease.
Taxonomy Ulmus americana was first described and named by Carl Linnaeus in his Species Plantarum, published in 1753. No subspecies or varieties are currently recognized.
Distribution and habitat The American elm is native to eastern North America, occurring from Nova Scotia west to Alberta and Montana, and south to Florida and central Texas. It is an extremely hardy tree that can withstand winter temperatures as low as −40 °C (−40 °F). The species occurs naturally in an assortment of habitats, most notably rich bottomlands, floodplains, stream banks, and swampy ground, although it also often thrives on hillsides, uplands and other well-drained soils. On more elevated terrain, as in the Appalachian Mountains, it is most often found along rivers. The species' wind-dispersed seeds enable it to spread rapidly as suitable areas of habitat become available. American elm fruits in late spring (which can be as early as February and as late as June depending on the climate), the seeds usually germinating immediately, with no cold stratification needed (occasionally some might remain dormant until the following year). The species attains its greatest growth potential in the Northeastern US, while elms in the Deep South and Texas grow much smaller and have shorter lifespans, although conversely their survival rate in the latter regions is higher owing to the climate being less favorable to the spread of DED. In the United States, the American elm is a principal member of four major forest cover types: black ash-American elm-red maple; silver maple-American elm; sugarberry-American elm-green ash; and sycamore-sweetgum-American elm, with the first two of these types also occurring in Canada. A sugar maple-ironwood-American elm cover type occurs on some hilltops near Témiscaming, Quebec.
Ecology The leaves of the American elm serve as food for the larvae of a number of species of Lepidoptera. These include such butterflies as the Eastern Comma (Polygonia comma), Question Mark (Polygonia interrogationis), Mourning Cloak (Nymphalis antiopa), Painted Lady (Vanessa cardui) and Red-spotted Purple (Limenitis arthemis astyanax), as well as such moths as the Columbian Silkmoth (Hyalophora columbia) and the Banded Tussock Moth (Pale Tiger Moth) (Halysidota tessellaris).
Pests and diseases The American elm is susceptible to Dutch elm disease and to elm yellows. In North America, there are three species of elm bark beetles: one native, Hylurgopinus rufipes ("native elm bark beetle"); and two invasive, Scolytus multistriatus ("smaller European elm bark beetle") and Scolytus schevyrewi ("banded elm bark beetle"). Although intensive feeding by elm bark beetles can kill weakened trees, their main impact is as vectors of DED. American elm is also moderately preferred for feeding and reproduction by the adult elm leaf beetle Xanthogaleruca luteola and highly preferred for feeding by the Japanese beetle Popillia japonica in the United States. U. americana is also the most susceptible of all the elms to verticillium wilt, whose external symptoms closely mimic those of DED. However, the condition is far less serious, and afflicted trees should recover the following year.
Dutch elm disease
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