American Elm
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{{Taxobox | color = lightgreen | name = American Elm | image = Img ulmus americana 2209.jpg | image_width = 240px | image_caption = Ulmus americana (American Elm) | regnum = Plantae | divisio = Magnoliophyta | classis = Magnoliopsida | ordo = Rosales | familia = Ulmaceae | genus = Ulmus | species = U. americana | binomial = Ulmus americana | binomial_authority = L. }}
The American Elm Ulmus americana is a species of elm native to eastern North America, occurring from Nova Scotia west to southeast Saskatchewan, and south to Florida and central Texas. It is also sometimes known as White Elm or American White Elm. It is an extremely hardy tree that can withstand harsh winters, even in northern and central British Columbia, Alberta and Saskatchewan, where they have been extensively planted as ornamental trees in cities across these Canadian provinces. Healthy trees can live for up to 275 years.
It is a deciduous tree, sometimes growing >40m tall with a trunk up to 3.5m in diameter. The crown forms a high, spreading canopy with open air space beneath. The leaves are alternate, 7-15 cm long, with double-serrate margins and an oblique base. The flowers are small, purple-brown with no petals, and produced in early spring before the leaves. The fruit is a flat samara 2 cm long and 1.5 cm broad, with a circular wing surrounding the single 4-5 mm seed; like the closely related European White Elm U. laevis, the flowers and seeds are borne on 1-3 cm long stems. The American Elm is unique within the genus in being tetraploid, ie. having double the normal number of chromosomes.
American Elm has been seriously affected by an introduced fungal disease, Dutch elm disease (DED), with heavy mortality in most of the range and in many areas outside of the natural range as far west as California. However, DED has had little impact in Florida, and has not spread to the northernmost areas of cultivation, such as most of Alberta and British Columbia.
Cultivation and uses
In years past, it was used widely as a shade tree and street tree because of its graceful, arching, vase-like growth form and its tolerance of stress. Furthermore, the cross-grained wood gives a level of strength to the branches that resists easy breaking.
Dutch elm disease(DED) has devastated the American Elm, causing catastrophic die-offs in cities across the range. It is a serious fungal disease that causes the trees to wither and eventually die by blocking its ability to take water. The disease was accidentally introduced from Europe, hence the name. Once this disease infected one tree on a street, other American elm trees close to it would die quickly because the fungus would infect them via the roots through root grafts that the trees had formed from. There still are many American Elms in the woods, but in cities, the ones that survived are generally those that are isolated from other elms. An example of this is Central Park in New York City, where a stand of several elms originally planted by Frederick Law Olmstead near the center of the park survives, although neighboring areas in New York had large die-offs.
In some areas still not yet populated by the Dutch Elm disease-carrying Elm bark beetle, the American Elm continues to thrive, particularly in most of Alberta and in British Columbia. It is reliable and recommended in places such as Calgary and Edmonton in Alberta. In fact, the province of Alberta has the largest number of DED-free American elms in the world. Aggressive measures are being taken to combat the spread of DED into Alberta, especially after a tree was found to have DED in southeastern Alberta in 1998. (the tree was immediately destroyed and this was an isolated case, as no other trees were affected) A smaller beetle, the European bark beetle, is also known to carry the disease. Although the European bark beetle is known to have been found across southern and central Alberta, they apparently do not seem to be carrying the disease.
Some cities such as Kansas City, Missouri had used mostly American elms in planting its city streets, and had some of the best-shaded residential streets in the nation until the disease almost obliterated these plantings in the late 1960s and early 1970s.
Attempts have been made over the last few decades to breed disease-resistant elm cultivars. Selections include 'Liberty', 'Valley Forge', and 'Princeton'. Other trees commonly used as substitutes for American Elm are Zelkova and Chinese Elm.