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American Elm

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iAmerican Elm
Ulmus americana (American Elm) at Longwood Gardens
Ulmus americana (American Elm) at Longwood Gardens
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Urticales
Family: Ulmaceae
Genus: Ulmus
Species: U. americana
Binomial name
Ulmus americana
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 (min. temperature of -42 °C), 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 potentially can live for hundreds of years.

Contents

[edit] Description

The American Elm is a deciduous tree, sometimes growing up to 30 m [100 ft] tall with a trunk up to 1.2 m [4 ft] 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 tree is hermaphroditic, having perfect flowers, i.e. with both male and female parts, and is therefore capable of self-pollination. The flowers are small, purple-brown and, being wind-pollinated, have no petals; they emerge 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 reaches sexual maturity at around 15 years of age and is unique within the genus in being tetraploid, i.e. having double the usual number of chromosomes.However, it is very likely that they won't reach over 10 years.

[edit] Dutch Elm Disease

The 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.

Dutch elm disease has ravaged the American elm, causing catastrophic die-offs in cities across the range. It is an extraordinarily efficient fungal disease that causes the trees to wither and eventually to die, by blocking the circulation of water and nutrients through their inner bark. The disease was accidentally introduced into North America in 1930, after it already had destroyed populations of Dutch elm hybrids in Europe and thus gained its name. Infection occurs when an elm bark beetle vectors the fungus from an infected tree and then burrows into the living tissue of a healthy tree. Once a tree becomes infected with this disease, other American elm trees close to it often die quickly, because the fungus can infect them via the roots through root grafts that the trees had formed underground; if a tree has root grafts with an adjacent, infected tree, the fungus may spread easily from one tree to the other, directly through the roots.

A fair number of mostly small to medium-sized American elms survive nowadays in woodlands, suburban areas, and occasionally cities, where most often the survivors had been relatively isolated from other elms and thus spared a severe exposure to the fungus. For example, inCentral Park and Tompkins Square Park in New York City, stands of several large elms originally planted by Frederick Law Olmstead survive due to isolation from neighboring areas in New York where there had been heavy mortalities from the epidemic. In historical areas of Philadelphia, Pennsylvania there are also a few mature American elms still standing -- notably in Independence Square and the Quadrangle at the University of Pennsylvania -- and at the nearby campuses of Haverford College and Swarthmore College.

The American elm's biology in some ways has helped to spare it from obliteration by the Dutch elm disease, in contrast to what happened to the American chestnut with the chestnut blight. The elm's seeds are wind-dispersed, and the tree grows quickly and begins bearing seeds at a young age. It grows well along roads or railroad tracks, and in abandoned lots and other disturbed areas, where it is highly tolerant of most stress factors. Elms have been able to survive and to reproduce in areas where the disease had eliminated old trees, although most of these young elms eventually succumb to the disease at a relatively young age. There is hope that these elms will preserve the genetic diversity of the original population, and that they eventually will hybridize with DED-resistant varieties that are being developed or that occur naturally.

In some areas still not 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.) Two species of elm bark beetle, one of them native, are known to carry the disease in North America. Although the European elm bark beetle is known to have occurred across southern and central Alberta, it does not appear to be carrying the disease in these areas.

Some cities such as Kansas City, Missouri had used mostly American elms in planting its city streets and had 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. Many cities in the United States still have some surviving American elms, but generally this species requires frequent attention to check for elm bark beetles and DED infection. (The National Park Service often checks on the hundreds of elm trees under its care in the Washington D.C. area for signs of illness.) Fungicidal injections can be administered by a qualified arborist to valuable American elms, to prevent the trees' becoming infected. Such injections generally are effective as a preventive measure for up to three years when performed before any symptoms have appeared, but they may not be so effective as a treatment once the disease is visibly present.

[edit] Disease-resistant cultivars

Attempts have been made over the last few decades, with some success, to breed disease-resistant American Elm cultivars. The few selections that have been made available to the public as yet include Valley Forge, New Harmony, Princeton, Jefferson, and a set of six different clones collectively known as American Liberty. The United States National Arboretum released the Valley Forge and New Harmony varieties to the public in 1996, after screening tests performed in 1992-93 showed each of these two varieties to have unusually high levels of resistance to DED. The Valley Forge variety had performed especially well in these tests. The Princeton variety has been in occasional cultivation since the 1920s, and it gained renewed attention after its performance in the same screening tests showed it also to have a high degree of DED resistance. A later test performed in 2002-2003 confirmed the DED resistance of these same three varieties and also showed the variety Jefferson to have high resistance to DED. The Jefferson elm was released to wholesale nurseries in 2004 and is becoming increasingly available for planting. Thus far, plantings of these four varieties generally appear to be successful. In 2005, 90 Princeton elms were planted near the White House and to date are healthy and thriving.

[edit] Cultivation and uses

In years past, the American elm was used widely as a shade tree and as a street tree, because of its graceful, arching, vase-like growth form and its tolerance of most stress factors. Furthermore, the cross-grained wood gives a level of strength to the branches that generally resists breaking. The species has been planted beyond its range as far north as central Alberta, and south to Lake Worth, Florida. It also survives low desert heat at Phoenix, Arizona.

Introductions across the Atlantic rarely prospered, even before the outbreak of Dutch elm disease. In the UK, it was noted in the 19th century that the foliage of American elms was far more prone to insect damage than native elms. Examples are still to be found at the Sir Harold Hillier Gardens and in the NCCPG elm collection held by Brighton and Hove City Council, which also includes the cultivars 'Beeb's Weeping' and 'Exhibition'. The cultivar 'Princeton,' which had become available commercially once again during the 1990's, was selected in 2002 by HRH The Prince of Wales to form an avenue from his residence Highgrove House to the garden boundary.

[edit] Ecology

The American elm occurs naturally in an assortment of conditions, most notably on bottomlands and floodplains, although it also can thrive in well-drained soils. On more elevated terrain, as in the Appalachian Mountains, it often prefers to grow along streams. In the United States, it is a major member of four cover types: Black Ash-American Elm-Red Maple; Silver Maple-American Elm; Sugarberry-American Elm-Green Ash; and Sycamore-Sweetgum-American Elm. The first two of these types also occur in Canada.[1] Some hilltops near Témiscaming, Quebec have a Sugar Maple-Ironwood-American Elm cover type.

[edit] References

  • Brown, Jean-Louis. (1981). Les forêts du Témiscamingue, Québec: écologie et photo-interprétation. Laboratoire d'écologie forestière, Université Laval, Québec.
  • Costello, L. R. (2004). A 10 -year evaluation of the performance of four elm cultivars in California, U. S. Journal of Arboriculture, March 2004. [2]
  • Elwes, H. J. & Henry, A. (1913). The Trees of Great Britain & Ireland. Vol. VII. pp 1848-1929. Private publication. [3]
  • Townsend, A. M., Bentz, S. E., and Douglass L. W. (2005). Evaluation of 19 American Elm Clones for Tolerance to Dutch Elm Disease. Journal of Environmental Horticulture, March 2005, Horticultural Research Institute, Washington, D.C.

[edit] External links

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