American Elm

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.

Description

The American Elm is a deciduous tree, sometimes growing over 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.

Dutch Elm Disease

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 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. Infection occurs when a beetle vectors the fungus from another, infected tree, and burrows into the living tissue of the tree. 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 underground; if an infected tree had root grafts with an adjacent tree, the fungus could spread easily from one tree to the other directly through the roots, removing the need for the fungus to be vectored by another beetle. There still are many American Elms in the woods and occasionally in the suburbs, but in cities, the ones that survived are generally those that are isolated from other elms. Examples of this are Central Park and Tompkins Square Park in New York City, where stands of several elms originally planted by Frederick Law Olmstead survive due to isolation from neighboring areas in New York where there were massive die-offs; there are also a few surviving, mature American elms standing in historic areas of Philadelphia, Pennsylvania-- notably in Independence Square-- and at the nearby campuses of Haverford College and Swarthmore College.

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. Some American cities still have some survivors, but generally this species requires frequent "checkups" by caretakers for beetles and DED infection. (The National Park Service often checks up on the hundreds of trees under its care in the Washington D.C. area for signs of illness.)

Attempts have been made over the last few decades to breed disease-resistant elm cultivars. Selections include 'Liberty', 'Valley Forge', and 'Princeton'. Thus far the plantings appear to be successful: in 2005 90 Princeton trees were planted near the White House and to date are healthy and thriving; the United States National Arboretum also plans on releasing a new cultivar known as 'Jefferson' within the next four years as this strain has proven to have high resistance to DED.

Fungicidal injections can be administered by a qualified arborist to valuable American Elms to prevent the trees from becoming infected, or to fight the disease once present. Correctly applied, such a systemic treatment can be effective for two or three years.

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. It has been planted beyond its range as far north as central Alberta, and south to Lake Worth, Florida. It survives low desert heat at Phoenix, Arizona.

Ecology

American elm is primarily found on bottomlands and floodplains. In hillier terrain, such as the Appalachian Mountains, it 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.^* Some hilltops near Témiscaming, Quebec have a Sugar Maple-Ironwood-American Elm cover type.

References

Brown, Jean-Louis. (1981). Les forêts du Témiscamingue, Québec: écologie et photo-interprétation. Laboratoire d'écologie forestière, Université Laval, Québec.
Elwes, H. J. & Henry, A. (1913). The Trees of Great Britain & Ireland. Vol. VII. pp 1848-1929. Private publication, Edinburgh. ^*

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