Ulmus americana

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Ulmus americana
Ulmus americana (American Elm) at Longwood Gardens, Kennett Square, Pennsylvania
Ulmus americana (American Elm) at Longwood Gardens, Kennett Square, Pennsylvania
Conservation status

Secure (TNC)[1]
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Urticales
Family: Ulmaceae
Genus: Ulmus
Species: U. americana
Binomial name
Ulmus americana
L.
Synonyms
  • Ulmus alba ( not Kit.), Raf.
  • Ulmus americana Planch.
  • Ulmus americana L. f. alba (Aiton) Fern.
  • Ulmus americana L. f. ascendens Slavin
  • Ulmus americana L. f. columnaris Rehd.
  • Ulmus americana L. f. intercedens Fern.
  • Ulmus americana L. f. laevior Fern.
  • Ulmus americana L. f. pendula (Aiton) Fern.
  • Ulmus americana L. f. viridis Seym.
  • Ulmus americana L. var. alba Aiton
  • Ulmus americana L. var. aspera Chapm.
  • Ulmus americana L. var. aurea Temple
  • Ulmus americana L. var. bartramii Planch.
  • Ulmus americana L. var. floridana (Chapm.) Little
  • Ulmus americana L. var. glabra Planch.
  • Ulmus americana L. var. pendula Aiton
  • Ulmus americana L. var. scabra Spach
  • Ulmus dentata Raf.
  • Ulmus floridana Chapm.
  • Ulmus mollifolia Marshall
  • Ulmus obovata Raf.
  • Ulmus pendula Willd.
  • Ulmus pubescens Walter

Ulmus americana, generally known as the American Elm or, less commonly, as the White Elm or Water Elm, is a species native to eastern North America, occurring from Nova Scotia west as far as British Columbia, from northern Alberta at the top of its range, south to Florida and central Texas. It is an extremely hardy tree that can withstand winter temperatures as low as −42 °C (−44 °F). Trees in areas unaffected by Dutch elm disease can live for several hundred years. A prime example of the species was the Sauble Elm [3], which grew in Ontario, Canada, to a height of 43 m (140 ft), with a d.b.h of 196 cm (6.43 ft) before succumbing to Dutch elm disease. Felled in 1968, a tree-ring count established that it had germinated in 1701.

Contents

[edit] Description

The American Elm is a deciduous tree, which, before the advent of Dutch elm disease, commonly grew to > 30 m (100 ft) tall with a trunk > 1.2 m (4 ft) d.b.h. 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, are apetalous; 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. As in the closely related European White Elm, U. laevis, the flowers and seeds are borne on 1–3 cm long stems. American Elm is wholly insensitive to daylight length (photoperiod), and will continue to grow well into autumn until injured by frost [2]. The tree 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, nowadays it is uncommon for the tree to reach over 10 years of age, such is its susceptibility to Dutch elm disease. The species is also the most susceptible of all the elms to verticillium wilt [3].

[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 [4] Furthermore, the cross-grained wood imbues the branches with great strength, and breakages were rare. The species has been planted beyond its natural 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. Introduced to the UK in 1752, it was noted that the foliage of the American Elm was far more susceptible to insect damage than native elms [5]. A few, mostly young, specimens survive in British arboreta. Introduced to Australasia, the tree was listed by nurseries in Australia in the early 20th century, and is known to have been planted along the Avenue of Honour at Ballarat and the Bacchus Marsh Avenue of Honour. It is only rarely found in New Zealand [6].

[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.[4] Some hilltops near Témiscaming, Quebec, have a Sugar Maple-Ironwood-American Elm cover type [7] The leaves of the American Elm serve as food for the larvae of various Lepidoptera. See List of Lepidoptera that feed on elms.

[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 has been estimated that only approximately 1 in 100,000 American elm trees is DED-tolerant, most known survivors simply having escaped exposure to the disease [5]. DED 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 the inner bark. The disease pandemic began in Europe shortly after the First World War, its cause formally identified by Dutch pathologists in the late 1920s, hence the name Dutch elm disease. It was accidentally introduced into North America in 1931, in shipments of logs from the Netherlands destined for use as veneer in the furniture industry of Cleveland and Columbus, Ohio.

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 grafts that the trees had formed underground. If a tree has root grafts with an adjacent, infected tree, the fungus can spread easily from one tree to the other, directly through the roots. The American Elm is particularly susceptible to disease because the period of infection often coincides with the period, approximately 30 days, of rapid terminal growth when new springwood vessels are fully functional. Spores introduced outside of this period remain largely static within the xylem and are thus relatively ineffective [8].

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, in Central Park and Tompkins Square Park in New York City [9], stands of several large elms originally planted by Frederick Law Olmsted survive because of their isolation from neighboring areas in New York where there had been heavy mortality. 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 also 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 largely 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 some reason to 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 a reliable tree and recommended in places such as Calgary and Edmonton in Alberta; 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 the south-east of the province in 1998 (the tree was immediately destroyed, and as this was an isolated case 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 finest 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] Cultivars

Numerous cultivars have been raised, originally for their aesthetic merit but more recently for their resistance to Dutch elm disease [10] The few disease-resistant 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' [11]. The United States National Arboretum released 'Valley Forge' and 'New Harmony' in late 1995, after screening tests performed in 1992–1993 showed both had unusually high levels of resistance to DED. 'Valley Forge' performed especially well in these tests. 'Princeton' has been in occasional cultivation since the 1920s, and 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 that of 'Jefferson'. 'Jefferson' 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 along Pennsylvania Avenue near the White House and to date are healthy and thriving. Introduced to the UK in 2001, 'Princeton' was selected by HRH The Prince of Wales to form the Anniversary Avenue from the Orchard Room reception centre to the Golden Bird statue at his Highgrove residence. In 2007, the Elm Recovery Project from the University of Guelph in Ontario, Canada, reported that cuttings from healthy surviving old elms surveyed across Ontario had been grown to produce a bank of resistant trees, isolated for selective breeding of highly resistant cultivars [6] .

The total number of named cultivars is circa 45, at least 18 of which have probably been lost to cultivation as a consequence of Dutch elm disease or other factors:

[edit] Hybrid cultivars

Hybrid cultivars are rare, and those that survive tend to be regarded with taxonomic suspicion. Two allegedly successful hybridizations were: 'Hamburg', and 'Kansas Hybrid', both with the Siberian Elm Ulmus pumila, but it has since been suggested that the American elm in question was more likely to have been Ulmus rubra, or Red Elm.

[edit] The American Elm in literature

“There was not in the whole countryside another tree which could compare with him. He was matchless. Never a stranger passed the elm but stopped, and stared, and said or thought something about it. Even dull rustics looked, and had a momentary lapse from vacuity. The tree was compelling. He insisted upon recognition of his beauty and grace. Let one try to pass him unheeding and sunken in contemplation of his own little affairs, and lo! He would force himself out of the landscape, not only upon the eyes, but the very soul……” from Six Trees by Mary Eleanor Wilkins Freeman.

“At last, all at once, when I was not thinking of it--I declare it makes my flesh creep when I think of it now--all at once I saw a great green cloud swelling in the horizon, so vast, so symmetrical, of such Olympian majesty and imperial supremacy among the lesser forest growths, that my heart stopped short, then jumped at my ribs as a hunter springs at a five-barred gate, and I felt all through me, without need of uttering the words, 'This is it!' . . . What makes a first-class Elm? Why, size in the first place, and chiefly. Anything over twenty feet of clear girth, five feet above the ground, and with a spread of branches a hundred feet across, may claim, that title, according to my scale.” from The Autocrat of the Breakfast Table by Oliver Wendell Holmes, Sr.

[edit] Arboreta etc. accessions

North America
Europe
Australasia

[edit] Nurseries

North America

Still widely available, although increasingly eclipsed by disease-resistant cultivars

Europe

Only as cultivar; see 'Princeton'

Australasia

[edit] Seed suppliers

[edit] References

  1. ^ Ulmus americana. NatureServe Explorer. NatureServe. Retrieved on 2007-07-06.
  2. ^ Downs, R. J. & Borthwick, H. A. (1956). Effects of photoperiod on growth of trees. Botanical Gazette, 117, 310-326
  3. ^ Pegg, G. F. & Brady, B. L. (2002). Verticillium Wilts. CABI Publishing. ISBN 0851995292
  4. ^ Stennes, M. (2003) Good news for the American Elm. Shade Tree Advocate 5(4). Minnesota Shade Tree Advisory Committee, St. Paul, MN.
  5. ^ Elwes, H. J. & Henry, A. (1913). The Trees of Great Britain & Ireland. Vol. VII. pp 1848–1929. Private publication. [1]
  6. ^ Auckland Botanical Society (2003). Journal Vol. 58 (1), June 2003. ISSN 0113-41332
  7. ^ Brown, Jean-Louis. (1981). Les forêts du Témiscamingue, Québec: écologie et photo-interprétation. Laboratoire d'écologie forestière, Université Laval, Québec.
  8. ^ Smalley, E. G. (1963). Seasonal fluctuations in susceptibility of young elm seedlings to Dutch elm disease. Phytopathology, 53, 846-853.
  9. ^ Barnard, E. S. (2002). New York City Trees. Columbia University Press.
  10. ^ 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.
  11. ^ 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]

[edit] External links

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