Ulmus americana

Ulmus americana
Ulmus americana (American elm) at Longwood Gardens, Kennett Square, Pennsylvania

Secure  (NatureServe)[1]
Scientific classification
Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Rosids
Order: Rosales
Family: Ulmaceae
Genus: Ulmus
Species: U. americana
Binomial name
Ulmus americana
L.
Synonyms
  • Ulmus alba Raf. (non Kit.)
  • Ulmus americana Planch.
  • Ulmus americana L. f. alba (Aiton) Fern.
  • Ulmus americana L. f. americana
  • 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. americana
  • 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,[lower-alpha 1] is a species native to eastern North America, occurring from Nova Scotia west to Alberta and Montana, and south to Florida and central Texas. The American elm 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,[2][3] which grew beside the banks of the Sauble River 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; when it was felled in 1968, a tree-ring count established that it had germinated in 1701.

For over 80 years, U. americana has 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 USDA 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 Dutch elm disease, suggesting the diploid parent trees, which have markedly smaller cells than the tetraploid, may too be highly resistant to the disease.[4][5]

Classification

Ulmus americana was first described and named by Carl Linnaeus in his Species Plantarum, published in 1753. No subspecies or varieties are currently recognized within the species.

Description

The American elm is a deciduous hermaphroditic tree which, before the introduction of Dutch elm disease, commonly grew to > 30 m (100 ft) tall with a trunk > 1.2 m (4 ft) d.b.h supporting a high, spreading umbrella-like canopy. The leaves are alternate, 7–20 cm long, with double-serrate margins and an oblique base. 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,[6] and emerge in early spring before the leaves. The fruit is a flat samara 2 cm long by 1.5 cm broad, with a circular papery wing surrounding the single 4–5 mm seed. As in the closely related European White Elm Ulmus 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.[7] Ploidy (2n = 56, or more rarely, 2n = 28).[8]

Ecology

The American elm 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.[9] On more elevated terrain, as in the Appalachian Mountains, it is most often found along rivers.[10] The species' wind-dispersed seeds enable it to spread rapidly as suitable areas of habitat become available.[9] American elm produces its seed crop in late spring (which can be as early as February and as late as June depending on the climate) and the seeds usually germinate right away 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 due to the climate being unfavorable for the spread of DED.

In the United States, the American elm is a major 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.[11] A sugar maple-ironwood-American elm cover type occurs on some hilltops near Témiscaming, Quebec.[12]

The leaves of the American elm serve as food for the larvae of various lepidopterans (butterflies & moths).

Pests and diseases

The American elm is highly susceptible to Dutch elm disease (DED) and elm yellows; it is also moderately preferred for feeding and reproduction by the adult elm leaf beetle Xanthogaleruca luteola,[13] and highly preferred for feeding by the Japanese beetle Popillia japonica[14] in the United States. Trees grown in Europe have proven very susceptible to damage by leaf-feeding insects in general, far more so than Asiatic elms.[15]

U. americana is also the most susceptible of all the elms to verticillium wilt,[16] whose external symptoms closely mimic those of DED. However, the condition is far less serious, and the tree should recover the following year.

Dutch elm disease

Dutch elm disease (DED) is a fungal disease which 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.[17] However, in some areas still not populated by the Dutch elm disease-carrying elm bark beetle, the American elm continues to thrive, notably in Florida, most of Alberta and British Columbia.

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.[18]

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 have been developed or that occur naturally. After 20 years of research, American scientists first developed DED-resistant strains of elms in the late 1990s.[17]

Fungicidal injections can be administered to valuable American elms, to prevent infection. Such injections generally are effective as a preventive measure for up to three years when performed before any symptoms have appeared, but may be ineffective once the disease is evident.

Cultivation

In the 19th and early 20th century, American elm was a common street and park tree owing to its tolerance of urban conditions, rapid growth, and graceful form. This however led to extreme overplanting of the species, especially to form living archways over streets, which ultimately produced an unhealthy monoculture of elms that had no resistance to disease and pests.[19] These trees' rapid growth and longevity, leading to great size within decades, also favored its horticultural use before the advent of Dutch elm disease.[9] Ohio botanist William B. Werthner, discussing the contrast between open-grown and forest-grown American elms, noted that:

In the open, with an abundance of air and light, the main trunk divides into several leading branches which leave the trunk at a sharp angle and continue to grow upward, gradually diverging, dividing and subdividing into long, flexible branchlets whose ends, at last, float lightly in the air, giving the tree a round, somewhat flattened top of beautifully regular proportions and characteristically fine twiggery.[9]

It is this distinctive growth form that is so valued in the open-grown American elms of street plantings, lawns, and parks; along most narrower streets, elms planted on opposite sides arch and blend together into a leafy canopy over the pavement. However, elms can assume many different sizes and forms depending on the location and climate zone, and the classic vase-like shape is far from the norm in naturally-occurring (as opposed to cultivated) specimens.

American elms have been planted in North America 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 by James Gordon in 1752, it was noted that the foliage of the American elm was far more susceptible to insect damage than native elms.[15] A few, mostly young, cultivars survive in British arboreta.[20] Introduced to Australasia, the tree was listed by Australian nurseries in the early 20th century. It is known to have been planted along the Avenue of Honour at Ballarat, Victoria and the Avenue of Honour in Bacchus Marsh, Victoria. In addition, a heritage-listed planting of American elms can be found along Grant Crescent in Griffith, Australian Capital Territory.[21] American Elms are only rarely found in New Zealand.[22]

Cultivars

See the list of Elm cultivars, hybrids and hybrid cultivars for more details.

Numerous cultivars have been raised, originally for their aesthetic merit but more recently for their resistance to Dutch elm disease[23] 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:

The National Elm Trial, begun in 2005, is currently evaluating 19 hybrid and species cultivars in scientific plantings across the United States to better assess their strengths and weaknesses.

The few disease-resistant selections 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'.[24] 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 planted by HRH The Prince of Wales to form the Anniversary Avenue from the Orchard Room reception centre to the Golden Bird statue at Highgrove House, however the trees succumbed to DED five years later and were felled and burned. In 2007, the 'Elm Recovery Project'[25] 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.[26]

Hybrids and hybrid cultivars

Thousands of attempts to cross the American elm with the Siberian elm U. pumila failed.[27] Attempts at the Arnold Arboretum using ten other American, European and Asiatic species also ended in failure, attributed to the differences in ploidy levels, and operational dichogamy,[6] although the ploidy factor has been discounted by other authorities.[28]

Success was finally achieved with the autumn-flowering Chinese elm Ulmus parvifolia by the late Prof. Eugene Smalley towards the end of his career at the University of Wisconsin-Madison after he overcame the problem of keeping Chinese elm pollen alive until spring.[29] Only one of the hybrid clones was commercially released, as 'Rebella' in 2011 by the German nursery Eisele GmbH; the clone is not available in the USA.

Other artificial hybridizations with American elm are rare, and now regarded with suspicion. Two such alleged successes by the nursery trade were 'Hamburg', and 'Kansas Hybrid', both with Siberian elm Ulmus pumila. However, given the repeated failure with the two species by research institutions, it is now believed that the "American elm" in question was more likely to have been the red elm, Ulmus rubra.[30]

Other uses

Wood

A wooden hand plane made of American elm.

The American elm's wood is coarse, hard, and tough, with interlacing, contorted fibers that make it difficult to split or chop, and cause it to warp after sawing.[9] Accordingly, the wood originally had few uses, save for making hubs for wagon wheels.[9] Later, with the advent of mechanical sawing, American elm wood was used for barrel staves, trunk-slats, and hoop-poles, and subsequently became fundamental to the manufacture of wooden automobile bodies, with the intricate fibers holding screws unusually well.[9]

Pioneer and traditional uses

Young twigs and branchlets of the American elm have tough, fibrous bark that has been used as a tying and binding material, even for rope swings for children, and also for making whips.[9]

Notable trees

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,[31] 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. The Olmsted-designed park system in Buffalo, NY [32] did not fare as well. A row of mature American elms graces Central Park along the entire length of Fifth Avenue from 110th St to 59th.[33] In Akron, Ohio there is a very old elm tree that has not been infected. 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, Swarthmore College, and The Pennsylvania State University, believed to be the largest remaining stand in the country.[34] There are several large American Elm trees in western Massachusetts. The large Massachusetts Champion Elm stands on Summer Street in the Berkshire County town of Lanesborough, Massachusetts has been kept alive by antifungal treatments. Rutgers University has preserved 55 mature elms on and in the vicinity of Voorhees Mall on the College Avenue Campus in New Brunswick, New Jersey in addition to seven disease-resistant trees that have been planted in this area of the campus in recent years.[35]

The largest surviving urban forest of American elms in North America is believed to be in the city of Winnipeg, Canada, where close to 200,000 elms remain. The city of Winnipeg spends $3M annually to aggressively combat the disease utilizing Dursban Turf[36] and the Dutch Trig vaccine,[37] losing 1500-4000 trees per year.

Treaty Elm

The Treaty Elm, Philadelphia, Pennsylvania. In what is now Penn Treaty Park, the founder of Pennsylvania, William Penn, is said to have entered into a treaty of peace in 1683 with the native Lenape Turtle Clan under a picturesque elm tree immortalized in a painting by Benjamin West. West made the tree, already a local landmark, famous by incorporating it into his painting after hearing legends (of unknown veracity) about the tree being the location of the treaty. No documentary evidence exists of any treaty Penn signed beneath a particular tree. On March 6, 1810 a great storm blew the tree down. Measurements taken at the time showed it to have a circumference of 24 feet (7.3 m), and its age was estimated to be 280 years. Wood from the tree was made into furniture, canes, walking sticks and various trinkets that Philadelphians kept as relics.[38]

Liberty Tree

Plaque marking the site of the original Liberty Tree in Boston

The Liberty Tree, an elm on Boston Common in Boston, Massachusetts, was a rallying point for the growing resistance to the rule of England over the American colonies.[39]

Washington Elm (Massachusetts)

The Washington Elm, Cambridge, Massachusetts. George Washington is said to have taken command of the American Continental Army under the Washington Elm in Cambridge on July 3, 1775. The tree survived until the 1920s and "was thought to be a survivor of the primeval forest". In 1872, a large branch fell from it and was used to construct a pulpit for a nearby church.[40] The tree, an American white elm, became a celebrated attraction, with its own plaque, a fence constructed around it and a road moved in order to help preserve it.[41] The tree was cut down (or fell — sources differ) in October 1920 after an expert determined it was dead. The city of Cambridge had plans for it to be "carefully cut up and a piece sent to each state of the country and to the District of Columbia and Alaska," according to The Harvard Crimson.[42] As late as the early 1930s, garden shops advertised that they had cuttings of the tree for sale, although the accuracy of the claims has been doubted. A Harvard "professor of plant anatomy" examined the tree rings days after the tree was felled and pronounced it between 204 and 210 years old, making it at most 62 years old when Washington took command of the troops at Cambridge. The tree would have been a little more than two feet in diameter (at 30 inches above ground) in 1773.[43] In 1896, an alumnus of the University of Washington, obtained a rooted cutting of the Cambridge tree and sent it to Professor Edmund Meany at the university. The cutting was planted, cuttings were then taken from it, including one planted on February 18, 1932, the 200th anniversary of the birth of George Washington, for whom Washington state is named. That tree remains on the campus of the Washington State Capitol. Just to the west of the tree is a small elm from a cutting made in 1979.[41]

Washington Elm (District of Columbia)

George Washington's Elm, Washington, D.C. George Washington supposedly had a favorite spot under an elm tree near the United States Capitol Building from which he would watch construction of the building. The elm stood near the Senate wing of the Capitol building until 1948.[40]

Logan Elm

The Logan Elm that stood near Circleville, Ohio, was one of the largest American elms in the world. The 65-foot-tall (20 m) tree had a trunk circumference of 24 feet (7.3 m) and a crown spread of 180 feet (55 m).[44] Weakened by Dutch elm disease, the tree died in 1964 from storm damage.[44] The Logan Elm State Memorial commemorates the site and preserves various associated markers and monuments.[44] According to tradition, Chief Logan of the Mingo tribe delivered a passionate speech at a peace-treaty meeting under this elm in 1774,[44][45]

"Herbie"

An April 21, 2008, picture of Herbie

Another notable American elm, named Herbie, was the tallest American elm in New England until it was cut down on January 19, 2010, after it succumbed to Dutch elm disease. Herbie was 110 feet (34 m) tall at its peak and had a circumference of 20.3 feet (6.2 m), or a diameter of approximately 6.5 feet (2.0 m). The tree stood in Yarmouth, Maine, where it was cared for by the town's tree warden, Frank Knight.[46]

When cut down, Herbie was 217 years old. Herbie's wood is of interest to dendroclimatologists, who will use cross-sections of the trunk to help answer questions about climate during the tree's lifetime.[46]

Central Park stand

American elms along The Mall and Literary Walk, Central Park

New York City's Central Park is home to approximately 1,200 American elms, which constitute over half of all trees in the park. The oldest of these elms were planted during the 1860s by Frederick Law Olmsted, making them among the oldest stands of American elms in the world. The trees are particularly noteworthy along the Mall and Literary Walk, where four lines of American elms dramatically stretch over the walkway forming a cathedral-like covering. The elms are an important part of New York City's ecology by improving air and water quality, reducing erosion and flooding, and lowering the air temperature during the hottest days of the year.[47]

While still vulnerable to the highly contagious Dutch elm disease, in the 1980s the Central Park Conservancy undertook aggressive counter-DED measures such as heavy pruning and removal of extensively diseased trees. These efforts have largely been successful in saving the majority of the trees, although several are still lost each year. Younger American elms which have been planted in Central Park since the outbreak are of the "Princeton" and "Valley Forge" variety of DED-resistant elms.[48]

National champion

The USA national champion, measuring 34 metres (112 ft) high in 2010, stands at Iberville, Louisiana.[49] Across the Atlantic, the TROBI champion grows at Avondale in Wicklow, Ireland; last measured in 2000, it was 22.5 metres (74 ft) high by 98 cm 98 centimetres (39 in) diameter at breast height.

The Glencorradale Elm

The Glencorradale Elm on Prince Edward Island, Canada, is a surviving 'wild' elm believed to be several hundred years old.[50]

Survivor Tree

The Survivor Tree at the Oklahoma City National Memorial

An American elm located in a parking lot directly across the street from the Alfred P. Murrah Federal Building in Oklahoma City survived the Oklahoma City bombing on April 19, 1995 that killed 168 people and destroyed the Murrah building. Damaged in the blast, with fragments lodged in its trunk and branches, it was nearly cut down in efforts to recover evidence. However, nearly a year later the tree began to bloom. Then known as the Survivor Tree, it became an important part of the Oklahoma City National Memorial, and is featured prominently on the official logo of the memorial.[51]

Ulmus americana in art and photography

The nobility and arching grace of the American Elm in its heyday, on farms, in villages, in towns and on campuses, were celebrated in the books of photographs of Wallace Nutting (Massachusetts Beautiful, N.Y. 1923, and other volumes in the series) and of Samuel Chamberlain (The New England Image, New York, 1962). Frederick Childe Hassam is notable among painters who have depicted American Elm.

See also

Accessions

North America
Europe
Austalazië

Notes

  1. ↑ The name "water elm" is also used for Planera aquatica, another species in the Ulmaceae.

References

  1. ↑ "Ulmus americana". NatureServe Explorer. NatureServe. Retrieved 2007-07-06.
  2. ↑ 'Lord of the Elms', web.archive.org/web/20110711013315/http://www.flyingsquirrels.com/sauble_elm/
  3. ↑ "Of Elms and Orioles". Emmitsburg News-Journal: The New Forest Society. Retrieved December 15, 2014.
  4. ↑ Whittemore, A. & Olsen, R. (2011). Ulmus americana (Ulmaceae) is a polyploidy complex. American Journal of Botany 98(4): 754–760. 2011. Botanical Society of America.
  5. ↑ Kaplan, K. (2011). Hidden elm population may hold genes to combat Dutch elm disease. ARS News, 30 March 2011. USDA.
  6. 1 2 Hans, A. S. (1981). "Compatibility and Crossability Studies in Ulmus". Silvae Genetica. 30: 4–5.
  7. ↑ Downs, R. J.; Borthwick, H. A. (1956). "Effects of Photoperiod on Growth of Trees". Botanical Gazette. 117: 310–326. doi:10.1086/335918.
  8. ↑ Whittemore, A. T.; Olsen, R. T. (2011). "Ulmus americana (Ulmaceae) is a Polyploid Complex". American Journal of Botany. 98 (4): 754–760. doi:10.3732/ajb.1000372.
  9. 1 2 3 4 5 6 7 8 Werthner, William B. (1935). Some American Trees: An Intimate Study of Native Ohio Trees. New York: The Macmillan Company. pp. xviii, 398.
  10. ↑ Strausbaugh, P.D.; Core, E.L. (1978). Flora of West Virginia (2 ed.). Morgantown, WV: Seneca Books, Inc. pp. xl, 1079.
  11. ↑ Bey, Calvin F. (1990). "Ulmus americana". In Burns, Russell M.; Honkala, Barbara H. Hardwoods. Silvics of North America. Washington, D.C.: United States Forest Service (USFS), United States Department of Agriculture (USDA). 2. Retrieved 14 December 2014 – via Northeastern Area State and Private Forestry (www.na.fs.fed.us).
  12. ↑ Brown, Jean-Louis (1981). Les forêts du Témiscamingue, Québec: écologie et photo-interprétation (in French). Quebec City, Canada: Laboratoire d'écologie forestière, Université Laval, Québec. ISBN 978-2-9201-0404-4.
  13. ↑ Miller, F.; Ware, G. (2001). "Resistance of Temperate Chinese Elms (Ulmuss spp.) to Feeding of the Adult Elm Leaf Beetle (Coleoptera: Chrysomelidae)". Journal of Economic Entomology. 94 (1): 162–166. doi:10.1603/0022-0493-94.1.162.
  14. ↑ Miller, F.; Ware, G.; Jackson, J. (2001). "Preference of Temperate Chinese Elms (Ulmuss spp.) for the Feeding of the Japanese Beetle (Coleoptera: Scarabaeidae)". Journal of Economic Entomology. 94 (2): 445–448. doi:10.1603/0022-0493-94.2.445.
  15. 1 2 Elwes, Henry John; Henry, Augustine (1913). The Trees of Great Britain & Ireland. 7. pp. 1855–1859.
  16. ↑ Pegg, G. F.; Brady, B. L. (2002). Verticillium Wilts. Wallingford, Oxfordshire, UK: CABI Publishing. ISBN 0-85199-529-2.
  17. 1 2 "New American Elms Restore Stately Trees". Agricultural Research Service, United States Department of Agriculture. July 1996. Retrieved December 18, 2014.
  18. ↑ Smalley, E. G. (1963). "Seasonal Fluctuations in Susceptibility of Young Elm Seedlings to Dutch Elm Disease". Phytopathology. 53 (7): 846–853.
  19. ↑ Stennes, Mark (2003). "Good News for the American Elm". Shade Tree Advocate. 5 (4): 6.
  20. ↑ Sir Harold Hillier Gardens accessions list
  21. ↑ ACT Government (2012-02-02). "ACT Tree Register, Grant Crescent" (PDF). ACT Tree Register, Grant Crescent. ACT Government. Retrieved 2016-07-07.
  22. ↑ Journal. Auckland Botanical Society. 58 (1). June 2003. Missing or empty |title= (help)
  23. ↑ Townsend, A. M.; Bentz, S. E.; Douglass, L. W. (March 2005). "Evaluation of 19 American Elm Clones for Tolerance to Dutch Elm Disease" (PDF). Journal of Environmental Horticulture: 21–24.
  24. ↑ Costello, L. R. (March 2004). "A 10-year Evaluation of the Performance of Four Elm Cultivars in California, U. S.". Journal of Arboriculture.
  25. ↑ Elm Recovery Project, University of Guelph
  26. ↑ "Scientists Breed Supertrees to Beat Dutch Elm". Canada.com. September 11, 2007. Retrieved December 21, 2014.
  27. ↑ "Ulmus americana". Fire Effects Information System. United States Forest Service. Retrieved December 14, 2014.
  28. ↑ Ager, A. A.; Guries, R. P. (1982). "Barriers to Interspecific Hybridization in Ulmus americana". Euphytica. 31: 909–920. doi:10.1007/bf00039231.
  29. ↑ Heybroek, H. M.; Goudzwaard, L.; Kaljee, H. (2009). Iep of olm, karakterboom van de Lage Landen (Elm, A Tree With Character of the Low Countries (in Dutch). Zeist, Netherlands: KNNV Uitgeverij. ISBN 978-9-0501-1281-9.
  30. ↑ "Ulmus 'Hamburg'". Morton Arboretum. Retrieved December 27, 2014.
  31. ↑ Barnard, Edward S. (2002). New York City Trees. New York: Columbia University Press. ISBN 978-0-2311-2835-3.
  32. ↑ Beveridge, Charles. "Buffalo's Park and Parkway System". Buffalo Architecture and History. Retrieved December 21, 2014.
  33. ↑ Trebay, Guy (February 22, 2014). "In the Treetops, a Winter Gift". New York Times. Retrieved December 23, 2014.
  34. ↑ Templeton, David (August 11, 2013). "Saving the Nation's Green Giants: Tall, Lush Trees". Pittsburgh Post-Gazette. Retrieved December 24, 2014.
  35. ↑ Manas, Steve (May 8, 2008). "Voorhees Mall Elms Add to Commencement's Ambience". Rutgers University. Retrieved December 24, 2014.
  36. ↑ "Elm Bark Beetle Control Program" (PDF). City of Winnipeg. August 7, 2009. Retrieved December 24, 2014.
  37. ↑ Rumbolt, Colin (November 17, 2009). "Dutch Elm Vaccine Tested In Winnipeg". The Manitoban. Retrieved December 24, 2014.
  38. ↑ "Penn Treaty Elm". Haverford College Arboretum. Retrieved December 28, 2014.
  39. ↑ Kessler, Ronald (October 3, 2011). "America Must Remember Boston's Liberty Tree". Newsmax. Retrieved December 27, 2014.
  40. 1 2 Platt, Rutherford (1992). 1001 Questions Answered About Trees. Courier Dover Publications. p. 19. ISBN 0-486-27038-6.
  41. 1 2 Jacobson, Arthur Lee. "Arthur Lee Jacobson: Trees of the Washington State Capitol Campus".
  42. ↑ "Big Day for Curio Hunter When Famous Elm is Cut". The Harvard Crimson. Harvard University. October 23, 1920. Retrieved December 24, 2014.
  43. ↑ Jack, J. G. (December 10, 1931). "The Cambridge Washington Elm" (PDF). Bulletin of Popular Information. Arnold Arboretum, Harvard University. Retrieved December 24, 2014.
  44. 1 2 3 4 "The Logan Elm". Over-land. Retrieved October 6, 2011.
  45. ↑ "The Roots of Tragedy". Ohio Magazine. Retrieved December 14, 2014.
  46. 1 2 Sharp, David (January 31, 2010). "Do Rings of Herbie the Elm Have Age, Climate Data?". Boston.com. Associated Press. Retrieved December 24, 2014.
  47. ↑ Central Park Conservancy. The Mall and Literary Walk.
  48. ↑ Pollak, Michael. The New York Times. "Answers to Questions About New York." 11 January 2013.
  49. ↑ "The 2012 National Register of Big Trees" (PDF). American Forests. 2012. Retrieved December 24, 2012.
  50. ↑ 'The Big Elm', Island Narratives Program, University of Prince Edward Island, vre2.upei.ca
  51. ↑ "Survivor Tree: Witness to Tragedy, Symbol of Strength". Oklahoma City National Memorial and Museum. Retrieved December 24, 2014.
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