American elm

''Ulmus americana'', generally known as the American elm or, less commonly, as the white elm or water elm, is a species of elm native to eastern North America, naturally 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 (DED) can live for several hundred years. A prime example of the species was the Sauble Elm, 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 DED; when it was felled in 1968, a tree-ring count established that it had germinated in 1701.

For over 80 years, ''U. americana'' had 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 DED, which ravaged American elms in the 20th century. This suggests that the diploid parent trees, which have markedly smaller cells than the tetraploid, may too be highly resistant to the disease.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.

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 monoecious tree which, before the introduction of DED, commonly grew to more than tall with a trunk whose diameter at breast height was more than , 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, 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. Ploidy is ''2n'' = 56, or more rarely, ''2n'' = 28.

File:Lovers' Elm, Gwynne estate, Dufferin Street.jpg, ''U. americana'', Toronto, c.1914
Image:ElmBark.jpg, Bark
File:Jardin jeanne arc.jpg, Boles
File:American Elm - Ulmus americana, Occoquan Regional Park, Lorton, Virginia.jpg, Flowers
Image:Ulmus americana (5102579562).jpg, Leaf
Image:Ulmus americana (5101983801).jpg, Foliage
Image:Ulmus americana seedlings.jpg, Seedlings

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. On more elevated terrain, as in the Appalachian Mountains, it is most often found along rivers. The species' wind-dispersed seeds enable it to spread rapidly as suitable areas of habitat become available. American elm fruits in late spring (which can be as early as February and as late as June depending on the climate), the seeds usually germinating immediately, 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 owing to the climate being less favorable to the spread of DED.

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

The leaves of the American elm serve as food for the larvae of a number of species of Lepidoptera (see List of Lepidoptera that feed on elms). These include such butterflies as the Eastern Comma (''Polygonia comma''), Question Mark (''Polygonia interrogationis''), Mourning Cloak ('' Nymphalis antiopa''), Painted Lady ('' Vanessa cardui'') and Red-spotted Purple (''Limenitis arthemis astyanax''), as well as such moths as the Columbian Silkmoth (''Hyalophora columbia'') and the Banded Tussock Moth (Pale Tiger Moth) (''Halysidota tessellaris'').

Pests and diseases

The American elm is highly susceptible to DED and elm yellows. In North America, there are three species of elm bark beetles: one native, '' Hylurgopinus rufipes'' ("native elm bark beetle"); and two invasive, '' Scolytus multistriatus'' ("smaller European elm bark beetle") and '' Scolytus schevyrewi'' ("banded elm bark beetle"). Although intensive feeding by elm bark beetles can kill weakened trees, their main impact is as vectors of DED.

American elm is also moderately preferred for feeding and reproduction by the adult elm leaf beetle '' Xanthogaleruca luteola'' and highly preferred for feeding by the Japanese beetle ''Popillia japonica'' in the United States.

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

Dutch elm disease

Dutch elm disease (DED) is a fungal disease that 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. However, in some areas still not infested by DED, the American elm continues to thrive, notably in Florida, Alberta and British Columbia. There is a notable grove of old American elm trees in Manhattan's Central Park. The trees there were apparently spared because of the grove's isolation in such an intensely urban setting.

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.

The American elm's biology in some ways has helped to spare it from obliteration by DED, 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.

Elms in forest and other natural areas have been less affected by DED than trees in urban environments due to lower environmental stress from pollution and soil compaction and due to occurring in smaller, more isolated populations.

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. Elms do not naturally form pure stands and trees used in landscaping were grown from a handful of cultivars, causing extremely low genetic diversity. These trees' rapid growth and longevity, leading to great size within decades, made them popular before the advent of DED. 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."

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. In 1926 the Klehm Nurseries of Arlington Heights, Illinois, wrote: "American Elms grown in the regular way from seedlings show extreme variability, growing up into trees of all shapes, some of them being very slow in growth while others are moderately rapid in development. The shapes run all the way from the true open excurrent growth to globular, or flat-topped, or pendant. As regards foliage, the leaves are from small to medium large, some shedding early and others late. This condition makes it difficult for the landscape architect to choose just the right trees to obtain the effect desired."'Three novelties for 1926: ''Ulmus americana'' 'Molinii', ''Ulmus americana'' 'Urnii', ''Populus alba'' 'Richardii' ' : bulletin of Klehms' Nurseries, Illinois
/ref> The classic vase-shaped elm was mainly the result of selective breeding of a few cultivars and is much less likely to occur in the wild.

Gallery of American elm trees

File:NIE 1905 Elm.jpg, Early photos of New England American elms, showing growth patterns, ''The New International Encyclopædia'' (1905).
File:Lafayette Street, Salem, MA.jpg, Lafayette Street, Salem, Massachusetts: 'high-tunnel effect' of ''U. americana'' avenues once common in New England (colorized postcard, 1910).
File:National Mall - walking path.JPG, Rows of American elm trees lining the sides of a path traversing the length of the National Mall in Washington, D.C. (April 2010).
File:Grant Crescent Griffith ACT April 2013.jpg, Grant Crescent, Griffith, Australian Capital Territory, Australia: American elms in autumn.
File:Winter scene of ulmus americana (Canberra, 2016).jpg, Grant Crescent, Griffith, Australian Capital Territory, Australia: American elms in winter.
File:PSM V78 D424 An old cambridge landmark.png, ''Ulmus americana'' as campus elm: Cambridge, Massachusetts.
File:American Elm in Old Deerfield, MA - December 2011.jpg, American elm, Old Deerfield, Massachusetts (2011). Girth was 19.3 ft at 4.5 ft above ground; height 106.8  ft; avg. crown spread 105 ft. ''This tree died in 2017.''
File:Massachusetts American Elm 1 - May 2012.jpg, Same American elm, Old Deerfield, Massachusetts (2012). ''This tree died in 2017.''
File:American Elm, Western Massachusetts - September 2016.jpg, Surviving American elm "street tree" in western Massachusetts (2016).
File:Elm Tree in Lee, MA - August 2020.jpg, Another view of the previous "street tree" in western Massachusetts (August 2020).
File:American Elm Tree (Closer), Hatfield, MA - June 2020.jpg, American elm, Hatfield, Massachusetts (2020). Measurements as of November 2019: girth 17.5 ft, 11 in at 4.5 ft above ground; height 86 ft; spread 87 ft.
File:Elm Tree in Massachusetts June 2012.jpg, American Elm in Massachusetts (June 2012). Girth 18 ft at 4.5 ft above ground; est. height 80 ft; spread 100 ft.
File:American Elm in Western Massachusetts - January 27, 2013.jpg, "The Grayson Elm" in Amherst, Massachusetts, in winter. American elm, Massachusetts (2013), with octopus-like limbs. Girth 17 ft; height 80 ft.
File:Grayson Elm Tree in Amherst, MA - August 2017.jpg, "The Grayson Elm" in Amherst, Massachusetts, in summer (2017).
File:"Grayson Elm" - American Elm Tree in Amherst, MA (October 2020).jpg, "The Grayson Elm" in Amherst, Massachusetts, in fall (2020).
File:JohnstownElm.jpg, American Elm in Johnstown, New York. (2013). 199 inches in circumference and 90 feet tall. ''Now deceased.''
File:American Elm Tree at Spring Grove Cemetery, Hartford, CT - May 26, 2012.JPG, American elm, Spring Grove Cemetery, Hartford, Connecticut (2012). Girth 15 ft at 4.5 ft above ground; height 83 ft; spread 75 ft.
File:C P Mall C67 spring elms jeh.jpg, American elms, Central Park, Manhattan, New York City (Spring 2011).
File:Large American Elm Tree, New Haven, CT - June 10, 2017.jpg, Large American elm in New Haven, Connecticut (June 2017). Girth over 18.5 ft; height 88 ft; spread 95 ft. ''This tree died in 2019.''
File:Elm Tree in West Hartford, Connecticut - May 2017.jpg, ''West Hartford Elm'' - Large American elm in West Hartford, Connecticut (May 2017). Girth 16 ft, 3 inches at 4.5 ft above ground; height 74 ft; spread 97 ft.
File:American Elm Tree, Charlottetown, PEI - August 2019.jpg, American elm in Charlottetown, Prince Edward Island, Canada (August 2019).
File:Old American elm tree near Horticultural Hall in Halifax Public Gardens - August 2019.jpg, Old American elm in Halifax Public Gardens, Nova Scotia, Canada (August 2019).
File:Elm Tree on Elm Street in Plaistow, NH - August 2019.jpg, Elm tree on Elm Street in Plaistow, New Hampshire, which was planted in the late 1800s (August 2019). Girth 13 ft at 4.5 ft above ground; height 85 ft; spread 80 ft.
File:Elm Tree in Vermont June 2017.jpg, Elm tree in Vermont (June 2017). Girth 13 ft at 4.5 ft above ground; Height 70 ft; Spread 75 ft.
File:Courthouse Elm Tree, Newport, Rhode Island - August 2015.jpg, American elm in front of the Florence K. Murray Courthouse in Newport, Rhode Island (August 2015).
File:Img ulmus americana 2209.jpg, ''Ulmus americana'' (American elm) at Longwood Gardens, Kennett Square, Pennsylvania (May 2004).
File:American Elm, Dartmouth College, Hanover, NH (June 2015).jpg, American elm at Dartmouth College in Hanover, New Hampshire (June 2015).
File:American Elm Tree in Greenwich, CT - May 2019.jpg, American elm in Greenwich, CT (May 2019).
File:Elm Tree at Milk Row Cemetery in Somerville, MA - August 2019.jpg, American elm at Milk Row Cemetery in Somerville, MA (August 2019).
File:Elm tree in Woodstock, Connecticut (2020).jpg, American elm near the Woodstock, CT, fairgrounds (May 2020).
File:American Elm at the Hill-Stead Museum, Farmington, CT - August 2021.jpg, American elm located at the Hill-Stead Museum in Farmington, Connecticut (August 2021).
File:American Elm Tree in Connecticut - May 13, 2020.jpg, American elm in Connecticut (May 2020).
File:Great Elm at Phillips Academy, Andover, MA - May 2020.jpg, American elm at Phillips Academy, Andover, MA (May 2020).
File:American Elm, Florence Street, Springfield, MA - June 2020.jpg, American elm tree, which survived the tornado that touched down in Springfield, Massachusetts, in 2011 (June 2020).
File:Elm Tree in Cummington, MA - August 2020.jpg, American elm tree in Cummington, Massachusetts (August 2020).
File:Elm Tree in Adams, MA - August 2020.jpg, American elm tree in Adams, Massachusetts (August 2020).
File:Elm Tree in Brattle Brook Park, Pittsfield, MA - August 2020.jpg, American elm tree in a park in Pittsfield, Massachusetts (August 2020).
File:American Elm Tree in Charlemont, MA - July 2021.jpg, American elm tree located in Charlemont, Massachusetts (July 2021).
File:American Elm at Sunderland Library, Sunderland, MA - June 2022.jpg, American elm tree in Sunderland, Massachusetts (June 2022).
File:Oval Summer.jpg, Alley of American elms, some from 1881, lining the central walk through The Oval on the campus of Colorado State University, Fort Collins (May 2004).
File:American Elm Tree in Easthampton, MA - August 2022.jpg, American Elm Tree in Easthampton, MA (August 2022). Across the street from the park in the rotary in which stood the "Pulpit Elm" until 1952.
File:Phillips Academy Elm, Andover, MA - November 2019.jpg, Phillips Academy Elm, Andover, MA (November 2019)

American elms have been planted in North America beyond its natural range as far north as central Alberta. It also survives low desert heat at Phoenix, Arizona.

Introductions across the Atlantic rarely prospered, even before the outbreak of DED. Introduced to the UK by James Gordon

in 1752, the American elm was noted to be far more susceptible to insect foliage damage than native elms. The tree was propagated and marketed in the UK by the Hillier & Sons nursery, Winchester, Hampshire from 1945, with 450 sold in the period 1962 to 1977 when production ceased with the advent of the more virulent form of Dutch elm disease.Hillier & Sons (1977). ''Catalogue of Trees & Shrubs''. Hillier, Ampfield, UK.Hillier & Sons ''Sales inventory 1962 to 1977'' (unpublished). 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. American Elms are only rarely found in New Zealand.

The American elm in literature

Mary Eleanor Wilkins Freeman, in her 1903 book of short stories, ''Six Trees'', wrote of the American elm:

Cultivars

Numerous cultivars have been raised, originally for their aesthetic merit but more recently for their resistance to Dutch elm disease The total number of named cultivars is circa 45, at least 18 of which have probably been lost to cultivation as a consequence of DED or other factors:
and others.

The disease-resistant selections made available to commerce to date include 'Valley Forge', 'New Harmony', 'Princeton', 'Jefferson', 'Lewis & Clark', 'Miller Park', 'St. Croix', 'Endurance', and a set of six different clones collectively known as 'American Liberty'. 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. 'Princeton' gained renewed attention after its performance in the 1992–1993 screening tests showed that it also had a high degree of disease resistance. A later test performed in 2002–2003 confirmed the disease resistance of 'Princeton', 'Valley Forge' and 'New Harmony', as well as that of 'Jefferson'. Thus far, plantings of these four varieties generally appear to be successful.

In 2005, approximately 90 'Princeton' elms were planted along Pennsylvania Avenue in front of the White House in Washington, D.C. The trees, whose maintenance the National Park Service (NPS) manages, remain healthy and are thriving. However, it has been noted that ''U. americana'' cultivars are not recommended for more than singular plantings as they have unresolved DED and elm yellows concerns.

It has also been noted that monoculture plantings of ''U. americana'' cultivars, such as those along Pennsylvania Avenue, have disproportionate vulnerabilities to disease. Further, long-term studies of 'Princeton' in Europe and the United States have suggested that the cultivar's resistance to DED may be limited (see Pests and diseases of 'Princeton').

The National Elm Trial evaluated 19 elm cultivars commercially available in the United States in scientific plantings throughout the nation to assess and compare the strengths and weaknesses of each. The trial, which started in 2005, lasted for ten years. Based on the trial's final ratings, the preferred cultivars of ''U. americana'' are 'New Harmony' and 'Princeton'.

'Jefferson' was released to wholesale nurseries in 2004 and is becoming increasingly available for planting. However, 'Jefferson' has not been widely tested beyond Washington, D.C. The National Elm Trial provided no data on ‘Jefferson’ because an error in tree identification had occurred earlier in the nursery trade. The error may still be causing nurseries to sell 'Princeton' elms that are mislabeled as 'Jefferson', although one can distinguish between the two cultivars as the trees mature.

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.

In 1993, Mariam B. Sticklen and James L. Sherald reported the results of NPS-funded experiments conducted at Michigan State University in East Lansing that were designed to apply genetic engineering techniques to the development of DED-resistant strains of American elm trees. In 2007, AE Newhouse and F Schrodt of the State University of New York College of Environmental Science and Forestry in Syracuse reported that young transgenic American elm trees had shown reduced DED symptoms and normal mycorrhizal colonization.

Hybrids and hybrid cultivars

* ''Ulmus'' 'Rebella' (''U. americana'' × ''U. parvifolia'')

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

Success was eventually 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. 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 United States.

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''.

Other uses

Wood

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. Accordingly, the wood originally had few uses, save for making hubs for wagon wheels. 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.

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.

Notable trees

A number of mostly small to medium-sized American elms now survive in woodlands, suburban areas, and occasionally cities, where the survivors have often 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, 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 did not fare as well.

A row of mature American elms lines Central Park along the entire length of Fifth Avenue from 59th to 110th Streets. 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 Pennsylvania State University, believed to be the largest remaining stand in the country.

There are several large American Elm trees in western Massachusetts. A large specimen, which 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

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