Elms are deciduous and semi-deciduous trees comprising the genus Ulmus, family Ulmaceae. Elms first appeared in the Miocene period about 40 million years ago. Originating in what is now central Asia, the tree flourished and established itself over most of the Northern Hemisphere, traversing the Equator in Indonesia. During the 19th and early 20th centuries, many species and cultivars were planted as ornamentals in Europe, North America, and parts of the Southern Hemisphere, notably Australasia.
Elm leaves are alternate, with simple, single- or, most commonly, doubly-serrate margins, usually asymmetric at the base and acuminate at the apex. The genus is hermaphroditic, having perfect flowers which, being wind-pollinated, are apetalous. The fruit is a round wind-dispersed samara. All species are tolerant of a wide range of soils and pH levels but, with one exception (the European White Elm U. laevis), demand good drainage.
The other genera of the Ulmaceae are Zelkova (Zelkova) and Planera (Water Elm). Celtis (Hackberry or Nettle Tree), formerly included in the Ulmaceae, is now included in the family Cannabaceae.
Species, varieties and hybrids
There are approximately 30 to 40 species of elm; the ambiguity in number is a result of difficulty in delineating species, owing to the ease of hybridization between them and the development of local seed-sterile vegetatively-propagated microspecies in some areas, mainly in the field elm group. Rackham describes Ulmus as the most difficult critical genus in the entire British flora. Eight species are endemic to North America, and a smaller number to Europe; the greatest diversity is found in China.
During the 18th and 19th centuries, elm cultivars enjoyed much popularity as ornamentals in Europe by virtue of their rapid growth and variety of foliage and forms. This 'belle époque' lasted until the First World War, when the consequences of hostilities, notably in Germany, and the outbreak of Dutch elm disease saw the elm slide into horticultural decline. The devastation caused by the Second World War, in particular the demise of the huge Späth nursery in Berlin which had once raised dozens of native elm cultivars, only accelerated the process. The outbreak of the new, three times more virulent, strain of Dutch elm disease Ophiostoma novo-ulmi in the late 1960s brought the tree to its nadir.
Since about 1990 however, the elm has enjoyed a slow renaissance through the successful development in North America and Europe (notably the Netherlands until 1992, and, more recently, Italy) of cultivars highly resistant to the new disease. Consequently, the total number of named cultivars, ancient and modern, now exceeds 300, although many of the older clones, possibly over 120, have been lost to cultivation. Unhappily, enthusiasm for the newer clones often remains low owing to the poor performance of earlier, supposedly disease-resistant Dutch trees released in the 1960s and 1970s. In the Netherlands, sales of elm cultivars slumped from over 56,000 in 1989 to just 6,800 in 2004 , whilst in the UK, only four of the new American and European releases were commercially available in 2008.
In 1997, a European Union elm project was initiated, its aim to coordinate the conservation of all the elm genetic resources of the member states and, among other things, to assess their resistance to Dutch elm disease. Accordingly, over 300 clones were selected and propagated for testing.
The classification adopted for Elm species, varieties, cultivars and hybrids is largely based on that established by Brummitt. A large number of synonyms have accumulated over the last three centuries, their Accepted Names can be found on Elm Synonyms and Accepted Names.
Resistant treesEfforts to develop resistant cultivars began in the Netherlands in 1928 and continued, uninterrupted by World War II, until 1992. Similar programmes were initiated in North America (1937), Italy (1978), and Spain (1990s). Research has followed two paths:
Hybrid cultivars from crossings of Asiatic with European species, or of Asiatic elms alone because of their innate resistance to Dutch elm disease. After a number of false dawns in the 1970s, this approach has produced some fine trees. Clones with immunity or very high resistance to disease have been raised in the USA, the Netherlands and Italy, and are now commercially available after many years of field trials. However, some of these trees, notably those with the Siberian Elm U. pumila in their ancestry, will probably have a comparatively small mature size and lack the forms for which the iconic American Elm and English Elm were prized. Several of the same have also proven unsuited to the maritime climate conditions in northwestern Europe, notably because of their intolerance of ponding on poorly-drained soils in winter. Dutch hybridizations included the Himalayan Elm U. wallichiana as a source of anti-fungal genes and have proved more tolerant of wet ground; they should also ultimately reach a greater size. A number of highly-resistant cultivars have been released since 2000, notably 'Nanguen' (Lutèce) which is effectively immune to Dutch elm disease.
Species and species cultivars. Careful selection has produced a number of trees not only resistant to disease, but also the droughts and extremely cold winters afflicting North America. Research in the USA has concentrated on the American Elm U. americana, resulting in the release of highly resistant clones, notably 'Valley Forge'. Much work has also been done into the selection of Asiatic species and cultivars.
Elms take many decades to grow to maturity, and as the introduction of these cultivars is relatively recent, their performance and ultimate size cannot be predicted with certainty. However, the National Elm Trial has been underway since 2005 as a large-scale, scientific effort to assess strengths and weakness of the leading cultivars over a 10-year timeframe.
The European White Elm. Finally, there is the unique example of the European White Elm Ulmus laevis. Whilst this species has little innate resistance to Dutch elm disease, it is not favoured by the vector bark beetles and thus only becomes colonized and infected when there are no other choices, a rare situation in western Europe. Research in Spain has suggested that it might be the presence of a triterpene, alnulin, that makes the tree bark unattractive to the beetle species that spread the disease. However this has not been conclusively proved.
Notable elm trees
The Biscarrosse Elm, France, planted 1350
•Sherwood Forest — the "Langton Elm" was a large tree that "was for a long time so remarkable as to have a special keeper", according to a book published in 1881.
•The Biscarrosse Elm. Planted in 1350, this Smooth-leafed Elm Ulmus minor subsp. minor survives in the centre of Biscarrosse in the Landes region of south-west France, well isolated from disease-carrying Scolytus beetles.
•Oxford — "Joe Pullen's Tree" was planted in about 1700 by the Rev. Josiah Pullen, vice president of Magdalen Hall. Josiah Pullen "used to Walk to that place every day, sometimes twice a day", according to diarist Thomas Hearne. The famous essayist Richard Steele (1672–1729) said his regular walks as an undergraduate to the elm with Pullen helped him to reach a "florid old age". The elm became famous at Oxford and its fame grew with its age. In November 1795, Gentleman's Magazine reported that "Joe Pullen, the famous elm, upon Headington hills, had one of its large branches torn off and carried to a great distance." When new parliamentary district boundaries were drawn after the Reform Act 1832, the tree was named as a landmark helping to mark the boundary of the Parliamentary Borough of Oxford. In early 1847, the owner of the property arranged to have the tree torn down, and work started on it before protests put an end to the plan. By 1892, however, rot had set in, and the tree was torn down to its (large and tall) "stump". Early in the morning of October 13, 1909, vandals set fire to the stump. A plaque was soon after installed on the side wall of Davenport House in Cuckoo Lane, marking the spot. It reads: Near this spot stood the famous elm planted by the Rev. Josiah Pullen about 1680 and known as Jo Pullen's Tree. Destroyed by fire on 13 October 1909.
"Herbie", New England's oldest and tallest elm
•"Herbie" in Yarmouth, Maine, has been standing by East Main Street (Route 88) since 1775. At 110 feet in height, it is believed to be, as of 1997, the oldest and tallest of its kind in New England. The tree, which stands in the front yard of a private residence, also has a 19-foot circumference and (until mid-2008) a 93-foot crown spread. As of 2003, only twenty of Yarmouth's original 739 elms had survived Dutch elm disease.