Elms are deciduous trees comprising the genus Ulmus, family Ulmaceae, native to temperate regions of the Northern Hemisphere. They have alternate, simple, single- or doubly-serrate leaves, usually with asymmetric bases, often rough with fine bristles. The fruit is a round samara.

There are between 20 to 45 species of elm; the ambiguity in the number is a result of difficult species delimitations in elms, 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.

Classification

• "Mountain elms": spring flowering; flowers subsessile; leaves usually rough above.
  • Ulmus bergmanniana - Bergmann's Elm.
  • Ulmus castaneifolia - Chestnut-leafed Elm.
  • Ulmus changii
  • Ulmus chumlia, syn. U. androssowii var. subhirsuta.
  • Ulmus elongata
  • Ulmus gaussennii
  • Ulmus glabra - Wych Elm.
  • Ulmus glaucescens
  • Ulmus laciniata - Manchurian Elm.
  • Ulmus lamellosa
  • Ulmus macrocarpa - Large-fruited Elm.
  • Ulmus microcarpa
  • Ulmus prunifolia
  • Ulmus rubra - Slippery Elm or Red Elm.
  • Ulmus uyematsui - Arishan Elm.
  • Ulmus wallichiana - Himalayan Elm or Kashmir Elm.

• "Field elms": spring flowering; flowers subsessile; leaves usually smooth above.
  • Ulmus chenmoui
  • Ulmus davidiana - David Elm.
    • U. davidiana var. japonica - Japanese Elm.
  • Ulmus harbinensis
  • Ulmus lanceaefolia or lanceifolia, lancifolia, syn. tonkinensis.
  • Ulmus minor - Field Elm.
    • U. minor subsp. angustifolia. syns. U. minor var. cornubiensis, U. stricta. - Cornish Elm.
    • U. minor subsp. minor, syn. U. carpinifolia - Smooth-leaved Elm.
    • U. minor var. plotii, syn. var. lockii - Plot's Elm, Goodyer's Elm or Lock Elm.
    • U. minor var. vulgaris, syn. U. procera - English Elm or Atinian Elm.
    • U. minor 'Sarniensis' - Guernsey Elm, also Jersey Elm, Wheatley Elm, Southampton Elm.
  • Ulmus pseudopropinqua.
  • Ulmus pumila - Siberian Elm.
  • Ulmus szechuanica.
  • Ulmus villosa - Cherry Bark Elm.
  • Ulmus wilsoniana - Wilson's Elm. Now treated as conspecific with U. davidiana.

• "White elms": spring flowering; flowers pedunculate
  • Ulmus alata - Winged Elm or 'Wahoo'.
  • Ulmus americana - American Elm.
  • Ulmus laevis - European White Elm or Fluttering Elm, Spreading Elm, (USA only) Russian Elm.
  • Ulmus thomasii - Rock Elm.

• "Autumn-flowering elms": autumn flowering.
  • Ulmus crassifolia - Cedar Elm.
  • Ulmus parvifolia - Chinese Elm or Lacebark Elm.
  • Ulmus serotina - September Elm.

• Hybrids and hybrid origin cultivars.
  • Ulmus × brandisiana. U. chumlia × U. wallichiana. Natural hybrid. India, Pakistan.
  • Ulmus × elegantissima 'Jacqueline Hillier'. Origin obscure, possibly U. glabra × U. minor var. plotii. ^*. UK.
  • Ulmus × hollandica. U. glabra × U. minor. Natural hybrid. Europe. Various cultivars including 'Major' ('Dutch Elm'), 'Vegeta' (Huntingdon Elm) syn. 'Chichester Elm', and 'Wredei' ('Golden Elm') ^*.
  • Ulmus 'Cathedral'. U. pumila × U. davidiana var. japonica. USA.
  • Ulmus 'Charisma'. (U. davidiana var. japonica × U. wilsoniana) × (U. davidiana var. japonica × U. pumila). USA.
  • Ulmus 'Clusius'. (U. glabra Exoniensis × U. wallichiana) × U. × hollandica 'Bea Schwarz'. Netherlands.
  • Ulmus 'Columella'. 'Plantyn' or 'Plantijn' selfed. Netherlands.
  • Ulmus 'Commelin'. U. × hollandica 'Vegeta' × U. minor. Netherlands.
  • Ulmus 'Coolshade'. U. rubra × U. pumila. USA.
  • Ulmus 'Dodoens'. U. glabra × U. wallichiana selfed. Netherlands.
  • Ulmus 'Frontier'. U. minor × U. parvifolia. USA.
  • Ulmus 'Green King'. U. rubra × U. pumila. USA.
  • Ulmus 'Groeneveld'. U. minor × U. glabra. Netherlands, 1960.
  • Ulmus 'Homestead'. U. pumila × (Commelin × (U. pumila × U. minor)). USA.
  • Ulmus 'Lobel'. (U. glabra Exoniensis × U. wallichiana) × U. × hollandica 'Bea Schwarz'. Netherlands, 1973.
  • Ulmus 'Morton' (Accolade ™). U. davidiana var. japonica × U. wilsoniana. ^*. USA.
  • Ulmus 'Morton Glossy' (Triumph ™). 'Morton' × 'Vanguard'. USA.
  • Ulmus 'Morton Plainsman' (Vanguard ™). U. pumila × U. davidiana var. japonica. USA.
  • Ulmus 'Morton Red Tip' (Danada Charm ™). U. davidiana var. japonica × U. wilsoniana. USA.
  • Ulmus 'Morton Stalwart' (Commendation ™). (U. davidiana var. japonica × U. wilsoniana) × (U. pumila × U. minor). USA.
  • Ulmus 'Nanguen' (Lutèce ™). 'Plantyn' × (U. × hollandica 'Bea Schwarz' × 'Bea Schwarz' selfed). ^*. Netherlands.
  • Ulmus 'New Horizon'. U. pumila × U. davidiana var. japonica . USA, 1995.
  • Ulmus 'Patriot'. 'Urban' × U. wilsoniana 'Prospector'. USA.
  • Ulmus 'Pioneer'. U. glabra × U. minor. USA
  • Ulmus 'Plantyn' or 'Plantijn'. (U. glabra Exoniensis × U. wallichiana) × (U. minor × U. minor). Netherlands, 1973.
  • Ulmus 'Plinio'. 'Plantyn' × U. pumila. Italy.
  • Ulmus 'Regal'. U. pumila × (Commelin × (U. pumila × U. minor 'Hoersholmiensis')). USA.
  • Ulmus 'San Zanobi'. 'Plantyn' × U. pumila. Italy.
  • Ulmus Sapporo Autumn Gold. U. pumila × U. davidiana var. japonica. Japan.
  • Ulmus 'Urban'. Commelin × U. pumila. USA.

• • and others without formal hybrid names

The other genera in the Ulmaceae are Zelkova (Zelkova) and Planera (Water-elm). Celtis (Hackberry or Nettle Tree), formerly included in Ulmaceae, is now treated in the family Cannabaceae.

Cultivation and uses

Elm wood is valued for its interlocking grain, and consequent resistance to splitting, with significant uses in chair seats and coffins. The wood is also resistant to decay when permanently wet, and was used for making water pipes during the mediaeval period in Europe. Elms also have a long history of cultivation for fodder, with the leafy branches cut for livestock. The bark, cut into strips and boiled, sustained much of the rural population of Norway during the famine in the mid-19th century.

From the 18th century to the early 20th century, elms were among the most widely planted ornamental tree in both Europe and North America. They were particularly popular as a street tree in avenue plantings in towns and cities, creating high tunneled effects.

In Europe, the Wych Elm U. glabra and the Smooth-leaved Elm U. minor were the most widely planted, with the former in northern areas (Scandinavia, northern Britain), and the latter further south. The hybrid between these two, Dutch Elm U. × hollandica, occurred naturally and was also commonly planted.

In North America the main species used was the American Elm U. americana, which has unique properties that made it ideal for such use; rapid growth, wide adaptation to a broad range of climates and soils, strong wood, resistant to wind damage, and vase-like growth habit requiring minimal pruning.

From about 1850 to 1920 the most prized small specimen elm was the Camperdown Elm, a contorted weeping cultivar of the Wych Elm Ulmus glabra 'Camperdown', grafted on a standard Wych Elm trunk to give a wide, spreading and weeping fountain shape in large garden spaces.

Large numbers of English Elms U. minor var. vulgaris were planted in Australia in the early 20th century, although the tree is not native to the island-continent.

Dutch elm disease

Dutch elm disease has devastated elms throughout Europe and North America. It is caused by the micro- fungus Ophiostoma novo-ulmi transmitted by two species of Scolytus elm-bark beetle which act as vectors. The disease affects all species of elm native to North America and Europe, but many Asiatic species have anti-fungal genes and are resistant. Fungal spores, introduced into wounds in the tree caused by the beetles, germinate in the vascular system, effectively blocking the flow from roots to leaves. Woodland trees in North America are not quite as susceptible to the disease because they usually lack the root-grafting of the urban elms and are somewhat more isolated from each other. In France, inoculation of over three hundred clones of the three European species with the fungus failed to find a single variety possessed of any significant resistance.

An earlier, less aggressive strain of the disease fungus, Ophiostoma ulmi, first appeared in Europe in 1910 and North America in 1928, but had declined by the 1940s. The second, far more virulent strain of the disease was identified in Europe in the late 1960s, and within a decade had killed over 20 million trees (approximately 75%) in the UK alone. The origin of the new strain remains a mystery; earlier believed to have been endemic to China, surveys there in 1986 found no trace of it, although bark beetles were common. The most popular hypothesis is that it arose from a hybrid between the original O. ulmi and another strain endemic to the Himalaya, O. himal-ulmi. While there is no sign of the current pandemic waning, there is some hope in the susceptibility of the fungus to a disease of its own caused by d-factors : naturally occurring virus-like agents that can severely debilitate it and reduce its sporulation.

Owing to its geographical isolation and effective quarantine enforcement, Australia has so far been unaffected by Dutch Elm Disease, and as such retains some of the world's best stands of English Elms; the long avenues of Royal Parade and St Kilda Road in Melbourne are perhaps the most beautiful examples ^*.

The provinces of Alberta and British Columbia in western Canada are also free of Dutch Elm disease, although in 1998, one tree in southeastern Alberta was found diseased and thus had to be destroyed immediately before the disease could spread any further. Aggressive means are being taken to prevent any occurrences of the disease in these two provinces. In fact, Alberta has the world's largest stands of elms unaffected by the disease, and many streets and parks in Edmonton and Calgary are still lined with large numbers of healthy mature trees.

Resistant Trees

Efforts to develop resistant trees began in the Netherlands in 1928. Research was later conducted in North America, and continues to this day in Italy. Research has followed two paths.

Hybridization between Asiatic species and European elms, or between Asiatic elms alone, has now, after a number of false dawns, produced some fine trees. Hybrids with immunity or very high resistance to disease are now commercially available after over 20 years of field trials; several originated in the USA, two in the Netherlands, and two in Italy. 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 and English Elms were prized. Several of the same have also proven unsuited to the oceanic 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 waterlogged ground; they should also ultimately reach a greater size.

Separately, efforts have been made in the USA to develop resistant cultivars of American Elm. The 'Liberty Elm', available commercially, represents the results of one such effort, and though marketed as a single product, consists of five cultivars chosen at random. These cultivars were the result of field selection of trees that survived in a region where the disease was endemic, followed by 2-3 generations of selection. Some of the cultivars are patented. The 'Valley Forge' and 'New Harmony' elms are similar cultivars, produced using selection techniques similar to those used for the 'Liberty Elm'.

Since elms take decades to grow to maturity, and these introductions are recent, the performance and ultimate size of these trees in the countryside is not known with certainty.

A related effort is the commercial reintroduction of the 'Princeton Elm', which is a cultivar selected in 1920 for its landscape qualities. Large plantings have survived the disease, and testing in laboratory conditions revealed that this cultivar has considerable resistance. It was selected in 2006 by HRH The Prince of Wales to create an avenue from his residence Highgrove House to the Golden Bird statue on the boundary of his garden. It has also been chosen to replace elms killed by disease along Pennsylvania Avenue in front of the White House.

Insect use

Many species of Lepidopteran larvae uses elm as a food plant; see list of Lepidoptera which feed on Elms. In Australia, introduced elm trees are sometimes used as food plants by the larvae of hepialid moths of the genus Aenetus. These burrow horizontally into the trunk then vertically down.

References

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• Armstrong, J. V. & Sell, P. D. (1996). A revision of the British elms (Ulmus L., Ulmaceae): the historical background. Bot. J. Linn. Soc. 120: 39-50.
• Bean, W. J. (1981). Trees and shrubs hardy in Great Britain, 7th edition. Murray, London.
• Brasier, C. M. (1996). New horizons in Dutch elm disease control. Pages 20-28 in: Report on Forest Research 1996. Forestry Commission. HMSO, London, UK.
• Brookes, A. H. (2006). An evaluation of disease-resistant hybrid and exotic elms as larval host plants for the White-letter Hairstreak butterfly Satyrium w-album, Part 1. Butterfly Conservation, Lulworth, UK.
• Burdekin, D. A. & Rushforth, K. D. (Revised by Webber J. F. 1996). Elms resistant to Dutch elm disease. Arboricultural Research Note 2/96. Arboricultural Advisory and Information Service, Alice Holt, Farnham, UK.
• Collin, E. (2001). Elm. In Teissier du Cros (Ed.) (2001) Forest Genetic Resources Management and Conservation. France as a case study. Ministry of Agriculture and Fisheries, Bureau of Genetic Resources. INRA DIC. France.
• Cornell University: Elm hybrids (pdf file)
• Elwes, H. J. & Henry, A. (1913). The Trees of Great Britain & Ireland. Vol. VII. pp 1848-1929. Private publication, Edinburgh. ^*
• Fu, L., Xin, Y. & Whittemore, A. (2002). Ulmaceae, in Wu, Z. & Raven, P. (eds) Flora of China, Vol. 5 (Ulmaceae through Basellaceae). Science Press, Beijing, and Missouri Botanical Garden Press, St. Louis, USA. ^*
• Melville, R. & Heybroek, H. (1971). Elms of the Himalaya. Kew Bulletin, Vol. 26 (1). Kew, London.
• Northern Arizona University: Elm trials.
• Richens, R. H. (1983). Elm. Cambridge University Press.
• Santamour, J., Frank, S. & Bentz, S. (1995). Updated checklist of elm (Ulmus) cultivars for use in North America. Journal of Arboriculture, 21:3 (May 1995), 121-131. International Society of Arboriculture, Champaign, Illinois, USA.
• Santini, A., Fagnani, A., Ferrini, F. & Mittempergher, L. (2002). 'San Zanobi' & 'Plinio' Elm Trees. HortScience, Vol. 37 (7) : 1139-1141. Dec. 2002.
• Ware, G. (1995). Little-known elms from China: landscape tree possibilities. Journal of Arboriculture, (Nov. 1995). International Society of Arboriculture, Champaign, Illinois, USA. ^*.

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