Elm
From Wikipedia, the free encyclopedia
-
For other senses of this word, see Elm (disambiguation).
 |
||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
 Mature Slippery Elm
|
||||||||||||
| Scientific classification | ||||||||||||
|
||||||||||||
|
|
||||||||||||
Elms are deciduous and semi-deciduous trees making up the genus Ulmus, family Ulmaceae, found throughout the Northern Hemisphere from Siberia to Indonesia, Mexico to Japan. They have alternate, simple, single- or doubly-serrate leaves, usually asymmetric at the base and acuminate at the apex. Elms are hermaphroditic, having perfect flowers, and which, being wind-pollinated, are without petals. The fruit is a round 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.
Contents |
[edit] Species, varieties and hybrids
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. Six species are endemic to North America and a similar number to Europe, but the greatest diversity is found in China.
During the 18th and 19th centuries, elm cultivars enjoyed huge popularity as ornamentals 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 and the outbreak of Dutch elm disease saw the elm slide into horticultural decline. The further disruption to horticulture caused by the Second World War and, 20 years later, the devastation wrought by the outbreak of a new, far more virulent strain of Dutch elm disease, brought the tree to its nadir.
Since circa 1980 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 strain of the disease. Consequently, the total number of named cultivars, ancient and modern, now exceeds 300, although most of the older clones have probably 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 UK for example, only three of the new American and European releases are commercially available.
[edit] Cultivation and uses
Elm wood was valued for its interlocking grain, and consequent resistance to splitting, with significant uses in wheels, chair seats and coffins. The wood is also resistant to decay when permanently wet, and hollowed trunks were widely used as water pipes during the medieval 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-tunnelled effects, and to this day, 'Elm Street' remains the most common road name in the USA. In North America the species most commonly planted was the American Elm U. americana, which had unique properties that made it ideal for such use: rapid growth, adaptation to a broad range of climates and soils, strong wood, resistant to wind damage, and vase-like growth habit requiring minimal pruning. In Europe, the Wych Elm U. glabra and the Smooth-leaved Elm U. minor var. minor were the most widely planted in the countryside, with the former in northern areas (Scandinavia, northern Britain), and the latter further south. The hybrid between these two, Dutch Elm U. × hollandica, occurs naturally and was also commonly planted.
In Australia large numbers of English Elms U. procera were planted as ornamentals in the early 20th century.
In parks and gardens, 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 Camperdownii, grafted on a standard Wych Elm trunk to give a wide, spreading and weeping fountain shape in large garden spaces.
[edit] Diseases
[edit] Dutch elm disease
Dutch elm disease devastated elms throughout Europe and North America in the 20th century. 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, invade the xylem or vascular system. The tree responds by producing tyloses, 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 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 [1]. Grattan St, Carlton, is also a very pleasing example of a stand of elms.
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 destroyed immediately before the disease could spread. 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.
[edit] Resistant cultivars
Efforts to develop resistant cultivars began in the Netherlands in 1928. Research was later initiated in North America in 1937, and continues to this day in Italy. Research has followed two paths:
Hybrid cultivars from crossings of Asiatic with European species, or of Asiatic elms alone. 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.
Species cultivars, mostly selected in North America. Careful selection has produced a number of trees not only resistant to disease, but also the droughts and extremely cold winters afflicting that continent. Research in the USA has concentrated on the American Elm U. americana. The American Liberty elm represents the results of one such effort, and though promoted as a single product, consists of six different genotypes collectively under a single name. Other species from which suitable cultivars have been selected in North America have included the Chinese Elm, U. parvifolia, the Siberian Elm, U. pumila, and the Japanese Elm, U. davidiana, var. japonica.
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.
[edit] 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.
[edit] References
- 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 [2]
- 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. [3]
- Heybroek, H. M. (1983). Resistant Elms for Europe. In Burdekin, D. A. (Ed.) Research on Dutch elm disease in Europe. For. Comm. Bull. 60. pp 108 - 113.
- Heybroek, H. M. (1993). The Dutch Elm Breeding Program. In Sticklen & Sherald (Eds.) (1993). Dutch Elm Disease Research, Chapter 3. Springer Verlag, New York, USA
- Melville, R. & Heybroek, H. (1971). Elms of the Himalaya. Kew Bulletin, Vol. 26 (1). Kew, London.
- Mittempergher, L. & Santini, A. (2004) The history of elm breeding. Investigacion agraria: Sistemas y recursos forestales 13(1): 161-177 (2004).
- 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.
- Smalley, E. B. & Guries, R. P. (1993). Breeding Elms for Resistance to Dutch Elm Disease. Annual Review of Phytopathology Vol. 31 : 325-354. Palo Alto, California.
- Ware, G. (1995). Little-known elms from China: landscape tree possibilities. Journal of Arboriculture, (Nov. 1995). International Society of Arboriculture, Champaign, Illinois, USA. [4].
[edit] External links
- [5]. Northern Arizona University: Elm trials.
