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Elms are deciduous and semi-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.
Species, varieties and hybrids
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 'Camperdownii', 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 *. 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. Resistant cultivars 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 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 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 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. These six clones were the result of field selection of trees that survived in a region where the disease was endemic, followed by another generation of breeding and selection. One of the six clones, a cultivar named Independence, was patented in 1988. The Valley Forge and New Harmony elms are similar cultivars that were selected after similar screenings were performed by the USDA in the early 1990's. The cultivar Jefferson was screened and selected by the USDA a few years later and was released to wholesale nurseries in 2004. 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. | |||||||||||||||||||||||||||||||||
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