Elms are deciduous and semi-deciduous trees comprising the genus Ulmus, family Ulmaceae, found throughout the Northern Hemisphere from Siberia to Indonesia, Mexico to Japan. Many species and cultivars have also been introduced as ornamentals to parts of the Southern Hemisphere, notably Australasia.
Elms have alternate, simple, single- or doubly-serrate leaves, usually asymmetric at the base and acuminate at the apex. They are hermaphroditic, having perfect flowers which, being wind-pollinated, are without petals. The fruit is a round wind-dispersed samara.
There are approximately 30 to 40 species of elm; the ambiguity in number 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. 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 circa 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.
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, resistance 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 England, it was the English Elm Ulmus procera that came to dominate the landscape; planted in hedgerows, it often occurred in densities of over 1000 per square kilometre. Such was its ubiquity, it almost always featured in the landscape paintings of John Constable. In Australia, large numbers of English Elms, as well as other species and cultivars, were planted as ornamentals following their introduction in the 19th century.
In parks and gardens, from about 1850 to 1920 the most prized small ornamental elm was the Camperdown Elm, Ulmus glabra 'Camperdownii', a contorted weeping cultivar of the Wych Elm grafted on a standard elm trunk to give a wide, spreading and weeping fountain shape in large garden spaces.
Many species of Lepidopteran larvae (butterflies and moths) uses elm as a food plant; see list of Lepidoptera that feed on elms. In Australia, introduced elm trees are sometimes used as foodplants by the larvae of hepialid moths of the genus Aenetus. These burrow horizontally into the trunk then vertically down.
The first, less aggressive strain of the disease fungus, Ophiostoma ulmi, appeared in Europe in 1910 and had spread to North America by 1928, but declined in the 1940s. The second, far more virulent strain of the disease Ophiostoma novo-ulmi was identified in Europe in the late 1960s, and within a decade had killed over 20 million trees (approximately 75%) in the UK alone. Approximately three times more deadly, 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, Ophiostoma 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 many stands of English Elms; the long avenues of Royal Parade and St Kilda Road in Melbourne, and Grattan Street in Carlton, Victoria, are three examples.
The provinces of Alberta and British Columbia in western Canada are also free of Dutch Elm disease. 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.
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 resistant hybrids have recently been developed, 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.
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 .