Trees & Shrubs – The Elms / Ulmus
Elms are deciduous and semi-deciduous trees comprising the genus Ulmus in the plant family Ulmaceae. The genus first appeared in the Miocene geological period about 20 million years ago, originating in what is now central Asia. These trees flourished and spread over most of the Northern Hemisphere, inhabiting the temperate and tropical-mountain regions of North America and Eurasia, presently ranging southward across the Equator into Indonesia.
The RHS list the following native species, IE: present at the end of the last ice-age.
Ulmus glabra (wych elm):35m, susceptible to Dutch elm disease
Ulmus minor (small-leaved elm): 30m, susceptible to Dutch elm disease, not readily available
Ulmus plotii (Plot’s elm): 30m, susceptible to Dutch elm disease, not readily available
Ulmus procera (English elm): 40m, susceptible to Dutch elm disease
Naturally occurring hybrids
Ulmus × elegantissima
Ulmus × hollandica
Ulmus × vegeta
Ulmus × viminalis
Elms are components of many kinds of natural forests. Moreover, during the 19th and early 20th centuries many species and cultivars were also planted as ornamental street, garden, and park trees in Europe, North America, and parts of the Southern Hemisphere, notably Australasia. Some individual elms reached great size and age. However, in recent decades, most mature elms of European or North American origin have died from Dutch elm disease, caused by a microfungus dispersed by bark beetles. In response, disease-resistant cultivars have been developed, capable of restoring the elm to forestry and landscaping.
Even though elm wood is a deciduous wood it is not a use-full wood for fuel.
Elm-wood burns like churchyard mould,
E’en the very flames are cold;
There are about 30 to 40 species of Ulmus (elm); the ambiguity in number results from 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 (Ulmus minor) group. Rackham describes Ulmus as the most difficult critical genus in the entire British flora, adding that ‘species and varieties are a distinction in the human mind rather than a measured degree of genetic variation’. Eight species are endemic to North America, and a smaller number to Europe; the greatest diversity is found in Asia.
The classification adopted in the List of 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 currently accepted names can be found in the list List of elm Synonyms and Accepted Names.
Botanists who study elms and argue over elm identification and classification are called pteleologists, from the Greek πτελέα (:elm).
As part of the sub-order urticalean rosids they are distant cousins of cannabis, hops, and nettles.
The name Ulmus is the classical name for these trees, with the English name “elm” and many other European names derived from it.
Elm wood is valued for its interlocking grain, and consequent resistance to splitting, with significant uses in wagon wheel hubs, chair seats and coffins. The bodies of Japanese Taiko drums are often cut from the wood of old elm trees, as the wood’s resistance to splitting is highly desired for nailing the skins to them, and a set of three or more is often cut from the same tree. The elm’s wood bends well and distorts easily making it quite pliant. The often long, straight, trunks were favoured as a source of timber for keels in ship construction. Elm is also prized by bowyers; of the ancient bows found in Europe, a large portion of them are elm. During the Middle Ages elm was also used to make longbows if yew was unavailable.
The first written references to elm occur in the Linear B lists of military equipment at Knossos in the Mycenaean Period. Several of the chariots are of elm (« πτε-ρε-ϝα », pte-re-wa), and the lists twice mention wheels of Elmwood. Hesiod says that ploughs in Ancient Greece were also made partly of elm.
The density of elm wood varies between species, but averages around 560 kg per cubic metre.
Elm wood is also resistant to decay when permanently wet, and hollowed trunks were widely used as water pipes during the medieval period in Europe.
Elm was also used as piers in the construction of the original London Bridge. However this resistance to decay in water does not extend to ground contact.
Wayside and Woodland Trees: A pocket guide to the British sylva by Edward Step – Elm trees.
The Wych Elm (Ulmus montana).
Of the two species of Elms commonly grown in these islands this alone is a native, though the Common or Small-leaved Elm (Ulmus campestris) was introduced from the Continent by the Romans, so that it has had time to get itself widely distributed over our country. Other names for the Wych Elm are Mountain Elm, Scots Elm, and Witch Hazel—the last-named being now more generally applied to an American plant, the Hamamelis. The philologists appear to be uncertain as to the origin and meaning of Wych, but it seems most probably a form of Witch. Just as a Hazel-rod is used by water-finders, who declare that its movements indicate the presence of hidden springs, so a wand of Ulmus montana may have furnished the Witch-finder with a Witch Hazel for the detection of witches!
Fruits of Wych Elm.
The names montana, campestris, and Mountain Elm must not be allowed to mislead us as to the habits of the two species, for though the Wych Elm is known to reach an altitude of 3300 feet in the Alps, here it ascends only to 1300 feet (Yorks.), whilst Ulmus campestris, which might be understood to be less a hill-climber, grows at an elevation of 1500 feet in Derbyshire. As a matter of fact, both species are much fonder of valleys than of mountains.
The Wych Elm forms a trunk of large size, from 80 to 120 feet or more in height, with a girth of 50 feet, and covered with rough bark that is often corky. Its long slender branches spread widely with a downward tendency, the downy forking twigs bearing their leaves in a straight row along each side. The leaves are somewhat oval in general form, but the two sides of the midrib are unequal in size and shape. Their edges are doubly or trebly toothed, and the surfaces are rough and harsh to the touch. The hairs that cover the strong ribs on the under surface serve for the protection of the breathing pores from dust. On leaves of the pendulous form of this tree, grown in the London parks and gardens, these hairs will be found to be quite black with the soot particles gathered from the air. Trees need carbon, but in this gross form they are too often suffocated by it. In March or April the brownish flowers are produced in bunches from the sides of the branches. They are a quarter of an inch long, bell-shaped, their edges cut into lobes, and finely fringed. The ovary, with its two awl-shaped styles, is surrounded by four or five stamens with purple anthers. They appear in March or April, before the leaf-buds have opened, and are dependent on the wind for the transfer of pollen. The fruit is an oblong samara, about an inch long. This consists of a single seed in the centre, invested by a thin envelope, which is extended all round as a light membranous wing, which gives it buoyancy and enables it to float through the air to a little distance. These seeds are not produced until about the thirtieth year of the tree’s life, and though they are ripened almost annually thereafter, good crops are biennial or triennial only. It has often been stated that the Wych Elm does not send up suckers, but it does, though not invariably; it does so chiefly as the result of root-pruning or some other check to the extension of the root-system.
Bole of Wych Elm.
Bole of Common Elm.
Small-leaved Elm (Ulmus campestris)
The Elm most frequently seen is the Small-leaved Elm (Ulmus campestris), which is therefore entitled to its alternative name of Common Elm.
Constantly grown as a hedgerow tree, it meets us at every turn, though it is much less plentiful in Scotland than in other parts of the United Kingdom. It is in all respects very similar to the Wych Elm, but its leaves are smaller—usually from two to three inches long, the twigs often covered with a corky bark, and the seed, instead of being in the centre of the samara, is much nearer to the notched end. The leaves are proportionately narrower than those of montana, and it will be found that the hairs which cover the midrib below possess in minor degree the irritating qualities of the Nettle’s stings. This is a fact not generally known, but I became painfully aware of it a few years ago when clearing away the suckers of an Elm that were encroaching too much upon my garden border. Examination of these hairs shows that they are constructed much on the same plan as those of the Nettle—a member of the same Natural Order, by the way. The fact that these leaves are browsed by cattle and deer may explain this development of the hairs, which, whilst they may serve to keep off sheep, have not yet reached a degree of acridity sufficient to protect them from the larger beasts. Both flowers and samaras are about a third smaller than those of montana; but seed is very seldom produced in this country, and the tree seeks to reproduce itself by throwing up abundant suckers round the base of the bole, and even from root-branches at a considerable distance from the trunk. These, of course, if allowed to grow, would soon surround the tree with copse. Campestris often attains a greater height with its straighter trunk than montana, but its girth is not so great, seldom being more than twenty feet. Its dark wood is harder and finer grained than that produced by the native tree. Its favour as a hedgerow tree is probably due to the fact that it gives shade which is not obnoxious to the growth of grass. Both species are subject to a great amount of variation, and in nurserymen’s catalogues these forms have appropriate names, but they are not regarded as of sufficient permanence to merit scientific distinction. In point of age—Elms are known to exceed five hundred years.
Among the insects that feed upon the Elm’s foliage, the most noteworthy is the caterpillar of the fine Large Tortoiseshell Butterfly (Vanessa polychloros). I have already mentioned the relationship subsisting between Elms and Nettles, and it is a point worth noting that nearly all our native species of Vanessa feed in the larval state upon the leaves of the Nettle. In London parks and squares the Elms are much infested by the caterpillars of the Vapourer-moth, whose wingless females may be seen like short-legged spiders on the bark, whilst the male flutters in an apparently aimless way on wings of rich brown with central white spots. In October the leaves, which have for some time assumed a very dull dark-green tint, suddenly turn to orange, then fade to pale yellow, and fall in showers. The name Elm was derived from the Latin Ulmus, and appears to indicate an instrument of punishment—probably from its rods having been used to belabour slaves. Prior remarks that the word is “nearly identical in all the Germanic and Scandinavian dialects, but does not find its root in any of them. It plays through all the vowels … but stands isolated as a foreign word which they have adopted.” This “playing through the vowels” may be thus illustrated—Alm, Ælm, and Elm (Anglo-Saxon and English); Ilme, Olm, and Ulme, in various German dialects.
Dutch Elm Disease
Dutch elm disease (DED) devastated elms throughout Europe and much of North America in the second half of the 20th century. It derives its name ‘Dutch’ from the first description of the disease and its cause in the 1920s by the Dutch botanists Bea Schwarz and Christina Johanna Buisman. Owing to its geographical isolation and effective quarantine enforcement, Australia has so far remained unaffected by Dutch Elm Disease, as have the provinces of Alberta and British Columbia in western Canada.
DED is caused by a micro-fungus 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 evolved 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 with the fungus of over three hundred clones of the European species failed to find a single variety possessed of any significant resistance.
The first, less aggressive strain of the disease fungus, Ophiostoma ulmi, arrived in Europe from the Far East in 1910, and was accidentally introduced to North America in 1928, but was steadily weakened by viruses and had all but disappeared in Europe by the 1940s.
The second, far more virulent rain 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 new strain arrived in Europe from the US on a cargo of Rock Elm; the hypothesis that it arose from a hybrid between the original O. ulmi and another strain endemic to the Himalaya, Ophiostoma himal-ulmi is now discredited.
Despite Dutch Elm disease London is still home to around 4000 elms.
There is no sign of the current pandemic waning, and no evidence of a susceptibility of the fungus to a disease of its own caused by d-factors: naturally occurring virus-like agents that severely debilitated the original O. ulmi and reduced its sporulation.
Efforts 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:
Species and species cultivars
In North America, careful selection has produced a number of trees resistant not only to disease, but also to the droughts and extremely cold winters afflicting the continent. Research in the US has concentrated on the American Elm U. americana, resulting in the release of highly resistant clones, notably the cultivars ‘Valley Forge’ and ‘Jefferson’. Much work has also been done into the selection of disease-resistant Asiatic species and cultivars.
In Europe, the European White Elm Ulmus laevis has received much attention. Whilst this elm 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 may be the presence of a triterpene, alnulin, which makes the tree bark unattractive to the beetle species that spread the disease. However this possibility has not been conclusively proven. ore recently, Field Elms Ulmus minor highly resistant to DED have been discovered in Spain, and form the basis of a major breeding programme.
Owing to their innate resistance to Dutch elm disease, Asiatic species have been crossed with European species, or with other Asiatic elms, to produce trees which are both highly resistant to disease and tolerant of native climates. After a number of false dawns in the 1970s, this approach has produced a range of reliable hybrid cultivars now commercially available in North America and Europe.
However, some of these cultivars, notably those with the Siberian Elm U. pumila in their ancestry, lack the forms for which the iconic American Elm and English Elm were prized. Moreover, several exported to northwestern Europe have proven unsuited to the maritime climate conditions there, notably because of their intolerance of anoxic conditions resulting from ponding on poorly drained soils in winter. Dutch hybridizations invariably included the Himalayan Elm U. wallichiana as a source of anti-fungal genes and have proven more tolerant of wet ground; they should also ultimately reach a greater size. However, the susceptibility of the cultivar ‘Lobel’, used as a control in Italian trials, to elm yellows has now (2014) raised a question mark over all the Dutch clones.
A number of highly resistant Ulmus cultivars has been released since 2000 by the Institute of Plant Protection in Florence, most commonly featuring crosses of the Dutch cultivar ‘Plantijn’ with the Siberian Elm to produce resistant trees better adapted to the Mediterranean climate.
Cautions regarding novel cultivars
Elms take many decades to grow to maturity, and as the introduction of these disease-resistant cultivars is relatively recent, their long-term performance and ultimate size and form cannot be predicted with certainty. The National Elm Trial in North America, begun in 2005, is a nationwide trial to assess strengths and weaknesses of the 19 leading cultivars raised in the US over a ten-year period; European cultivars have been excluded. Meanwhile, in Europe, American and European cultivars are being assessed in field trials started in 2000 by the UK charity Butterfly Conservation.