| Ulmus laevis | |
|---|---|
| European White Elm in winter | |
| Scientific classification | |
| Kingdom: | Plantae |
| Division: | Magnoliophyta |
| Class: | Magnoliopsida |
| Order: | Rosales |
| Family: | Ulmaceae |
| Genus: | Ulmus |
| Species: | U. laevis |
| Binomial name | |
| Ulmus laevis Pall. |
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| Synonyms | |
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Ulmus laevis Pall., the European White Elm, Fluttering Elm, Spreading Elm and, in the USA, Russian Elm, is a large deciduous tree native to Europe, from France[1] northeast to southern Finland, east as far as the Urals, and southeast to Bulgaria and the Crimea; there is also a disjunct population in the Caucasus. Moreover, a small number of trees found in Spain is now considered a relict population rather than an introduction by man, and possibly the origin of the European population.[2]
Essentially endemic to alluvial forest, it is rarely encountered at elevations above 400 m.[3] Most commonly found along rivers such as the Volga and Danube, it is one of very few elms tolerant of prolonged waterlogged, anoxic ground conditions. The White Elm is allogamous and is most closely related to the American Elm U. americana.
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The tree is similar in stature to the Wych Elm, if rather less symmetric, with a looser branch structure and less neatly rounded crown. It typically reaches a height and breadth of > 30 m, with a trunk < 2 m d.b.h. The extensive shallow root system ultimately forms distinctive high buttresses around the base of the trunk. The leaves are deciduous, alternate, simple ovate with a markedly lop-sided base, < 10 cm long and < 7 cm broad, comparatively thin, often almost papery in texture and very translucent, smooth above with a downy underside. The apetalous wind-pollinated flowers appear before the leaves in early spring, produced in clusters of 15-30; they are 3–4 mm across on 20 mm long stems. In England, trees grown from seed flower in March, commencing at ages of between 7 and 12 years. The fruit is a winged samara < 15 mm long by 10 mm broad with a ciliate margin, the single round 5 mm seed maturing in late spring. U. laevis sheds its leaves earlier in the autumn than other species of European elm.
The tree is most reliably distinguished from other European elms by the long flower stems, and is most closely related to the American Elm U. americana, from which it differs mainly in the irregular crown shape and frequent small sprout stems on the trunk and branches, features which also give the tree a winter silhouette; a useful diagnostic feature at this season.[4][5][6]
U. laevis suckers from roots, but not stools. Although the species is protandrous, levels of self-pollination are high[7]
Like other European elms, natural populations of the European White Elm have little innate resistance to Dutch elm disease, although research by Cemagref has isolated clones able to survive inoculation with the causal fungus, initially losing < 70% of their foliage, but regenerating strongly the following year. Moreover, the tree is not favoured by the vector bark beetles, which colonize it only when there are no other elm alternatives available,[8] an uncommon situation in western Europe. Research in Spain[9] has indicated that it is the presence of a triterpene, alnulin, that renders the tree bark unattractive to the beetles. Ergo: the tree's decline in western Europe is chiefly owing to woodland clearance in river valleys, not disease.
The species has a slight to moderate susceptibility to Elm Yellows, but a very low susceptibility to the elm leaf beetle Xanthogaleruca luteola.[10]
Although ideally suited to wet ground conditions, the tree can still grow, albeit more slowly, on drier sites including chalk downland. However, one overriding factor in choosing a site is Exposure. White Elm is comparatively weak-wooded, much more so than Field Elm Ulmus minor, and thus an inappropriate choice for windy locations. In trials in southern England by Butterfly Conservation, young trees of < 5 m height were badly damaged by gusts of 40 knots (75 km/h) in midsummer.
Ulmus laevis is occasionally planted as an ornamental tree. In the UK examples are few and far between but sometimes of great age. Mature specimens survive today amid diseased native elm near Torpoint in Cornwall;[11] others can be found in Edinburgh (The Meadows area and Powderhall Road), London (Riverside Walk, near Royal Botanic Gardens, Kew, and at Peckham and Tooting), Chelmsford (beside the Chelmer at the Rivermead Campus) [2009], Brighton & Hove, and near St. Albans.
The species was never widely introduced to the USA, but is represented at several arboreta. In the Far East, the tree has been planted in Xinjiang province and elsewhere in Northern China; planting in Tongliao City is known to have been particularly successful. White Elm is also known to have been introduced to Australia.[12]
In recent years, the tree has enjoyed a small renaissance in England. A popular larval host plant of the White-letter Hairstreak Satyrium w-album butterfly across Europe, the elm is now being planted by Butterfly Conservation and other groups to restore local populations decimated by the effects of Dutch elm disease on native or archaeophytic elms.[13] The Cheshire Wildlife Trust, for example, is planting 1,000 White Elms on its reserves in the former Vale Royal district of the county.
The timber of the White Elm is of poor quality and thus of little use to man, not even as firewood. Its density is significantly lower than that of other European elms.
Usually easy to grow from seed sown to a depth of 6 mm in ordinary compost and kept well-watered. However, as seed viability can vary greatly from year to year, softwood cuttings taken in June may be a more reliable method. The cuttings strike very quickly, well within a fortnight, rapidly producing a dense matrix of roots.
The three largest known trees in Europe are at Gülitz in Germany (3.1 m d.b.h.), at Komorów in Poland (2.96 m d.b.h.), and at Bergemolo in Piedmont, Italy. The "Olmo di Bergemolo", over 200 years old, has a girth of 6.2 m (2008) and a height of 26 m. [8] A veteran between 300 and 500 years old, with a girth of 4.5 metres, survives in Rahnsdorf near Berlin.[14] The UK Champion is at Ferry Farm, Harewood, Cornwall (27 m high, 1.8 m d.b.h.). In the USA, a reputedly magnificent tree grows in Portland, Oregon, but its dimensions and age are not known.
Several putative varieties have been identified. A variety celtidea from what is now the Ukraine was reported by Rogowicz in the middle of the 19th century, but no examples are known to survive. Another variety parvifolia has been reported from Serbia.[15]
In Russia several ornamental forms are recognized: f. aureovariegata, f. argentovariegata, f. rubra, and f. tiliifolia.
Compared with the other European species of elm, U. laevis has received scant horticultural attention, there being only five recorded cultivars, none of which are known to remain in cultivation, with the possible exception of 'Colorans' and 'Pendula': Aureovariegata, Colorans, Ornata, Pendula, Punctata, Urticifolia.
U. laevis does not hybridize naturally, in common with the American Elm U. americana to which it is closely related. However, in experiments at the Arnold Arboretum, it was successfully crossed with U. thomasii and U. pumila; no such crosses have ever been released to commerce.
None known