Ulmus laevis

Ulmus laevis Pall., variously known as the European white elm, fluttering elm, spreading elm, stately elm and, in the USA, the Russian elm, is a large deciduous tree native to Europe, from France northeast to southern Finland, east beyond the Urals into Kyrgyzstan and Kazakhstan, and southeast to Bulgaria and the Crimea; there are also disjunct populations in the Caucasus and Spain, the latter now considered a relict population rather than an introduction by man, and possibly the origin of the European population. U. laevis is rare in the UK, however its random distribution, together with the absence of any record of its introduction, has led at least one British authority to consider it native. NB: The epithet 'white' elm commonly used by British foresters alluded to the timber of the wych elm. The species was first identified, as Ulmus laevis, by Pallas, in his Flora Rossica published in 1784. The tree is allogamous and is most closely related to the American elm U. americana. Endemic to alluvial forest, U. laevis is rarely encountered at elevations above 400 m. 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 species is threatened by habitat destruction and disturbance in some countries, notably Spain. Flood control schemes are particularly harmful, as seed dispersion is reliant on floods, while abstraction from aquifers lowering ground water levels has compromised the development of the trees. Although not possessed of an innate genetic resistance to Dutch elm disease, the species is rarely infected in western Europe. Ulmus laevis is similar in stature to the wych elm, if rather less symmetric, with a looser, untidy, branch structure and less neatly rounded crown. The tree 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 bark is smooth at first, then in early maturity breaks into thin grey scales, which separate with age into a network of grey-brown scales and reddish-brown underbark, and finally is deeply fissured in old age like other elms. The leaves are deciduous, alternate, simple ovate with a markedly asymmetric base, < 10 cm long and < 7 cm broad, comparatively thin, often almost papery in texture and very translucent, smooth above with a downy underside. Significantly, the leaf veins do not divide from the central vein to the leaf margin. The leaves are shed earlier in autumn than other species of European elm. The tree is most reliably distinguished from other European elms by its long flower stems, averaging 20 mm. Moreover, the apetalous wind-pollinated flowers are distinctively cream-coloured, appearing before the leaves in early spring in clusters of 15-30; they are 3–4 mm across. 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. The seeds have a generally high rate of germination, 45–60% for Serbian trees examined by Stilinović. Although the species is protandrous, levels of self-pollination can be high The tree can grow very rapidly; where planted in persistently moist soil, trunk width of 13-year-old trees increased by 4 cm per annum at breast height (d.b.h.). The species differs from its closest relative, the American elm, mainly in the irregular crown structure and frequent epicormic shoots, features which also give the tree a distinctive winter silhouette. The American elm also has less acute leaf buds, longer petioles, narrower leaves, and a deeper apical notch in the samara which reaches the seed. Like other European elms, natural populations of the European white elm have little innate resistance to Dutch elm disease, although research by Irstea 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, an uncommon situation in western Europe. Indeed, in a study of elm in Flanders, not one example of U. laevis was found to be afflicted by Dutch elm disease. Research in Spain has indicated that it is the presence of an antifeedant triterpene, alnulin, at a concentration of 200 μg/g {dried bark} which renders the tree unattractive to the beetles. Ergo: the tree's decline in western Europe is chiefly owing to woodland clearance in river valleys, and river management systems eliminating flooding, not disease.