Ulmus pumila
Ulmus pumila, the Siberian elm, is a tree native to Central Asia, eastern Siberia, the Russian Far East, Mongolia, Tibet, northern China, India (northern Kashmir) and Korea. It is also known as the Asiatic elm and dwarf elm, but sometimes miscalled the 'Chinese Elm' ("Ulmus parvifolia"). It is the last tree species encountered in the semi-desert regions of central Asia. Described by Pallas in the 18th century from specimens from Transbaikal, "Ulmus pumila" has been widely cultivated throughout Asia, North America, Argentina, and southern Europe, becoming naturalized in many places, notably across much of the United States . Description. The Siberian elm is usually a small to medium-sized, often bushy, deciduous tree growing to 25 m tall, the d.b.h. to 1 m; the bark is dark gray, irregularly longitudinally fissured. The branchlets are yellowish gray, glabrous or pubescent, unwinged and without a corky layer, with scattered lenticels. The winter buds dark brown to red-brown, globose to ovoid. The petiole is 4–10 mm, pubescent, the leaf blade elliptic-ovate to elliptic-lanceolate, 2-8 × 1.2-3.5 cm, the colour changing from dark green to yellow in autumn. The perfect, apetalous wind-pollinated flowers bloom for one  week in early spring, before the leaves emerge, in tight fascicles (bundles) on last year's branchlets. However, flowers emerging in early February are often damaged by frost, consequently the species was dropped from the Dutch elm breeding programme. Each flower is about 3 mm across and has a green calyx with 4–5 lobes, 4–8 stamens with brownish-red anthers, and a green pistil with a two-lobed style. Unlike most elms, the Siberian elm is able to self-pollinate successfully. The wind-dispersed samarae are whitish tan, orbicular to rarely broadly obovate or elliptical, 1-2 × 1-1.5  cm, glabrous except for pubescence on stigmatic surface; the stalk 1–2 mm, the perianth persistent. The seed is at centre of the samara or occasionally slightly toward apex but not reaching the apical notch. Flowering and fruiting occur March to May. Ploidy: 2"n" = 28. The tree also suckers readily from its roots. The tree is short-lived in temperate climates, rarely reaching more than 60 years of age, but in its native environment may live to between 100 and 150 years. A giant specimen, 45 km southeast of Khanbogt in the south Gobi, with a girth of 5.55 m  in 2009, may exceed 250 years (based on average annual ring widths of other "U. pumila" in the area). Pests and diseases. The tree has considerable variability in resistance to Dutch elm disease; for example, trees from north-western and north-eastern China exhibit significantly higher tolerance than those from central and southern China. Moreover, it is highly susceptible to damage from many insects and parasites, including the elm leaf beetle "Xanthogaleruca luteola", the Asian 'zigzag' sawfly "Aproceros leucopoda", Elm Yellows, powdery mildew, cankers, aphids, leaf spot and, in the Netherlands, coral spot fungus "Nectria cinnabarina". However, "U. pumila" is  the most resistant of all the elms to verticillium wilt. Cultivation and uses. "U. pumila" was introduced into Spain as an ornamental, probably during the reign of Philip II (1556–98), and from the 1930s into Italy. In these countries it has naturally hybridized with the Field Elm "U. minor" (see below). In Italy it was widely used in viniculture, notably in the Po valley, to support the grape vines until the 1950s, when the demands of mechanization made it unsuitable. Three specimens were supplied by the Späth nursery of Berlin to the Royal Botanic Garden Edinburgh in 1902 as "U.  pumila", in addition to specimens of the narrow-leaved "U. pumila" cultivar 'Pinnato-ramosa' (see 'Cultivars' below). One was planted in RBGE; the two not planted in the Garden may survive in Edinburgh, as it was the practice of the Garden to distribute trees about the city. Kew Gardens obtained specimens of "U. pumila" from the Arnold Arboretum in 1908 and, as "U. pekinensis", via the Veitch Nurseries in 1910 from William Purdom in northern China. A specimen obtained from Späth and planted in 1914 stood in the Ryston Hall arboretum, Norfolk, in the early 20th century. The tree was  propagated and marketed by the Hillier & Sons nursery, Winchester, Hampshire, from 1962 to 1977, during which time over 500 were sold. More recently, the popularity of "U. pumila" in the Great Britain has been almost exclusively as a bonsai subject, and mature trees are largely restricted to arboreta. "U. pumila" is said to have been introduced to the United States in 1905 by Prof. J. G. Jack, and later by Meyer, though 'Siberian elm' appears in some 19th-century US nursery catalogues. The tree was cultivated at the USDA Experimental Station at Mandan, North Dakota, where it flourished. It was  consequently selected by the USDA for planting in shelter belts across the prairies in the aftermath of the Dustbowl disasters, where its rapid growth and tolerance for drought and cold initially made it a great success. However, the species later proved susceptible to numerous maladies. Attempts to find a more suitable cultivar were initiated in 1997 by the Plant Materials Center of the USDA, which established experimental plantations at Akron, Colorado, and Sidney, Nebraska. The study, no. 201041K, will conclude in 2020. The seeds lose their viability rapidly after maturity unless placed on suitable germination conditions or dried and  placed at low temperatures. The species has a high sunlight requirement and is not shade-tolerant; with adequate light it exhibits rapid growth. The tree is also fairly intolerant of wet ground conditions, growing better on well-drained soils. While it is very resistant to drought and severe cold, and able to grow on poor soils, its short period of dormancy, flowering early in spring followed by continuous growth until the first frosts of autumn, renders it vulnerable to frost damage. As an ornamental "U. pumila" is a very poor tree, tending to be short-lived, with brittle wood and  poor crown shape, but it has nevertheless enjoyed some popularity owing to its rapid growth and provision of shade. The Siberian Elm has been described as "one of the world's worst... ornamental trees that does not deserve to be planted anywhere". Yet in the US during the 1950s, the tree was also widely promoted as a fast-growing hedging substitute for privet, and as a consequence is now commonly found in nearly all states. Food. The unripe seeds have long been eaten by the peoples of Manchuria, and during the Great Chinese Famine they also became one of the  most important foodstuffs in the Harbin region. The leaves were also gathered, to the detriment of the trees, prompting a prohibition order by the authorities, which was largely ignored. The leaves eaten raw are not very palatable, but stewed and prepared with Kaoliang or Foxtail millet make a better tasting and more filling meal. Invasiveness and spontaneous hybridization. In North America, "Ulmus pumila" has become an invasive species in much of the region from central Mexico northward across the eastern and central United States to Ontario, Canada. It also hybridizes in the wild with the native "U. rubra" (Slippery Elm ) in the central United States, prompting conservation concerns for the latter species. In South America, the tree has spread across much of the Argentine pampas In Europe it has spread widely in Spain, and hybridizes extensively there with the native field elm ("U. minor"), contributing to conservation concerns for the latter species. Research is ongoing into the extent of hybridisation with "U. minor" in Italy. "Ulmus pumila" is often found in abundance along railroads and in abandoned lots and on disturbed ground. The gravel along railroad beds provides ideal conditions for its growth: well-drained, nutrient poor soil, and high  light conditions; these beds provide corridors which facilitate its spread. Owing to its high sunlight requirements, it seldom invades mature forests, and is primarily a problem in cities and open areas, as well as along transportation corridors. The species is now listed in Japan as an alien species recognized as established in Japan or found in the Japanese wild. Varieties. Two varieties were traditionally recognized: var. "pumila" and var. "arborea", the latter now treated as a cultivar, "U. pumila" 'Pinnato-ramosa'. Cultivars. Valued for the high resistance of some clones to Dutch elm disease, over a dozen selections have been  made to produce hardy ornamental cultivars, although several may no longer be in cultivation: Some authorities consider the cultivar 'Berardii' a form of "Ulmus pumila". For the 19th-century cultivar called 'Siberian elm' by Castle Nurseries, Nottingham, see 'Nottingham elm'. Hybrid cultivars. The species has been widely hybridized in the United States and Italy to create robust trees of more native appearance with high levels of resistance to Dutch elm disease: Notable trees. Roerich describes a specimen discovered on his travels through Mongolia: The US National Champion, measuring high in 2011, grows in Berrien County, Michigan. In the UK the  TROBI Champions grow at Thorp Perrow Arboretum, Yorkshire, × in 2004, and at St Ann's Well Gardens, Hove, Sussex × in 2009.