Ulmus americana

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Ulmus americana, generally known as the American Elm or, less commonly, as the White Elm or Water Elm, is a species native to eastern North America, occurring from Nova Scotia west as far as British Columbia, from northern Alberta at the top of its range, south to Florida and central Texas. It is an extremely hardy tree that can withstand winter temperatures as low as −42 °C (−44 °F). Trees in areas unaffected by Dutch elm disease can live for several hundred years. A prime example of the species was the Sauble Elm [3], which grew in Ontario, Canada, to a height of 43 m (140 ft), with a d.b.h of 196 cm (6.43 ft) before succumbing to Dutch elm disease. Felled in 1968, a tree-ring count established that it had germinated in 1701.

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Description

The American Elm is a deciduous tree, which, before the advent of Dutch elm disease, commonly grew to > 30 m (100 ft) tall with a trunk > 1.2 m (4 ft) d.b.h. The crown forms a high, spreading canopy with open air space beneath. The leaves are alternate, 7–20 cm long, with double-serrate margins and an oblique base. The tree is hermaphroditic, having perfect flowers, (i.e. with both male and female parts) and is therefore capable of self-pollination. The flowers are small, purple-brown, and, being wind-pollinated, are apetalous; they emerge in early spring before the leaves. The fruit is a flat samara 2 cm long and 1.5 cm broad, with a circular wing surrounding the single 4–5 mm seed. As in the closely related European White Elm, U. laevis, the flowers and seeds are borne on 1–3 cm long stems. American Elm is wholly insensitive to daylight length (photoperiod), and will continue to grow well into autumn until injured by frost [2].

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