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 to Alberta and Montana, and south to Florida and central Texas.
The American elm is an extremely hardy tree that can withstand winter temperatures as low as -42C. Trees in areas unaffected by Dutch elm disease can live for several hundred years. A prime example of the species was the Sauble Elm, which grew beside the banks of the Sauble River in Ontario to a height of 43 m (140 ft) before succumbing to the Dutch elm disease, when it was felled in 1968; a tree ring count established that it had germinated in 1701.
The American elm is a deciduous hermaphroditic tree which, before the introduction of the Dutch elm disease, commonly grew to >30m tall with a trunk >1.2m d.b.h.(diameter at breast height) supporting a high, spreading umbrella-like canopy.
The leaves are alternate, 7-20 cm long, with double-serrate margins and an oblique base. The perfect flowers are small, purple brown and being wind pollinated apetalous. The flowers are also protogynous, the female parts maturing before the male, thus reducing, but not eliminating, self-fertilization.
The American elm occurs naturally in an assortment of habitats, most notably rich bottomlands, floodplains, stream banks, and swampy ground, although it also often thrives on hillsides, uplands and other well drained soils.
The American elm is highly susceptible to the Dutch elm disease and elm yellows. It is also moderately preferred for feeding and reproduction by the adult elm leaf beetle Xanthogaleruca luteola.
Dutch elm disease (DED) is a fungal disease which has ravaged the American elm, causing catastrophic die-off in cities and across the country side in its range. It has been estimated that only approximately 1 in 100,000 American elm trees is DED tolerant, most known survivors simply having escaped exposure to the disease.
Dutch elm disease is caused by a member of the sac fungi (Ascomycota) affecting elm trees, and is spread by the elm bark beetle. Although believed to be originally native to Asia, the name “Dutch elm disease” refers to its identification in 1921 and later in the Netherlands by Dutch phytopathologists Bea Schwarz and Christine Buisman. The disease was first reported in the United States in 1928, with the beetles believed to have arrived in a shipment of logs form the Netherlands. DED reached eastern Canada during the Second World War, and spread to Ontario in 1967.