American elm

Ulmus americana
Ulmus americana (American elm) at Longwood Gardens, Kennett Square, Pennsylvania
Conservation status
Secure (NatureServe)
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Rosids
Order:	Rosales
Family:	Ulmaceae
Genus:	Ulmus
Species:	U. americana
Binomial name
Ulmus americana L.
Synonyms
Ulmus alba Raf. (non Kit.) Ulmus americana Planch. Ulmus americana L. f. alba (Aiton) Fern. Ulmus americana L. f. americana Ulmus americana L. f. ascendens Slavin Ulmus americana L. f. columnaris Rehd. Ulmus americana L. f. intercedens Fern. Ulmus americana L. f. laevior Fern. Ulmus americana L. f. pendula (Aiton) Fern. Ulmus americana L. f. viridis Seym. Ulmus americana L. var. alba Aiton Ulmus americana L. var. americana Ulmus americana L. var. aspera Chapm. Ulmus americana L. var. aurea Temple Ulmus americana L. var. bartramii Planch. Ulmus americana L. var. floridana (Chapm.) Little Ulmus americana L. var. glabra Planch. Ulmus americana L. var. pendula Aiton Ulmus americana L. var. scabra Spach Ulmus dentata Raf. Ulmus floridana Chapm. Ulmus mollifolia Marshall Ulmus obovata Raf. Ulmus pendula Willd. Ulmus pubescens Walter

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 −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, which grew beside the banks of the Sauble River 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; when it was felled in 1968, a tree-ring count established that it had germinated in 1701.

For over 80 years, U. americana has been identified as a tetraploid, i.e. having double the usual number of chromosomes, making it unique within the genus. However, a study published in 2011 by the Agricultural Research Service of the USDA revealed that about 20% of wild American elms are diploid, and may even constitute another species. Moreover, several triploid trees known only in cultivation, such as 'Jefferson', are possessed of a high degree of resistance to Dutch elm disease which ravaged American elms in the 20th century. This suggests that the diploid parent trees, which have markedly smaller cells than the tetraploid, may too be highly resistant to the disease.

Ulmus americana was first described and named by Carl Linnaeus in his Species Plantarum, published in 1753. No subspecies or varieties are currently recognized within the species.

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Wikipedia