American elm (Ulmus americana)

Description

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,naturally 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.The American elm is a deciduous hermaphroditic tree which,before the introduction of Dutch elm disease,commonly grew to > 30 m (100 ft) tall with a trunk > 1.2 m (4 ft) d.b.h 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,and emerge in early spring before the leaves.The fruit is a flat samara 2 cm long by 1.5 cm broad,with a circular papery wing surrounding the single 4?5 mm seed.As in the closely related European White Elm Ulmus 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.Ploidy (2n = 56,or more rarely,2n = 28).