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The re-emergence of an old tree killer By Joe Boggs and Francesca Peduto Hand, PhD T he rise and fall of American elms (Ulmus americana) in urban forests is well-known to arborists. The story is often used as a cautionary tale illustrating the elevated risk posed by plant monocultures to support devastating pest and disease outbreaks; in this case, the over-use of a single tree species in urban plantings. Conversely, the American elm story is also used to demonstrate the value of plant diversity. 6 May 2014 Dense, dark green foliage coupled with fast growth and a stately vase-shaped silhouette were prized characteristics that inspired the journey of this North American native from forests to city streets. The rise of American elms to become ‘Canada’s street tree’ was further enhanced by the ability of this species to cope with a wide array of human-induced urban abuses. The pinnacle of American elm’s arching trajectory as an urban tree occurred when Dutch Elm Disease (DED) was discovered in North America. All images courtesy Joe Boggs Elm yellows: