Description and habitat
White Elm or American Elm (Ulmus americana) is a large deciduous tree in the Walnut Family (Juglandaceae), reaching up to 40 m tall at maturity, forming a broad, vase-shaped crown with hanging branches. It is one of Ontario’s three native elm species – the others being Slippery / Red Elm (Ulmus rubra), and the much rarer Rock Elm (Ulmus thomasii). All elms in Ontario have alternate, simple leaves. Along Hackberry (Celtis occidentalis), elms are part of the nettle-lineage (the Urticales or Urticalean rosids), a large group of over 2500 species worldwide. As the name indicates, elms are related to nettles (Urticaceae)
The leaves of American Elm are 7-20 cm long, deep green, with doubly-toothed leaf margins. They are asymmetrical, with one side typically slightly larger than the other, especially at the base. Autumn foliage is yellow or rusty brown, giving it a more muted, subtle display compared to many other hardwood species. White Elm bark is comprised of grey, comprised of flat-topped ridges divided by furrows. Flowers appear in spring, sometimes as early as late March, though more typically in early April. The fruit of American Elm, which consists of a 1-1.5 cm winged seed (samara). The seeds ripen in late May or early June. Elm seeds are oval to round, with the wing surrounding the seed.
Across North America, White Elm has an extremely large distribution, from northern Ontario to Florida west to Texas and North Dakota, though the largest populations occur in swampy lowland regions with large rivers. In Ontario, White Elm most abundant is found in swamps and on floodplains, though it is adaptable tree in many types of ecosystems. In swamps, it most often grows with Silver Maple (Acer saccharinum), Red Maple (Acer rubrum), Green Ash (Fraxinus pennsylvanica), Black Ash (Fraxinus nigra), and Eastern Cottonwood (Populus deltoides). On floodplains, it can grow on riverbanks with Sycamore (Platanus occidentalis) and Cottonwood. Slightly further from watercourses, it can grow in seasonally flooded bottomlands with Green Ash, Slippery Elm, Black Walnut (Juglans nigra), Bur Oak (Quercus macrocarpa), and Hackberry. It can also be found on mesophytic floodplains with a wide variety of species, such as Black Maple (Acer nigrum), Blue Ash (Fraxinus quadrangulata), Bitternut Hickory (Carya cordiformis), Shagbark Hickory (Carya ovata), Black Walnut, Hackberry, Slippery Elm, Green Ash, White Ash (Fraxinus americana). The understories of elm forests vary substantially depending on ecosystem but typically include mesophytic plant species adapted to moist or wet conditions.
Wildlife importance
Elms are the preferred nesting habitat for Baltimore Orioles (Icterus galbula). This preference is largely generated by the birds preference for hanging branches, where it can easily weave a nest into a location away from large branches easily accessible by predators. The seeds provide an early-season food source for small mammals and seed-eating birds. Foliage is a preferred food source for many species of butterflies and moths (lepidoptera), including Mourning Cloak Butterfly (Nymphalis antiopa), Question Mark Butterfly (Polygonia interrogationis), Eastern Comma Butterfly (Polygonia comma), Red-spotted Purple / White Admiral (Limenitis arthemis astyanax), Painted Lady (Vanessa cardui). Moth species feeding on White Elm including Polyphemus Moth (Antheraea polyphemus), Cecropia Moth (Hyalophora cecropia), Columbian Silkmoth (Hyalophora columbia), Elm Casebearer (Coleophora ulmifoliella), Spring Cankerworm (Paleacrita vernata), Fall Cankerworm (Alsophila pometaria), Forest Tent Caterpillar (Malacosoma disstria), Saddled Prominent (Heterocampa guttivitta), and Nerice bidentata.
Dutch Elm Disease
Dutch Elm Disease was introduced to North America in Ohio (1930), and New York (1933). Dutch Elm Disease reached Ontario in 1946. The disease is caused by a fungus (Ophiostoma ulmi and Ophiostoma novo-ulmi) most likely from Japan. It is called “Dutch Elm Disease” because it was first observed in the Netherlands in Europe before being introduced to North America. Dutch Elm Disease is vectored by 2 elm bark beetles (the native Hylurgopinus rufipes and the introduced Scolytus multistriatus), which facilitates the spread of the fungus. Infected trees have >99% mortality rate, usually with progressive branch dieback occurring within a few weeks to a few years. A small number of trees either escape infestation or possess inherent resistance to the tree. Most elm trees reach reproductive maturity before being killed by Dutch Elm Disease, thus seeds continuously being produced. White Elm maintains a large presence on the landscape, though large, mature individuals remain uncommon, though they are more abundant in some areas. Disease-resistant cultivars have been selected for horticultural purposes, leading to the gradual reintroduction of White Elm trees in urban environments throughout Ontario.
Fun facts
- If unimpacted by DED, healthy White Elms were known to be very long-lived trees. Old specimens could reach ages of 200 to 300 years or more, growing into truly massive trees with impressive trunk diameters.
- White Elm wood is tough and strong, highly resistant to splitting, especially when wet. It is highly valuable for uses requiring durability and resistance to impact.
- White Elms can tolerate temperatures down to -45°C. These trees are also very drought tolerant, in part due to the large spreading root systems, being cultivated in the Canadian Prairie Provinces such as Manitoba, Saskatchewan, and Alberta.
- Calgary and Edmonton have the largest urban elm populations in North America, where they comprise a substantial proportion of the urban tree canopy.
- After DED infection, while seemingly dead, White Elm trees often eventually resprout from the base.
Written by Matthew Palarchio, HBSc in Environmental Sciences, Western University.
All photos by Matthew Palarchio
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