Elm trees (Ulmus) are deciduous trees known for their distinctive vase-like shape and attractive foliage. They were once a common and iconic sight, gracing urban landscapes and forests.
Identifying Elm Trees
Elm trees often exhibit a vase-shaped or spreading canopy, with branches that gracefully arch outwards. Their leaves are typically oval or egg-shaped with a pointed tip and distinctive doubly serrated edges. A notable feature is their asymmetrical base, where one side of the leaf blade is slightly broader than the other. Leaves range from light to dark green, often turning yellow in autumn.
The bark of mature elm trees is generally dark gray-brown and deeply furrowed, displaying a rough texture with intersecting ridges. Younger trees may have smoother bark that becomes more fissured with age. Elm trees also produce unique seeds called samaras, which are small, flat, papery, circular or oval discs. These samaras mature in the spring and are dispersed by wind.
Elms in History and Nature
Elm trees have long held significant roles in natural ecosystems and human societies. Ecologically, they provide habitat and food sources for various wildlife, contributing to biodiversity and forest health. Their large canopies offer ample shade, making them valuable components of urban and natural landscapes.
Historically, elms were favored in urban planning and landscaping for their aesthetic appeal and extensive shade. Many cities once featured elm-lined streets, creating picturesque, arching canopies. Beyond their ornamental value, elm wood was prized for its strength, durability, and resistance to splitting. Its interlocking grain made it suitable for diverse applications, including furniture, wagon wheels, and boat keels.
The Impact of Dutch Elm Disease
The primary reason for the vanishing of elm trees is Dutch Elm Disease (DED). This fungal disease is caused by two closely related fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi. First identified in the Netherlands in the 1920s, DED has decimated elm populations worldwide. It is primarily spread by elm bark beetles.
These beetles breed in the bark of weakened or dying elm trees, picking up fungal spores as new adults emerge. When infected beetles feed on healthy elms, they introduce spores into the tree’s vascular system. The fungus rapidly multiplies, clogging the xylem and impeding water and nutrient flow. Symptoms begin with wilting and yellowing leaves on branches, often called “flagging,” which then turn brown and shrivel but remain on the tree. The disease progresses quickly, and an infected tree can die within weeks to a few seasons.
DED has had a profound impact, especially on American and English elms. Over 75% of North America’s elms were lost by 1989 after the disease’s introduction in the 1930s. In the United Kingdom, a virulent strain in the late 1960s led to the death of over 25 million trees. The widespread planting of elms as monocultures in urban areas inadvertently facilitated DED’s rapid spread, highlighting the risks of limited species diversity.
Protecting and Restoring Elms
Efforts are underway to combat Dutch Elm Disease and restore elm populations. A primary strategy involves developing and planting DED-resistant elm varieties. Cultivars like ‘Princeton’ and ‘Valley Forge’ American elms show high resistance and are increasingly utilized. Many hybrid Asian elms also exhibit strong resistance.
Research continues into biological controls, such as the biopesticide Dutch Trig®. When injected into healthy elms, this agent stimulates the tree’s natural defense mechanisms, protecting it from DED. Management strategies also include sanitation pruning of infected branches and destroying diseased wood to eliminate beetle breeding sites.
Preventative fungicide injections can protect high-value elm trees, though these treatments are expensive and require repeated application. Genetic diversity and selective breeding play a role in long-term resilience, with ongoing efforts to identify and propagate resistant elm genotypes. These combined approaches offer hope for the preservation and re-establishment of these trees.