What Grows Next?

Dutch elm disease arrived in North America nearly a century ago. We knew what it could do. We watched it dismantle the great canopies of Montreal, Toronto and Winnipeg. And still, we planted our streets and parks with a monoculture of more than 91,000 American Elm. Tolerant of pollution, salt and clay, we found the species too perfect, too well-suited and too beautiful not to plant — believing, perhaps, that distance or vigilance might spare us. They cannot. And now, with confirmed cases of Dutch elm disease inside city limits, we’re confronting a long-deferred truth: what takes decades to grow can be undone in seasons.

Dutch elm disease is a fungus. Spread through beetles, contaminated tools, and intertwined roots, the fungus blocks water flow, killing leaves, then branches, then limbs, and finally the base, potentially wiping out trees as old as the city itself in just two years. Imagine neighbourhoods like Strathcona, Wîhkwêntôwin, Westmount and Highlands without their elm-lined boulevards. That’s why, in August 2024, when Dutch elm disease was discovered in Killarney and along the Yellowhead Corridor, Edmontonians were understandably alarmed.

But even as we respond to Dutch elm disease, an arguably more severe threat, the emerald ash borer — an invasive and destructive beetle — continues to advance, threatening just as many ash trees, but at a much faster rate. The former is getting more attention because of its threat to civic symbolism, but the latter could completely overwhelm our budget, resources and capacity to replant. Combined, the two diseases could wipe out close to 40 per cent of the City of Edmonton’s public tree inventory, in addition to countless trees growing on private property.

Edmonton’s urban forest is vulnerable by design. That vulnerability has a shape: broad boulevards lined with genetically identical trees, canopies that feel quintessentially Edmontonian. So, it’s understandable why the instinct is to fight these diseases — and we are. However, even if we’re successful in stopping our most recent outbreaks, we’re still left with a vulnerable urban forest. Monocultures, even beautiful ones, are fragile, offering little buffer against time.

Trees are living infrastructure, storing carbon in their trunks and roots, moderating temperatures, slowing stormwater. The list goes on. As the climate becomes more volatile, more extreme, we risk losing mature trees just when they’re most needed — and hardest to replace. In other words, if we focus on maintaining a monoculture, we risk overlooking the deeper imperative: to cultivate a resilient urban forest. That means encouraging tree diversity — not single-species rows planted all at once, but a polyculture of trees planted over decades. It means a shared responsibility for citizens and the municipality to preserve trees while growing denser, more climate-adaptive communities. And it means recognizing that this won’t be the last crisis.

But this isn’t just about what we might lose. It’s about what we still have time to grow: an urban forest for the next 100 years — messier, more complex, more alive and more valuable to future generations. The monoculture will fail; that much is certain. The questions we need to ask ourselves are: how quickly, and what will replace it? Do we have the dedication, resources and budget to transform the makeup of our urban forest for the next century? Or will we trap ourselves in the perpetual maintenance of monoculture?