Urban forests under threat from climate change

Picture this: a quaint suburban street with row upon row of mature ash trees, their canopies growing and weaving together to form a tunnel of green, through which you can just make out a row of houses being shaded from the hot afternoon sun.

Before 2002, much of Toronto fit this description perfectly.

Today, thanks to the Emerald Ash Borer, all but 8500 of Toronto’s 850,000+ Ash trees have disappeared[1]. The Ash Borer is now part of a growing list of insects and diseases threatening Canada’s urban forests, a list which includes Dutch Elm Disease, whichlike the Emerald Ash Borer cannot be stopped once it takes hold of an American Elm tree, and the Mountain Pine Beetle, which feeds ruthlessly on Lodgepole Pine.

Like many invasive pests, the Ash Borer’s arrival is a consequence of globalization. Endemic to Asia, the bugs likely made their way to North America via one of the millions of container ships, airplanes and trucks that connect the global economy. However, a changing climateparticularly the move to hotter, longer summerscan have a noticeable impact on how quickly the bugs reproduce and spread. In Toronto, for example, recent hot summers have allowed Borer populations to complete two breeding cycles in one summer, doubling the normal rate of infestation[2]. Furthermore, these same warm and dry conditions are stressing the trees, leaving them with less energy to deal with infection.

Meanwhile, in Victoria, British Columbia, the self-proclaimed “City of Gardens,” efforts are now underway to find new tree species to fill the gaps left by aging and ailing Ash and Cypress trees. For decades, these two species thrived in the relatively moderate climate of Victoria. Now, thanks in part to climate change, hotter temperatures and longer dry spells mean that Ash and Cypress are no longer a good match for the city. The city of Victoria estimates that over 20% of its urban forest cover will disappear over the next 20 years[3]. Cherry trees can better handle the heat but are far less likely to survive to maturity. And so the search continues for a replacement.

Victoria isn’t the only Canadian city looking for new climate-resilient trees to plant. The Emerald Ash Borer has made its way east, all the way to Winnipeg, where it is expected to wipe out a good portion of the city’s 350,000 Ash trees[4]. This is especially unfortunate timing for Winnipeg, as Dutch Elm Disease is currently killing off American Elm trees in record numbers across the city[5]. Like Victoria, Winnipeg’s summers are expected to continue to get hotter and drier in the future. But unlike Victoria, winters in Winnipeg will still have bouts of extreme cold, the kind of weather for which the city is infamous. This creates a difficult problem for the city’s urban forest managers because it’s easy enough to find a tree that can handle the heat and dryness but very difficult to select one that can also handle the cold.

The threat climate change poses to our urban forests is ironic, given that urban forests are one of the best tools available for mitigating the risk climate change poses to human health. On top of offering privacy and making our neighbourhoods more beautiful, trees provide shade from the summer heat and protection from high winds; they pull carbon from the air; they release moisture that cools the local climate; they stabilize the soil and prevent erosion; they help with water absorption and retention; and they can remove pollution from the air[6].

Back in Toronto, city officials are planning for the future[7]. Much more money is going to be needed to protect the city’s 10 million remaining trees. But given how valuable trees are as ecological services, these added expenses will likely be well worth it. By some estimates, Toronto’s urban canopy is worth over $7 billion and provides roughly $80 million in environmental savings each year[8]. What’s more, investing in our urban forests helps reduce the future costs associated with climate change, keeping our cities affordable, clean and pleasing to the eye and mind for the next generation.

Ryan Smith


Prairie Climate Centre

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[1] http://www.cbc.ca/news/canada/toronto/toronto-s-ash-trees-face-extinction-1.1001010

[2] https://toronto.ctvnews.ca/hot-dry-weather-accelerated-toronto-s-emerald-ash-borer-tree-crisis-1.3044828

[3] http://www.victoria.ca/assets/City~Hall/Media~Releases/2012/120817_MR.pdf

[4] http://www.cbc.ca/news/canada/manitoba/winnipeg-emerald-ash-borer-1.4438061

[5] http://www.cbc.ca/news/canada/manitoba/dutch-elm-trees-city-of-winnipeg-1.4300963

[6] https://glslcities.org/wp-content/uploads/2015/09/Climate_Change_Adaptation_Options_for_Torontos_Urban_Forest_2007.pdf

[7] https://www.toronto.ca/city-government/accountability-operations-customer-service/long-term-vision-plans-and-strategies/parks-forestry-recreation/strategic-forest-management-plan/

[8] http://business.financialpost.com/news/economy/torontos-urban-forest-worth-7-billion-or-700-a-tree