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For 20 years, Toronto has been building biodiversity and tree cover by slowly replacing invaders such as Norway Maple and European Buckthorn with native species.
When I was a kid growing up in downtown Toronto, I spent a lot of time in the Rosedale Valley Ravine, which was close to where I lived — riding my bike along the paths with my dad, climbing trees, clambering up their slopes, building forts out of fallen branches, glimpsing cool new birds, and looking for salamanders under rocks.
In some ways, Toronto’s ravines are the same ones I knew in the 1970s. Their trees still help clean the air and suck CO2 out of it. They provide habitat for countless insects, birds, and small mammals. They serve as havens where people can walk and ride bikes and generally escape the busyness and noise of urban life. And they play an important role in stormwater management by allowing runoff to drain away from city streets into local waterways like the Don River and, ultimately, Lake Ontario.
But there have been important changes in the ravines since I was a kid. They get far more visitors than they used to, so dirt paths are now paved over to accommodate the increased foot and bicycle traffic. But the biggest transformations in the ravines have occurred in their vegetation. Many of the ravines’ native trees and shrubs have been crowded out by invasive species such as European buckthorn and Norway maple, which is particularly nasty because it is poisonous to insects and other plants. If there are no insects, there are no birds, and if no other plants can grow nearby, the greenery on the forest floor withers away. Thus, if Norway maples and other invasives proliferate, they will ultimately take over and slowly degrade the important ecological services provided by the ravines and the diversity of the wildlife that lives there. The forests will become all but barren of the life that once teemed there.
If Norway maples and other invasives proliferate, they will ultimately take over and slowly degrade the important ecological services provided by the ravines and the diversity of the wildlife that lives there. The forests will become all but barren of the life that once teemed there.
This alarming prospect is why, in 2005, in collaboration with a nonprofit organization called Forests Ontario, the City of Toronto launched the Tree Seed Diversity Program. The initiative is unique in Canada. As far as Kristen Vincent — the City of Toronto’s supervisor of Natural Resource Management — knows, no other municipality works with a nonprofit on this level to regenerate urban forests. In this program, private seed collectors hired by the city gathered acorns from High Park and the Glen Stewart Ravine, and a couple of nurseries north of the city nurtured the acorns into seedlings in controlled growing conditions. The idea was that as invasive tree species died or were selectively removed by arborists, they could be replaced with healthy young oaks that, over time, would help restore the compromised ecosystem balance. That replanting started in 2009.
Over the next 15 years, spurred in part by the adoption by the city of several urban planning strategies that recognized the public value of green spaces, the program expanded to 10 different tree species –– including black, silver, and sugar maple, bitternut hickory, and red, white, bur, and black oak –– and eight parks and ravines across the city. Each year, depending on seed availability, city workers planted between 500 and 5000 seedlings. Since it’s common to lose seedlings to frost, pests, and predation, the workers overplanted to get the final success rates they wanted.
But modern tree planting is no longer a matter of simply growing seedlings and planting them where the seeds were gathered. In the 1960s, one could safely assume that the climate the young seedlings grew up in would be more or less the same as the climate their parents grew up in. But climate change has upended all that. These days, if a seedling is planted in Toronto today, it will mature in a warmer climate than existed in the past.
In 2013, the City of Toronto approved a forest management strategy that, among other goals, aims to create a tree canopy that covers 40 percent of the city and to increase biodiversity. To that end, in 2016, the City of Toronto began working with the Forest Gene Conservation Association to identify different growing zones –– roughly half in other parts of Ontario south and west of Toronto, and the other half as far south as Ohio –– whose current climate is similar to what Toronto’s is predicted to be in 70 years. Seeds collected in these areas have genes that are already adapted to warmer conditions, so they’ll have a better chance of growing to maturity when they’re replanted in Toronto.
In 2013, the City of Toronto approved a forest management strategy that, among other goals, aims to create a tree canopy that covers 40 percent of the city and to increase biodiversity.
By 2020, the Tree Seed Diversity Program was planting far more trees than it did in the early days. But problems began to surface after the closures of the Ontario nurseries that housed most of the city’s seed banks and supplied its seedlings. Orders for new seedlings were not being fulfilled, partially fulfilled, or incorrectly fulfilled.
“We were having trouble getting the species and the quantities we wanted,” recalls Vincent. “And some nurseries were growing seedlings by taking cuttings from existing plants. That provides less genetic diversity than if you grow the plant directly from seed.”
As a result of these issues, Toronto rethought its planting program and, in November 2021, inked a 10-year contract with Forests Ontario. Under the new deal, Forests Ontario is responsible for collecting the desired seeds and working with Ontario nurseries to provide a reliable supply of the young native trees in the quantities Toronto wants. The new program focuses not only on trees but shrubs, since certain invasive varieties of the latter, such as Japanese Knotweed, are also proliferating in the ravines and choking out native shrubs. (It also is now known as the Seed Diversity Program, dropping “Tree” from its name, since the program now includes more than just tree seeds.)
The idea is that as invasive tree species died or were selectively removed by arborists, they could be replaced with healthy young native species that, over time, would help restore the compromised ecosystem balance.
“It's important to kind of place your bets on having a couple of different tree species [when replanting] because one thing can come through and wipe out your whole canopy,” says Jess Kaknevicius, Forest Ontario’s executive director.
“We do have plantings that don’t succeed and see the negative effects of the urban environment on our restoration,” says Vincent. “But the positives definitely outweigh the negatives.”
