The Toronto Valley rules are based on the idea that as a species grows over generations, it develops characteristics specific to its location, so that a tree grown from a seed collected in Toronto is more likely to flower when native pollinators are active than a seed of the same species grown at a lower latitude.
Foresters say there’s another good reason to try to preserve as many native trees as possible: For some indigenous peoples and indigenous peoples with deep ties to certain tree species, phasing them out could be the latest in a long history of cultural and physical land dispossession.
For example, in the Pacific Northwest, Western Red Cedar (written as one word because it is not a true cedar) is central to Native American cultural practices for many local tribes, with some groups calling themselves the “Cedar People” and using the logs for canoes, basketry and medicine.
But the tree no longer thrives in many areas of Portland, Oregon, due to drying soils, said Jen Cairo, the city’s urban forestry manager. The city has faced deadly heat waves and drought in recent years. As a result, Portland has recommended that the species be planted only in optimal conditions on its list of approved street trees. “We’re not going to eradicate the tree, but we are being careful about where we plant it,” she said.
A similar strategy is being adopted in Sydney, where Port Jackson fig trees are struggling but a closely related Moreton Bay fig is thriving. Urban forestry manager Karen Sweeney said the city was considering irrigated parkland as habitat for native species dying elsewhere in the city. “We often say we’re happy to do it where we can find a place,” she said.
When introducing new tree species to supplement urban canopies, we need to ensure that the newcomers do not spread invasively, dominating new habitats and damaging native species.
Examples of what to avoid abound. Norway maples, native to Europe and Western Asia, have spread beyond the boundaries of North American cities, casting excessive shade and crowding out understory plants. They are one of several invasive species displacing native species in Toronto’s canyons. Black locust, native to China, deposits chemicals in the soil, damaging nearby plants and creating dense overgrowth that crowds out native species. It’s illegal to plant black locust in parts of the U.S., and in Indiana residents are urged to pull them out if they see them. Highly flammable eucalyptus, native to Australia, has taken root around the world and also increases the risk of wildfires.
Urban tree experts don’t believe invasive species will cause much disruption to native wildlife. Done right, adding some diversity to a city dominated by one type of tree could help mitigate problems caused by waves of pests and disease. A mix of trees could act as a buffer against infections among the same trees. Esperon Rodrigues, an ecologist at the University of Western Sydney, says it’s possible that new plant species could replace plants used by animals that rely on one type of plant for survival, but such cases are the exception rather than the rule.
