Toronto’s ravine rules are based on the idea that a species will develop site-specific traits as it grows over many generations. As a result, trees grown from seeds collected in Toronto may be more likely to flower when native pollinators are active than seeds of the same species grown at a lower latitude.
Foresters say there is another valid argument for trying to conserve as many native trees as possible. For some First Nations and Indigenous peoples with deep ties to particular varieties, their phasing out could add to the long history of cultural and physical dispossession.
In the Pacific Northwest, for example, western red cedar (spelled as a single word because it is not a true cedar) is central to the cultural practices of the Native Americans of many local tribes. Some groups refer to themselves as “thecedar people”, using the records for canoesbasketry and medicine.
But dry soils mean the tree no longer thrives in many parts of Portland, Oregon, said Jenn Cairo, the city’s urban forestry manager. The city has faced deadly heat domes and drier conditions in recent years. As a result, Portland recommends planting the species only under optimal conditions on its list of approved street trees. “We’re not eliminating them,” she said, “but we’re being careful where we plant them.”
A similar tactic is being used in Sydney, where the Port Jackson fig is struggling, but a close relative, the Moreton Bay fig, is thriving. Urban Forestry Chief Karen Sweeney said the city is considering irrigated parks as possible homes for native species that are dying in other parts of the city. “We often say we’re happy to do it wherever we can find a location,” she said.
When introducing new tree species to complement the urban canopy, they must ensure that the newcomers do not spread invasively, dominating their new habitats and causing damage to native species.
There are many examples of what to avoid. The Norway maple, native to Europe and Western Asia, has escaped the limits of North American citiescreating excessive shade and displacing understory plants: they are one of the invasive species that expel native ones in the ravines from Toronto. Tree of heaven, native to China, deposits chemicals in the ground that damage nearby plants, allowing it to establish dense thickets and drive out native species; It is illegal to plant in some parts of the US, including Indiana, where residents are urged to lift it wherever they see it. Eucalyptus, native to Australia and highly flammable, has taken root around the world, bringing with it an increased danger of forest fires.
Urban tree experts do not expect the introduced species to cause major disturbances to native wildlife. Well done, adding some variety to cities dominated by a type of tree could reduce problems caused by waves of pests or diseases. A mosaic of species could create a buffer against tree-to-tree infection between the same species. While it is possible for new plant species to displace those used by animals that depend on one type of plant to survive, such cases are the exception, said Quieron-Rodriguez, an ecologist at Western Sydney University.