Native vs invasive trees in the Glebe
by Jennifer Humphries
Does the Glebe have an invasive tree problem? Do we have a concerning overabundance of non-native trees in our midst?
I reached out to Owen Clarkin, whose tree expertise is valued by Ottawans including Friends of the Farm for whom he leads Arboretum walks, and John Stewart, principal consulting arborist with Urban Tree Management Group and president-elect of the International Society of Arborists, Ontario.
What is an invasive tree? As with plants such as loosestrife and dog-strangling vine, invasive trees are those that are non-native to our region and, more critically, have aggressive tendencies; they beat out competitors and if unchecked can kill off local native trees and vegetation.
“Native trees are those that grow naturally in a particular locale. Less obviously, they can include trees that grow typically in adjacent territory or, in the case of Eastern Ontario, in the eastern US. Political borders are not nature’s borders,” Owen Clarkin says. “With climate change, it’s possible to grow trees here that are more typical in the southeastern forest, possibly helping ensure the survival of species at risk.”
Meeting with Clarkin at a Bank Street coffee shop, he immediately pointed out the prospector elm out front. It is not a native tree – it’s an elm cultivar from Japan – but there is reason to plant some of this species. They are hardy, do well in hardscape environments and are resistant to Dutch elm disease. Clarkin cautions that it’s one thing to plant a few, but when you go from zero to 1,000 of any species, you risk unintended consequences.
In the Glebe, as in Ottawa generally, city street planting has focused on what Clarkin calls “a cocktail of the same species” creating a homogenous treescape across the urban area. A typical example of large-scale use is the Japanese tree lilac. It is not native and many don’t find it aesthetically appealing and don’t appreciate its fragrance, but it is used due to its smaller size as it can fit where an oak or maple cannot. Other examples of regularly planted species in Ottawa are honey locust and blue spruce. We have gotten away from the monoculture approach whereby an entire street is planted with a particular species, such as American elm, ash, linden that can leave us vulnerable to potential massive loss from species-specific diseases or pests. But we continue to see a limited number of species being planted, which is not ideal either.
Diversity is essential to ensure a strong tree population and a healthy sustainable canopy. We can’t know what may impact our street trees and forests in the future, so we need to hedge our bets. Clarkin urges us to experiment with different species, not necessarily exotic, but those native trees and cultivars that are less frequently seen.
Are there any species that should be avoided? Norway maple was planted here in earlier decades due to its longevity and great shade but it is hard to deny that it is invasive, given its tendency to supplant other species. On the other hand, Manitoba maple, which many people dislike, is likely native to Ontario and is a beautiful, hardy tree, albeit “messy.” If planting a maple is your interest Clarkin recommends another native maple, the black maple.
John Stewart, an arborist for 25 years who has worked with city forestry in Waterloo and Brampton, says, “It’s great to plant native trees, but it can be difficult in a harsh urban environment. Consider cultivars that have properties that make them urban-tolerant. For example, the autumn blaze maple is a hybrid of red and silver maples that is more tolerant than sugar maple.”
I asked Stewart whether banning certain trees is a good idea. “In my opinion, banning is counterproductive,” he says. “In some cases temporary bans make sense. For instance, there is no point in planting ash trees unless you are prepared to treat them constantly to keep emerald ash borer (EAB) at bay. But in future, when the food source is gone, the EAB may decline sufficiently that we can plant ash again.” The ash debacle underscores the importance of tree diversity.
Stewart also urges that bans or regulation, if applied, be the purview of local conservation authorities who will take into account localized realities. Cities need to take an inventory and look at matters such as canopy size – some trees yield big canopies, others may be very small – and age. We need not only species differentiation but age differentiation as well, if we are to ensure a strong canopy for the next 30 or 40 years.
Don’t miss: Fall Tree Festival, September 23-24, 10 a.m. to 4:30 p.m., Brewer Pond in Brewer Park.
Jennifer Humphries is co-chair of the Glebe Community Association’s Environment Committee, which, in cooperation with Ecology Ottawa, is promoting the planting of 150 trees in the neighbourhood in 2017. Write to Jennifer at environment@glebeca.ca.
