Tropical storm Nicole didn’t hit with the oomph of Hurricane Fiona, but its remnants sure brought enough wind through Halifax over the weekend and into Monday to stir up memories.
Winds gusted to 65 km/h on Monday morning at the Halifax Shearwater airport, according to Environment Canada. On Saturday at noon, those gusts reached 73 km/h. (“I’m closing my booth until this goes away,” one Macdonald Bridge toll collector quipped early Monday, as wind whipped across The Narrows.)
While streets throughout the HRM have largely been cleared of downed trees and fallen branches since Fiona, one question that hasn’t been swept away is what happens to our region’s tree canopy the next time a hurricane hits. Halifax has a plan to add more than 26,000 trees on HRM lands in the next decade—and every decade thereafter. The region’s urban forest master plan, last updated in 2013, envisions expanding tree coverage in virtually every community but Dartmouth, Bedford and Sackville.
But what does that mean in a region prone to hurricanes? Just how feasible are those plans—and can the HRM future-proof against further storm damage? Should a storm like Fiona change how we think about a region’s tree canopy and building resilience?
In search of answers, The Coast spoke with Peter Duinker, a professor emeritus at Dalhousie University, the lead researcher on the HRM’s 2013 urban forest plan and head of the Halifax Tree Project.
This interview has been edited and condensed for length and clarity.
The Coast: Which of Nova Scotia’s big three hurricanes of the last 20 years—Juan, Dorian or Fiona—was most destructive to Halifax’s tree canopy?
Peter Duinker: I would say Juan—and that may be partly because Juan came first, after no hurricanes had hit Halifax for quite a while. Juan may have actually taken out the worst of the structural problems in the city canopy.
But the three hurricanes were quite different. Juan came straight at us from the south and smacked the city right in the face: It put down about three quarters of the trees in Point Pleasant Park. The estimates of the trees downed around the core of the city was between five and 10%. Hurricane Dorian kind of came from Yarmouth straight up the middle of the province and did lots of damage across a larger area. Then Fiona hit us somewhere along the Eastern Shore, and our winds in Halifax were from the north.
And when I looked at all the trees that were down right after Fiona, most of the trees that came down [fell] from the north to the south. I think a lot of the trees in Fiona's path had not had a strong north wind in a long time, and so that strong north wind really tested the trees that didn't get taken down by Juan and Dorian.
Halifax wants to plant more trees and increase its tree canopy across the HRM. And given global heating, we are virtually assured to see more hurricanes—which, in turn, will down more trees. How do we plan a tree canopy that can survive these future storms?
I'll broaden the conversation here by talking about resilience of the urban forest. We've been doing a lot of thinking about, “What are the strong things that we need to do to make our urban forests more resilient in general,” but also in the face of wind. So, you know, I can probably enumerate three or four major things to do.
If you look up into a Norway maple, it may have three stems starting five, six metres up from the ground and sticking out into the sky. And an elm tree might have eight to 10 of those, and maybe the other major species in our streets, which is the European linden, might have two. But arborists don't like that—they want a single stem, because a single stem is much more stable up in the sky. When the wind blows, there's no big crotches to let go and half the tree goes down.
It used to bother me that they had that point of view, until now. And right now I'm thinking, “Yep, that's exactly what we need to do when those little trees go in the ground.” The city needs to mobilize a structural pruning program, so that for every one of those trees, the training of its crown starts early… to make sure that crown is going to grow in the absolute strongest way possible. So I'm leaning now towards the notion that we can build wind resistance by really careful structural pruning.
There’s also what we call cyclical pruning, instead of only responding to 311 calls when a citizen says, “Hey, my tree has a broken branch, can you come in and make it safe?” Or “My tree died, can you take it down?” Most cities in North America would like to put their trees on a seven- or eight-year cycle, where every tree gets visited with a bucket truck and an arborist in that bucket. And they're lightening the crown; they’re taking out the dead wood. If you make the crown more porous, the wind can come through the crown, not press against it, and thereby [not] start to push the tree down.
The other thing that does, of course, is to lighten the weight—and the less weight you have high up in the tree, the greater the chance that tree can stand when it gets buffeted by strong winds.
You were the lead researcher in the HRM’s urban forest master plan in 2013. That report mentions the aftermath of Hurricane Juan as an “opportunity lost,” because in the cleanup, the region could’ve taken stock of the ages and health of trees that were downed to get a better sense of how to build back stronger. Instead, it didn’t. Has that changed with subsequent hurricanes?
No, it hasn't. And I've long lamented my own lack of preparedness, because I really wish we could find a way that when trees come down in a storm, that we would immediately have a basic data-taking protocol ready.
What I would like to see happen is that when trees are down, and they're cut off, that our research team would be notified immediately of those trees having come down where they are, and probably what time they were. But if they couldn't determine what time they were downed, then we would determine that and count the rings on the stump before the stump is removed, because that's where you get the proper age count on the rings on the stump. But we've never done that.
The urban forest team at HRM is still pretty small. And when they get a calamity, the last thing they're thinking about is records. They’re thinking about helping people out by getting the trees off the power lines and out of the way for the streets. So one of the things I might do in my proper retirement now is to help the city build those kinds of protocols to make sure we don’t miss any more opportunities like this.
Nova Scotia isn’t the only place in North America prone to hurricanes. Researchers in Florida have looked into wind-resistant species less likely to lose limbs in a storm. Should the HRM be prioritizing some tree species over others in future planning?
I'd say in all of the downtowns of Halifax and Dartmouth and… in the Centre Plan area, about a third of it is Norway maple. And Norway maple is a really bad choice in a hurricane, for a variety of reasons.
A key reason is that the wood is not strong enough. When we get storms that are maybe a little less powerful than a hurricane, I would say that 90% of the tree damage that the city has to clean up is Norway maple. And so that's a clear sign that Norway maple is not good, structurally, when the trees get big.
Norway maple also has a bad rooting system that's hard to correct when you get it into the ground. But species like red oak, for example, have much more robust root systems. And it’s my impression that of all the trees that come down in a hurricane in Halifax, the oaks are the ones that stand the best.
The other thing I would like to say is that we have spent almost no time at all, just like most cities across North America, at trying to get a better soil environment than what the engineers give us when they do road building or road recapitalization. And if you have shallow soils, well, no wonder your trees fall down—because they don't have a chance to get roots way down, if indeed that's the kind of rooting system they have. And so I've been talking with our urban forestry people at the HRM about putting fewer trees in the streetscape and spending more on each one.
That means getting the soil better—so you excavate the junk that's down there from construction; getting structural pruning done right, early; and making sure that for the first two, three years, those trees get adequate water. If you were paying attention this summer to the weather in Halifax, it was great for tourism but terrible for small trees in the streetscape. Getting the trees water lets the roots get out into the soil and start to firm up that long-term pattern of root extension and stability.
If Norway maples are a bad fit for Halifax’s weather, why did the region plant so many all those years ago?
I have some hypotheses. Norway maple is easy to grow in the nurseries. It was fashionable, just like elm was fashionable 60 to 80 years ago. And after elm ravaged so many towns and cities and populations, ash was favourite. It was fashionable. And people overplanted with ash, and then the emerald ash borer comes along and takes those trees out.
Norway maple didn’t really have any pests that would knock it out. It’s tough as nails when you plant it in the streets. And so folks thought, “Well, if Norway maple is cheap, abundant and does well, let’s do it.” But now we know that there are all kinds of issues with Norway maple—and it’s also invasive. Where you have Norway maple next to woodlands, Norway maples creep in there and start taking over the growing space.
The second thing, it's not wise to have so much of the tree canopy made up of one species—it just gives you all kinds of risk problems. And research shows that Norway maple does not support the kind of insect biodiversity that you would want in a tree, so that the tree becomes attractive for birds to hang around in it and feed. If we take our domestic maples, like sugar maple and red maple, those are far better, both from the standpoint of structural integrity and all that kind of thing. But also they support native biodiversity way better, because they are native.
In your experience, can a city be both hurricane-prone and maintain an extensive tree canopy? Can those two things coexist?
Of course, they can coexist. And they do coexist now. I mean, just because we get hurricanes doesn’t mean people want to see the city devoid of trees. Of course, there’s a risk if you get a tree up into the powerlines or over the houses and the big winds blow as they will, you know, you’ll get that kind of calamity. But it doesn’t stop us from wanting to have trees in the city. I feel nervous to say this, but potentially it means choosing species that don't grow so tall.
The other thing we need to really work on is the notion that the power lines come first and the trees come second. In my opinion, the trees come first and the power lines come second, even though we would say, “Oh no, the power lines have to be first because they're the lifeblood of maintaining household integrity.”
I think it’s high time we called upon Nova Scotia Power to be a little more robust and build a resilient powerline system in the face of hurricanes—not only expect that we're going to rebuild the urban forests to accommodate the crappy powerline infrastructure we have in Halifax.