Building a better urban forest

Trees and vegetation are crucial to preventing the buildup of urban heat islands where temperatures rise beyond that of the broader environment.

That cooling process comes from “evapotranspiration,” wherein water moves from the ground, to a plant, and from a plant’s leaves to the environment. Its release into the air helps cool that air (since the water is inevitably cooler than the air).

But during drought and increasingly dry summer, the moisture and rain available to maintain plants and vegetation inevitably becomes scarce. So how can we create cities with vegetation that produces such evapotranspiration effects when water is so rare?

And how do we create the urban forests we need, while also building denser communities—with less private land devoted to yards and greenspace?

The Current recently put those questions to UBC forestry professor Andrew Almas.

This interview was conducted several months ago, during research for our story on the future of lawns and grass in Fraser Valley neighbourhoods. You can find that story here.

Evapotranspiration is the process by which moisture on the surface of plants evaporates into the surrounding air, helping to cool it. The process helps reduce the urban heat island effect that can leave cities considerably hotter than the surrounding landscape. But as Lower Mainland cities face increasingly long summer drougths, some plants are better suited than others to helping mitigate the heat.

FVC: “You want to have that water release into the environment, but you need it to happen in a very delayed way so water that fell a month ago or two weeks ago is still being [released]. How can does that happen?”

Almas: “So the way you’re going to do that is you’re going to have larger trees, because those larger trees have a larger root system; they have access to way more soil moisture, they have access to groundwater and these things that we want to tap into.

“When we remove our large trees, we remove those services—the capacity of the greenery to cool our areas… When we’re trying to account for what’s the best thing to do in a drought, the best thing we can do is we can maintain the existing greenery that we have, especially the mature trees.”

Those large trees have to be accommodated, Almas noted.

“That’s a challenge when we think about densifying urban locations. When we think about having large trees next to houses, there’s often very little appetite of having a 100-foot Douglas fir beside your house; people don’t like the thought of it crashing in their bedroom while they’re sleeping. So how do we strike that balance is the question. Some people prefer to have those large trees and don’t see it as a huge risk. But some people see it ultimately as something that’s untenable and as a result don’t maintain the large trees. And from a municipal perspective, it’s extremely important that they maintain their large trees.”

FVC: What do cities and communities need to do with their urban green spaces to adapt to climate change and more frequent extreme heat events?

Almas: That's the million dollar question, isn't it? I think they need a strategic and nuanced approach to, to managing the various issues that arise? We make it this sort of bubble issue of ‘Changing climate’, but there are so many issues that we have to manage in different ways, as a result of the changing climate.

So what what we need is a strict strategic vision. Municipalities tend to write out these planning documents, and we see it with Vancouver where we have like climate change strategies, and we've got the the rain city plans, right. So these are different ways of dealing with different aspects of the impacts of climate change so when we get sort of a co-ordinated effort, then we can start planning our cities around these changing scenario.

So it's quite a complex thing, especially in cities, because there are always various and competing demands for how space is used and for how resources are used. So when we're talking about what cities can do, well, the first thing that they need to do is devote tax dollars to coming up with a solution. Right. And when we think about devoting tax dollars to it, okay, well, where did those tax dollars go?

Well, we can think about planning documents that, incorporate sort of engineering solutions, or in urban forestry, we would think of greening solutions—nature-based solutions. This is anything from: So if it's getting hotter in cities, what can we do to mitigate that?

Well, we can have more albedo effect, right? Not as much in terms of pavement that's reflecting the sun, we can have more greenery within that particular area. And we know that greenery reduces heat by a significant amount. So when we look at sort of the urban heat island effect, we see that in areas where there are large pockets of green, for instance, in Vancouver with Pacific spirit Park and Stanley Park, we can see that that it's significantly cooler around those green spaces than on Robson Street or something like that.

So we need to think about how can we get the best of both worlds where we have sort of a livable city, where we have industry happening, commerce happening, people moving and working in this area, but also that it's a nice, comfortable place to live. And these are sort of the questions that we wrestle with as people dealing with climate change and urban foresters, because, of course, everybody has a different potential solution.

We also have to think about the scale that we're working at? Because when we're working at, like a parcel scale, like a site scale, the interventions that we think about are, are much more basic than if we're thinking about, what's the strategy for Metro Vancouver.

The other aspect of it is that even if we can all agree that climate change is happening, and we've modelled it as a certain way and, this is how it's going to happen. It doesn't mean that we're going to agree on the usage of land in terms of what we want to do with it.

So when we think about parks, for instance, and park planning, well, we have a whole bunch of parks in Metro Vancouver. So if we say, well, one of the ways that we can mitigate climate change is to more more canopy cover because more trees equals cooling, well, that's not what everybody wants done with their green spaces. People want large areas where they can sit out and put out a blanket. We get a lot of overcast days, a lot of rain, so we have to account for those days as well and what people want at those times. People want dog parks, people want rose gardens, people want golf courses. They don't just want treed forest areas. So how do we sort of balance and account for everybody's particular sort of interest. These are some of the things that we come up against,

FVC: I'm interested in how that relates to calls for denser forms of housing, and the need to alleviate the housing crisis. Because obviously, you have more residential lots being filled by housing, which seems to be needed. Abbotsford’s forestry plan has kind of confronted this. You’ve got just less space for greenery, and you need to do better with your public spaces or create more public space. Can you talk about those kinds of challenges that we're facing across BC?

Almas: Yeah. That's a huge push-pull dynamic. This aspect of ‘Well, we need affordable housing, we need areas for people to live; we have single family homes, and we want to increase the density, we want to incorporate row houses, we want to incorporate laneway houses, and we want …. build larger multi-unit, condo blocks, and so forth.

When we do that, of course, we lose the green space. And we also lose a lot of the vegetation with a lot of the redevelopment that's happening. Even when we have single family parcels, often what's happening is that house was built in 1975, and the way the housing market is working, people go and they buy that property, they tear down the house, and they take out all the vegetation at the same time. They build a new house and then put in new vegetation. But when you remove old vegetation, of course, what's happening is you're losing all of those services that you are getting from those from that mature vegetation, and you're planting these smaller things that have to grow and are not providing you with those same services.

When you extrapolate that to a Regional District scale, you have a bit of a conundrum in terms of the ecosystem services that are being provided. So this really comes down to ‘Okay, well, how do we plan for and an account for keeping our green spaces?’

This is why it's important to have city plans where we go through and we say, well, this area is going to be designated for parkland, for instance.

Oftentimes, what we see when we have new developers come in … the city essentially negotiates with the developer, saying that we need you to devote a certain amount of your development land to park, so that the people that live in your development have access to this.

So when we look at Maple Ridge, there are interesting examples of that. For instance, where they've built up in the Silver Valley area of Maple Ridge, they've referred to [the development that has happened] as eco-clusters.

What they did was they developed into this fairly undeveloped land, and they retained like a few-acre parcel, and the houses around this parcel all had access to this few-acre parcel of green space. And this was sort of the development ethos of this particular property.

Now, these developments are for the most part, all single-family homes again, so you're not really densifying, you're just you're just increasing the footprint of suburbia at that point. But when we think about this, then we have to get creative. If we're getting more dense, but this is where people live. So if we don't have large parks, like, Stanley Park, what can we do? Well, we can have smaller parks. But we can also green our streets. We can green our alleyways, we can have green walls that we incorporate.

It's really a bit of a jigsaw puzzle in terms of how we can do this. I mean, we can go to extremes, where we look at examples like in Milan with the Bosco Verticale where they essentially put large trees on skyscrapers.

But that's a pretty expensive endeavor. So what are some less costly endeavors that we can incorporate? The bottom line is that when we see densification, in cities, we tend to see less canopy cover. But it doesn't mean that we can't come up with certain solutions to try and incorporate as much greenery as possible into these areas. But they need to be thought about strategically prior to the development happening, because it's always harder to carve out green space after the development has already happened.”