The Urban Tree Canopy: How Trees in Cities Keep Temperatures Low

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In this episode of The Evergreen Thumb, Erin Hoover talks with Tim Kohlhoff about the role trees play in creating healthier, more livable communities. They explore why urban tree canopies matter, the challenges communities face in protecting them, and how thoughtful tree planting can help shape the future of our neighborhoods.

• WSU Spokane County Extension Website
• Favorite social media profile: Master Gardeners of Spokane County – WSU | Spokane WA
• A Guide to Washington State’s Urban Tree Canopy
• Urban Tree Canopy | Conserve WA Resources
• Urban Tree Canopy Data Now Available for Download
• Seattle’s Canopy Cover | Seattle.gov
• Washington Tree Equity Insights
• Managing Winter Issues in Trees with Tim Kohlhauff – Episode 10
• Unearthing the Secrets of Soil Carbon Sequestration – Episode 14
   

Erin Hoover (00:00)

Welcome to episode 83 of The Evergreen Thumb. My guest today is Tim Kohlhoff

Tim is the Urban Horticulture Program Coordinator for Washington State University Extension Spokane County Office. He has led the local Master Gardener Volunteer Program since 2012. He is an adjunct instructor at Spokane Community College, where he has taught arboriculture and plant problem diagnosis classes. For 10 years, Tim was the arborist at the Kalispell Golf and Country Club and has been an ISA-certified arborist since 2004. And today, Tim is here to talk to us about the urban tree canopy and its effects on our communities.

Erin Hoover (00:39)
Tim, thanks for joining me today. Welcome back.

Tim Kohlhauff (00:41)
Absolutely. I’m happy to be here.

What is an Urban Tree Canopy?

Erin Hoover (00:44)
So today we’re talking about urban tree canopy. So what is an urban tree canopy?

Tim Kohlhauff (00:50)
It’s really, simply put, it’s all of the trees in a community. So we might think about all the public trees, you know, the parks and the boulevards and the street trees that are out there, but it’s more than that. It includes all of the trees in the commercial areas and all of the trees in in everybody’s backyard. So if we think of that all together as a forest, but in the middle of a community, that’s our urban tree canopy.

The Benefits of the Urban Tree Canopy

Erin Hoover (01:20)
So what is the function that a canopy provides to the community?

Tim Kohlhauff (01:26)
So there are, and here I might get in trouble. You might have to cut me off because I will talk about the benefits of trees and shrubs and plants, you know, all day long, following people out to their cars after the class is over.

So roughly we can categorize the benefits into a few different things. There’s the health and wellness benefits to people, to humans. There are the economic benefits to commerce, but really, I think what we gardeners think of are the environmental services that they provide. And so, trees in a community are a lot like trees in your backyard. You have shade in your backyard, but if you have a lot of trees in a community, you have the accumulation of the cooling that provides. If you think about a tree in your backyard buffering sound or providing a little bit of a sound break.

A lot of trees in a community also provide a sense of privacy and maybe deadening some of the noise in the area. There are some other benefits; tree-lined streets tend to slow drivers down. And so sometimes you will even see trees intentionally planted around schools so that people will naturally slow down. I mean, they hopefully are anyway, but where there are trees, people will kind of subconsciously think, I’m not sure if I have all the visibility I need. Maybe I should slow down. And yes, you should probably.

Some of the really big benefits are that a lot of trees in our community do a good job of intercepting air pollution, different types of air pollution. One of the ones they do a really good job of intercepting are what we call particulate matter. Dust, pollen, mold spores, things like that that are fairly large, and they do a good job of just acting like well, acting a little bit like my bookcase, right? Every time I go to my bookcase, I don’t know where all that dust came from, but somehow the dust just shows up there. Trees and to a lesser extent shrubs are like that a little bit.

So they’re cleaning the air; they’re taking in CO2 and not exhaling, but they’re exuding oxygen, which is great for us. Also, they’re sequestering carbon. They’re capturing CO2 out of the air using that and then returning oxygen to us, and some of that CO2 then becomes stored long term in the stems, the branches, all of that. So as long as the tree is still there, then we have that CO2 that’s locked up.

Which is great because we have a little too much of it in the atmosphere.

And then a couple other ones. One is mitigating stormwater. I know a lot of folks are aware of when it rains, it lands on streets. Sometimes pollution then washes off the street or off the building roof, and it can get into surface water; streams, lakes, the sound. Trees have a way of slowing down that water. I sometimes hear about storms where it rains an inch in an hour. And some of that water is caught by trees and slowed down so that that whole surge of water doesn’t enter a community’s storm drain system or sewer system at the same time.

I hate to use this term, but flattening the curve of water, and I know that kind of calls us back to COVID and that’s an unpleasant memory, but flattening the curve of how fast water comes into our storm system or s or sewer system helps protect that system. And then also trees are absorbing some of that water. So not letting it become runoff that can pollute.

And going back to some of the reasons that we plant trees in our backyard, shade, cooling, our communities have a tendency to get hotter than the surrounding rural areas. And if we plant enough trees, we can make an impact on how hot our cities and towns get.

How Heat Urban Heat Islands Affect Communities

Erin Hoover (05:42)
Well, that was a great lead-in to my next question about the urban heat island effect and how that affects communities.

Tim Kohlhauff (05:50)
Yeah. yes. So a lot of us have heard of that now. Unfortunately, we’re becoming more familiar with the idea that all of the human activity in cities, so we’re driving around and our cars are exuding heat, and we’re in buildings and we’re using energy that has a byproduct of heat. Ironically, air conditioning to keep our buildings cool in the summer also generates heat.

And then things like manufacturing and other processes that are just a part of city life that generate heat, all of that adds up, and so that when you’re in a city or a town and the effect is bigger the larger the area is, we actually can we can measure a difference in the temperature in say a downtown urban area versus the rural area outside the city.

Unfortunately, too much heat is not good for humans. And we have a lot of people who are impacted by heat. If we think back to the heat dome of June 2021, we unfortunately lost a lot of people because the stress to their system from that excess heat was too much. And so that’s the urban heat island effect where we live.

Where there are a lot of us, heat tends to build up. And where we plant trees in those areas where we have shade over roads and sidewalks, and sometimes we have green roofs that aren’t necessarily trees, but still plants. So there’s shade, there’s some cooling to help combat that urban heat island effect.

One of the things that I’ve learned recently is that where trees are planted tend to be in the higher-end parts of town, the richer areas of a town, and often areas where lower-income people are living don’t have as much access to trees. This is an issue we call tree equity, meaning everyone deserves to have shade around them. They deserve to have a place to go and be cool when it’s especially hot.

Some cities in Washington are working really hard to address this tree equity issue by intentionally planting trees in areas where they should go, where people need that cooling. And it works out really well because those are often the places where we can plant trees, right?

A lot of places, parks and the higher-end or higher-income parts of towns often have trees already, but if we intentionally plant in some of the lower-income areas, we’re not only helping people there, but we’re also adding to the urban tree canopy, which hopefully will benefit the entire community.

Climate Change and The Urban Canopy

Erin Hoover (08:51)
How are climate trends putting more pressure on urban canopies in different areas?

Tim Kohlhauff (08:58)
So unfortunately, they are putting more pressure on urban canopies. Not every tree is really well adapted to living around people, if that makes sense. There are some trees that just get huge root systems that want to heave sidewalks up or break up asphalt. Some trees just can’t adapt to the soil conditions that we have, especially in urban areas.

There are some that just aren’t well adapted to the heat that we seem to be experiencing now. I’m thinking about native plants like the Western Red Cedar or the Big Leaf Maple, where we see a lot of decline in population because it’s just too hot. Or our precipitation cycles are different than they used to be, and there isn’t as much or enough water for some trees now to really thrive.

So there is pressure on our urban tree canopy because they’re already in an environment that’s kind of challenging for them. And now we’re changing things up a little bit. We’re making it a little hotter, maybe extending the growing season so it’s a longer period of time that they have to adapt to, or maybe changing how much water falls in the root zone.

One of the things that arborists and urban foresters think about is how can I prepare my urban forest, my urban tree canopy for the future? What do I need to do to get the forest ready for this? And I know a lot of work has gone into trying to understand how we can do this.

Is it treating the trees differently? Is it what we sometimes call assisted migration, where we get trees from warmer, drier areas and we try them up here to see how well they exist in urban areas? And the answer seems to be we’re going to try it all. And the things that work we hope we’ll keep doing.

The Urban Tree Canopy and Pest and Disease Pressure

Erin Hoover (11:02)
What about like pest and disease pressure too on the canopy?

Tim Kohlhauff (11:06)
That’s something that, because of the changing climate that we’re seeing, and also a lot more trade with other parts of the world than we’ve ever had before, we’re seeing new pests and diseases come into urban areas and impact trees.

So I can give an example that people might be familiar with: not too long ago here in Washington, we had the first recorded instance of the Emerald Ash Borer, a tree pest that’s killed tens of millions of trees in the Midwest and East Coast. We don’t have as many ash trees here as they have back east, but we still have our native Oregon Ash.

And that pest represents a threat to those native trees that, in a lot of areas, people are already talking about replanting different native trees because the ash tree may not be around there. So we think that the Emerald Ash Borer came here possibly in shipping containers as long ago as maybe the late 90s or early 2000s.

So we have pests and diseases moving around the world in ways that they haven’t before. But we also have pests that are now able to move into areas that they couldn’t before because of the changing weather.

So I remember even in my lifetime there’s a pest called the Walnut Twig Beetle that sometimes carries a disease called Thousand Cankers. That disease is fatal to plants that are in the Walnut family: Black Walnut, English Walnut, Butternut.

When I started in this industry, when I started as a Master Gardener, our winters in Spokane were too cold and the Walnut Twig Beetle couldn’t live over winter in our area. But in the last 10 years, our winters are not as cold as they used to be. And now we’re starting to see, or think we see, a lot of decline in those species of trees because of Thousand Canker disease.

And so it’s kind of coming from two directions. A lot of it is just us doing the best we can to catch pests before they can come in and establish and then managing the ones that do establish and just doing our best to keep our canopy strong, even if it means changing what the canopy looks like by trying different trees.

Aging Urban Canopies

Erin Hoover (13:33)
What about aging canopies, like in older cores of cities that might be in decline?

Tim Kohlhauff (13:41)
So one of the things that is so great, and I know it’s not unique to Washington, but I’m going to say Washington is great anyway. We’ve always had trees in our communities, in our cities. So, there are a lot of really big trees that people really appreciate in cities and towns. And they give us the most, I’ll call it bang for our buck, but the most environmental services. So those mature trees do a lot for us.

It takes a while for a tree that we plant to really get to be big enough to give us the benefits that we’re hoping for. What we hope we can do is that we can protect those trees from things like redevelopment or from incoming pests and disease that might be showing up, and just take care of the mature trees in our communities. We do a pretty good job of planting new trees. I mean, we could always do better.

But one thing that’s a little harder to do is to replace a mature tree that maybe is threatened by development or, as I said, some new pest. So it really pays for us to take care of those trees.

At the same time, trees do have a life cycle like everything living, and so some of them get to the end of that life cycle, and there is just isn’t anything we can do to save them. So the best we can do is manage their decline, hopefully, so that they’re not a risk or a threat to anybody. And then if they become a problem, then we have to remove them. But hopefully we do something that replants with new trees.

I know that we had a community tree, a willow in the Finch Arboretum in Spokane, that reached the end of its lifespan and it fell down. People had for years had wedding pictures, senior pictures in high school had been taken with that tree. So there’s a movement to try and rescue what’s left of the tree and turn it into maybe an outdoor playground.

So keeping some of that wood and that carbon that’s stored in there, keeping that carbon stored, but also doing something positive with the tree when it reaches the end of its lifespan. And I know they took a lot of cuttings from the tree to hopefully propagate and replace.

Erin Hoover (16:13)
My next question was if they if anybody took cuttings because willows are so easy to propagate.

Tim Kohlhauff (16:18)
Yeah. And I think even though they had it fenced off, I think some residents who were maybe not technically authorized to cross the fence, I think there were some cuttings that maybe went home with people just because that that tree was such a huge part of some people’s lives. I took pictures of my niece and nephews at that tree, and so there may or may not be a cutting somewhere in my yard, but if anyone asks, I’ll deny where it came from.

Keeping up with Canopy Loss

Erin Hoover (16:50)
So is there any concern about the canopy loss outpacing replacements in some of the older neighborhoods?

Tim Kohlhauff (16:58)

Yes, and it’s sometimes hard to tell where we are in our canopy. And let me back up and say, so most of the larger cities in Washington now have canopy coverage goals. The idea of canopy coverage is that if I fly over the city anywhere and I can’t see the ground because there’s something green there, or a tree there, then that’s our canopy coverage.

And in general, more canopy coverage is better. We want to plant things in the right places, but we want more canopy coverage for more shade, more environmental benefits. So some of the bigger cities, Seattle, Tacoma, Spokane, and I think Vancouver, have all set canopy coverage goals of about 30%.

That’s kind of aspirational in most cases. We might not get there. The slogan is 30% by 2030 or 30 by 30. What Seattle found was that in a couple of areas, they lost pretty significant canopy coverage in some redevelopment. So they lost some bigger trees. And so I think it was a couple of years into this initiative to try and cover more of the city, they found their canopy coverage was actually going down because of some trees that were lost in some pretty big areas.

So there is that push because we also have a lot of people who need places to live. And we really need to house people. That means more houses, less space for trees. I know there are some changes to zoning, so we talk a little bit about something called the missing middle in housing, meaning that instead of single-family dwellings, we move more towards triplexes, duplexes, small apartment buildings, big apartment buildings, so that we can house more people in a fairly small space, but that means less space for trees.

It’s absolutely n necessary that we find space for people to live. But there’s that tension between housing and is there still room for trees, especially in a larger urban community. It’s hard to reconcile those sometimes. So we sometimes lose an older area of where houses were, and we redevelop, and that can be really good, really exciting, more places for people to live, but then we find the only spaces for trees that are left allow very small trees instead of the taller ones that might give us more benefits.

Selecting Trees for The Home Garden

Erin Hoover (19:44)
So what are some criteria or some things to think about for species selection for home gardeners who want to add more trees to their yard?

Tim Kohlhauff (19:54)

I think the thing that has been drilled into me ever since I was in school for this was to know the landscape, know your landscape, meaning go out and take a really good look at how much sun this tree will actually get. Is it going to get full sun? Is it going to get is it shaded part of the day? Understand your particular growing area. Is there shelter there from winds, or is it really exposed?

And something that a lot of folks don’t think of because it’s just not on our radar is what’s the soil condition like in your landscape? Half or maybe more than half of the mass of a tree could be below ground. And I would argue a really important part of the tree is below ground. So we want to make sure to match the tree to the site that we have.

And I know I’m talking about this, and I’m saying, you know, choose the right tree for your location. And then I’m as guilty as anyone of going to the nursery, and I see, holy cow, that, you know, that tree has blue flowers. I’ve never seen that before. And okay, well it’s native to a subtropical tropical region somewhere. Well, I’ll just I’ll take care of it. I’ll change the environment for it. And no. And I wish I could say that I’ve learned and I’m better now.

I still sometimes go into the nursery, and I see something that no one else has, and there’s that little I say, maybe a plant hoarder part of me that or a plant collector- that’s a nicer thing to say. Plant collector that just really wants, you know, to take that palm tree home, even though where I live, palm trees are just they could live indoors, but yeah, no, a palm tree isn’t a good choice for me. So, understanding your environment is probably the best thing that you can do.

Trying to match the tree species to the conditions that already exist. As we grow, as our population increases, we will have maybe less water for our landscapes than we used to because humans deserve to have water. We need water. And so, water has to come first for humans. And landscapes maybe in the future won’t get as much water, supplemental water. They’ll still get what falls from the sky. So if you can find something that’s adapted to the precipitation in your area, that’s ideal.

In Eastern Washington, that sometimes really limits the palette of trees. Native trees in Eastern Washington are not all well adapted to living in somebody’s backyard. I have native cottonwood near my house, and they’re really wonderful, beautiful trees, but I wouldn’t have one in my backyard because every time the wind blows, big branches come crashing out of them. And I have a hard head, but not that hard, so it’s just not the tree for me.

So native trees are great, but understand your conditions and then understand: am I able to give this tree supplemental water if I’m living in eastern Washington? How likely will that be to continue? And what is it that I want out of the tree? In other words, am I looking for shade? Am I looking for a screen from my neighbor or from the garbage cans along the alley? Try to find something that matches those.

So I think I’m giving people a really long list, and that’s why people like me then just go into the nursery and say, blue flowers, and because that list is not nearly as fun to check off when you’re in the nursery and you see something really wonderful. But I promise you will be happier if you buy something that’s adapted to the landscape where you’re living.

And if you want to think ahead a little bit, think about temperatures getting warmer. So we in the past have always been worried about cold tolerance and how hardy cold-hardy trees are, but there’s more attention being paid to heat tolerance now.

So think about or look for trees that are adapted to the heat that we get, not just now, but if it were to get warmer in the future, would they do well under those conditions? And that’s a hard thing to predict because I am not a good predictor of the future. I don’t know how warm it might get or might not get, but the trend seems to be getting warmer, so think about trees that are heat-tolerant.

Erin Hoover (24:43)
Right. That kind of reminds me one more thing to add to the list is if you’re planting a shade tree, be sure that you’re aware of the mature size of the tree and know that that tree is going to get larger than what the plant tag says.

Tim Kohlhauff (24:58)
Yes. I’m so glad you brought that up because in some commercial developments around the area, I think maybe the folks who did the design don’t live here, so they weren’t able to come out and maybe stand where the tree’s going to be and then look up. Gosh, those power lines are right above here. Maybe I should not plant the Douglas fir under the power lines. Maybe something else would be a good choice. So yes.

Think in terms of how large that tree is going to grow because a lot of us buy trees when they’re small and cute and wonderful and don’t think about this being, you know, if the tag says fifty feet in some parts of the state, it’s going to be seventy-five or maybe more. And I know in my area it might not get; it’s going to be at the smaller end of its normal growth range, but still lots bigger than it looks when it’s at the nursery and when you first plant it and think, see it it’s perfect right there.

Erin Hoover (26:05)
Until it’s not.

Tim Kohlhauff (26:06)
Until it’s not, until you planted the tall tree under the eaves of your house and suddenly, you’re out there pruning every 15 minutes because the poor tree is just trying to reach the sun and you don’t want it to go into your attic.

Tips for Taking Care of Young Trees

Erin Hoover (26:20)
So, let’s talk about some of the core practices for young trees since we’re talking about new trees. You mentioned watering and making sure that’s a tree you can give supplemental water to, but what other kinds of things to take into consideration? What not to do when it comes to planting, some things like that?

Tim Kohlhauff (26:39)
Yes. So one of the things that we still sometimes see, and I think this is getting better, but I still warn people about this, is that when you plant a new tree, you want to make sure that the root flare that’s the part of the tree at the base of the trunk where the trunk flares out into the root system. That root flare should be at grade level. And there are a few situations where there might be a difference there, but in general and in all of the circumstances I can think of right now, that root flare needs to be right at the top of the soil.

That’s where it’s adapted or where the tree wants to be. We still see a lot of folks who are planting maybe too deep. In other words, that root flare where the trunk widens out gets buried below ground. And that causes problems for the tree down the road. Sometimes they don’t really establish well; they just don’t grow very well. Sometimes they grow well for a while, and then they’re more prone to pests and disease later on.

I know that occasionally you will get trees that, when they’re planted low in the soil, they’ll send out a bunch of roots right at the soil line, and over time, because those roots, we call them adventitious roots; they don’t grow like the normal roots do. They eventually will girdle the tree when it gets to be much larger.

And you want to do everything you can to give your tree the best chance it has at living a full lifespan. So plant it at grade level. If you can mulch around the base of your tree out at least to the drip line with something like wood chips. Organic mulch in general is better. If you’re in a fire-prone area, maybe you want to use something like river rock or some sort of rock mulch, but mulch around the base of the tree that will help protect the soil and protect the tree roots.

And then for the first year at least, make sure your tree has enough water because when we transplant trees, they lose some roots. Sometimes when trees are harvested, they can lose up to ninety percent of their original root system. So when they come to our landscape, they’ve lost a lot of their roots. They’re trying to grow roots as fast as they can to support the part of the tree that’s above ground.  So we want to make sure that there’s water there.

We don’t want to flood the tree, but we want to give it a little bit of water regularly so that the roots don’t dry out, especially that first year. And then depending on where you are in the state, there may be enough water in the form of precipitation to keep it happy the second year. In the drier eastern side of the state, we say at least three years; plan on giving it supplemental water. That’s even if it’s a native tree. And then some trees, even after those three years, you’re giving it supplemental water. But maybe you’re doing it less often.

So my first year I might be doing it weekly or every three or four days. My second year, maybe I’m stretching that out a little bit. And third and fourth years, then I’m giving it periodic deep watering. So I’m really saturating the soil, but then I let the top of the soil dry out a little bit so that it kind of encourages deeper root growth and gives the tree what it needs.

Final Thoughts About the Urban Tree Canopy

Erin Hoover (30:19)
All right. Do you have any final thoughts about the urban tree canopy?

Tim Kohlhauff (30:24)
My gosh, I, you know, if we could plant our way out of climate change, you know, I would be the first person to try and do that. I wish we could plant trees and reverse some of the things that are happening. We can’t, but trees can make things better, and they can give us those environmental benefits that will hopefully protect us from some of the worst impacts of climate change or at least make us more resilient.

And so, I think the best thing to do would be to have planted a tree 20 years ago. But the second-best thing is to plant one tomorrow. So go out and plant more trees where it’s appropriate. And always look up for power lines. And other things, yes.

Erin Hoover (31:10)
And other things. All right. Thanks so much, Tim. This was great.

Tim Kohlhauff (31:18)
You bet it was a pleasure. Thank you.