Episode 538 Our Urban Trees

Stephen Sheppard was a founder of the Urban Forestry department, U of British Columbia, where Lorien Nesbitt is now an assistant professor. David Price retired after a career in forestry modeling for Canada’s Natural Resources department. We discuss with several Pugwashites the health challenges of urban trees and the value of planting them as a means of cooling the cities, mainly through their transpiration, “exhaling” of water vapor. The foresters all agree that the health and comfort of living in cities would be improved if most of the lawns are replaced with trees, and possibly whole local blocks can be devoted to small urban neighborhood forests. For the video, audio podcast, transcript and public comments: https://tosavetheworld.ca/episode-538-our-urban-trees.


Stephen Sheppard

David Price

Lorien Nesbitt

Peter Meincke


trees, people, soil, cities, plant, cooling, big, benefits, urban forestry, lawn, canopy, water, lorien, stephen, street, neighborhood, urban, shade, transpiration, conditions


Metta Spencer, Robin Collins, Lorien Nesbitt, Stephen Sheppard, Peter Meincke, David Price, Bill Bhaneja


Lorien Nesbitt’s research examines disparities in access to urban forest canopy cover and the potential harms of tree planting. Addressing potential objections to tree planting, she explained that fears often stem from poor choices in planting sites and poor tree management. She stressed the need to mitigate issues such as infrastructure damage or nuisance caused by trees.

Metta and Lorien agreed on the value of active community involvement in tree planting and care, with emphasis on establishing a system where individuals and cities can work together to ensure equitable maintenance of urban trees, particularly in low-income areas.

Metta Spencer cites a Toronto initiative where children collect and grow tree seeds in tin cans, showing the value of community engagement in tree conservation. Lorien Nesbitt and Stephen Sheppard point out that urban trees struggle because of urban soil conditions that often lack necessary nutrients, as urban development typically removes original, nutrient-rich soil, replacing it with sand. The mortality rate for newly planted trees in these conditions can be up to 50% in the first 10-15 years.

An experimental field the size of a football field was mentioned, where different types of soil and tree species are tested to understand optimal conditions for tree health. However, such experimental initiatives are not widespread.

Sheppard elaborates on tree health issues, mentioning factors such as climate change, increasing heat, and drought stress. Western red cedar trees are notably failing, possibly due to drought stress or other factors. Other challenges include loss of canopy due to urban development and lack of space for trees.

Nesbitt and Sheppard also mention ‘Gator bags’, designed to provide long-duration watering for young, newly planted trees. These bags are placed around the base of trees and gradually release water to prevent excessive runoff. It is clear, though, that urban trees face many challenges, including leaf blowers, which pollute and remove essential nutrients that trees need for soil improvement and moisture retention. Sheppard advocates leaving leaves in place so they can decompose naturally and improve the soil, emphasizing the need for landscapes designed to support this.

The discussion then moves to urban trees, where Price suggests converting entire city blocks into small urban forests to provide a self-contained ecosystem that supports wildlife, absorbs water, and allows leaves to decompose naturally, benefiting the soil. Price indicates that these clusters of trees can cool down the city and provide carbon benefits due to transpiration.

If rundown areas are converted into small urban forests, economies of scale could reduce the cost per tree.

Stephen Sheppard emphasizes the potential benefits of neighborhood-level cooling achieved by increasing tree cover, which can decrease temperatures, improve mortality rates, and enhance access to nature. However, he notes this is a complex issue, with three domains of the urban forest to consider: parks, streetscapes, and private yards. Each has different potentials and challenges in terms of contributing to canopy cover. The optimal level of canopy is around 40% to achieve neighborhood cooling effects, and Sheppard suggests focusing on better management of private spaces and the promotion of tree planting among residents.

He acknowledges the challenge of cultural barriers and weak tree protection bylaws that are often bypassed in redevelopment. He advocates for programs to educate citizens about the benefits of trees, including reduced energy bills and improved health. Old trees also need to be maintained as they provide a buffer before young trees can develop a full ecosystem.

Sheppard asserts that Canada lacks an effective system for integrating urban forest plans and community engagement. He suggests the need for better engagement with communities and stakeholders and highlights programs that offer solutions such as de-paving yards and reclaiming street spaces.

Metta Spencer mentions the impending advent of electric taxis that could eliminate the need for parking spaces, which could then be used for tree planting. Sheppard agrees, suggesting the need for formal strategies from governments to project these future changes and prepare for a reduction in space for cars and an increase in green, pervious areas.

The participants discuss whether the space taken by parallel parking could be utilized for planting trees, with Sheppard explaining how bulge-outs for traffic calming could support tree growth. While larger trees are more beneficial in the long run, it’s also possible to grow small or medium-sized trees in small spots along the street.

A question is raised about the most suitable tree species for urban areas, given that certain trees might not have a long lifespan due to soil suitability, weather resilience, and susceptibility to diseases. Drought-resistant trees are identified as the most suitable for urban planting. Sheppard suggests that advanced cities are working on guidelines for selecting tree species that will have a long lifespan and high adaptability.

Price explains that the cooling effect of trees is primarily due to transpiration – water circulation – while the shade from tree canopies can lower the surface temperature of the soil.

The participants acknowledge the different conditions across Canadian cities, where the West Coast’s humid climate differs significantly from the drier conditions in Alberta. Thus, the role of tree cover should be considered in relation to the city’s climate and conditions. The conversation ends with a question about the CO2 absorption capabilities of urban forestry, with participants expressing curiosity about a study claiming New York City’s green cover absorbs as much CO2 as the city produces.


The following transcript has been machine generated using “otter.ai.” Prior to using information from the transcript, please watch the video to catch any obvious errors.

Metta Spencer  00:00

Hi, do you think your city has enough trees? I don’t think we’re going to ever get too many trees in the city. And I think it would do us a world of good if we just planted a few more. So that’s what we’re going to talk about today. I have a team of people who are experts on urban forestry. And we’re going to have a couple of Pugwash members who will join me in interrogating them about the value if any, and the potential downside of urban trees. So, I guess, in Vancouver is Stephen Shepherd. You are in Vancouver. That’s correct. Very good. And Stephen was, as I understand, the founder of the Department of Forestry at UBC, is that right?

Stephen Sheppard  00:55

Oh, well, it was quite a team of us at the time, but we did. We did get that going a few years back now.

Metta Spencer  01:01

Okay, very good. And also we’ve got British Columbia all over the place today. Also in British Columbia, more in the hinterland, I think, is David Price. Is that right, David? You’re out in the boonies?

David Price  01:16

Well, it’s hardly in the boonies. But yeah, in the southern interior, between Kamloops and Kelowna, basically, and I wish to point out that I am assuredly not an expert on urban forestry.

Metta Spencer  01:29

All right, but you know what tree when you see one. You spent a lot of your life modeling forests and crunching numbers having to do with trees, right.

David Price  01:44

Yep, that’s basically true. Yes.

Metta Spencer  01:47

All right, very good. And also in Vancouver, is Lorien Nesbitt, who is an assistant professor of urban forestry at the University of British Columbia. So she and Steven know each other. And we will get back to Lorien right away because I want to give her the first word. And, but in Ottawa are two of my friends who are members of the Canadian Pugwash group, Robin Collins and Bill Bhaneja. Bill is a retired bureaucrat in the Canadian government? Is that an insulting word to you, Bill? Or would you like to describe yourself in other terms?

Bill Bhaneja  02:33

I used to be in science and technology.  foreign affair.

Metta Spencer  02:38

But Now Lorien, you’re the only woman here and I think we need to do more to get women. We don’t have enough of you on this show. And I’m glad to welcome you because I understand you have to go in about 15 minutes. So I’d like to have you start off.

Lorien Nesbitt  02:58

I guess I should start by saying that. I’m in the traditional unceded territories of the Musqueam. Squamish and first nations. And aside from sort of just the importance of acknowledging that,  I say that also because my research is about Urban Forestry and Environmental Justice. And so part of what is involved in environmental justice is the participation of different perspectives and nations and the ways in which we live together and work together in an equitable way. In urban forestry, I guess my research kind of spans the more basic kind of social ecological system of urban forestry and understanding how we live together with trees, the benefits we receive from them, but then I, as I sort of started observing some disparities in access, for example, or the ways in which different knowledge systems are brought into the discipline, I became more interested in the environmental justice aspects of urban forestry and so looking at things like who has better access to tree canopy cover, which is particularly important for things like climate change adaptation, whether urban forest canopy cover or parks are accessible for people of different identities mobilities – that kind of thing, as well as the potential harms of planting trees although I personally think that trees are always good sometimes the systems within which we plant them can cause problems. So it’s not the trees, it’s the humans always. But something to kind of think through

Metta Spencer  04:35

I’m predisposed to agree with you, but I just had a conversation yesterday with one of my other Pugwashite, who is anti tree. He looks at all the downsides. So just tell me what do you think? Seriously, I just completely overlook any reason one might have for not wanting to plant trees. So let’s start with that,

Lorien Nesbitt  05:05

 think we should start by thinking through what planting a tree is. So you plant a tree. And that’s the thing that everyone talks about. But we also need to provide adequate space for that tree, we need to provide adequate soils for that tree, we need to then provide management care for that tree so that it’s healthy thereafter and not causing damage. So one of the things that people are afraid of when planting trees is that there will be some kind of tree failure branches will drop and cause damage, the tree itself may come down, a lot of that, in my opinion, is due to poor choice of planting site, poor choice of tree and then poor management of the tree thereafter. And so I would love us to move towards more of a relationship with trees where we understand that they are living beings and not a bridge, you know, or a car. Well, even a car needs maintenance, for example, right. And so investing in that management and that ongoing relationship with the trees. When we’re expecting those trees to give us benefits back like shading, like stormwater control, we need to give them benefits as well and care.  I guess the other things that people often talk about are more related to kind of those conflicting values or priorities that we have for what we want to see happening in our cities. So you know, I hear that people are upset about trees, conflicting with infrastructure or causing sidewalks to heave. They want to remove a tree because it’s close to their home, and they’re feeling nervous about the roots causing damage to the pipes in the house, for example, or even things like trees dropping fruits. That, once again, I think is about choosing the right tree for the right place. And then kind of balancing those different values that we have. I personally would like, rip out at least half of the roads in Vancouver and plant trees in them, you know, but I know not everyone wants to do that. So it’s about kind of balancing those different perspectives, but also having that conversation. I don’t really see us having that conversation in cities, we’re taking a business as usual approach. And we are seeing that our canopies are declining. So I think we need to lean into this. I think it’s great that you’re having this conversation right now.

Metta Spencer  07:10

Okay, well, one guy said, All the trees are always dropping their leaves. And it’s a nuisance. And besides, the branches can fall and knock a hole in your roof and or start a fight with your neighbors because they fall in somebody else’s property. And he just didn’t like trees. So we’ve got to work on him.

Lorien Nesbitt  07:36

I’m interested, more recently in sort of the relationship that people have to nature and cities. And there’s some scholarship that suggests that living in a city can alienate you from nature. And I understand that because we don’t necessarily have a lot of opportunities to engage actively in stewarding aspen trees or other plants in cities. So the fact that trees drop leaves, I mean, it’s like, who cares, you get a chance to go outside and rake leaves, or you can even hire someone to do that yourself if you have the means. I think it has a lot to do with whether we’re alienated from nature or not whether we think it’s something important in our lives, or whether we just want to live amongst a bunch of skyscrapers.

Metta Spencer  08:14

Well, one of the things that one of the angles that we with the Pugwash proposal have in mind is trying to set up a system where we have people actively engaged in a big campaign to plant trees in their neighborhood. You know, I could imagine, the Canadian government announces that such and such a date, please, those of you who are interested, please show up at Ridelle and Marlee. And we will have spades and trees and dirt, and biochar and, you know, whatever other rocks that you want, and we will show you where there is a place along the street for you. In fact, if you need help digging, we’ve got a steam shovel that’ll come and help break up the concrete or something. So you have a place to plant your trees. And then after that, you and your high school kids will each own responsible for the care and upkeep of these trees because you got to water them for a couple of years. And everybody’s going to take care of the trees and you’ll get acquainted with your neighbors and you’ll have a good time. So please show up next Saturday. How about that?

Lorien Nesbitt  09:40

I mean, I think that’s great. I like that  there’s the commitment to sort of at least some management or stewardship thereafter. I know Stephen does a lot of work getting people to relate to the trees in their streets and in the neighborhood. So he can probably talk more about that. In some of the cities I’ve found some resistance to tree planting unless the city or some other organization will commit to providing some of that sort of larger maintenance, like pruning, or removal if necessary in the future. And so there’s a lot of tree planting and tree giveaway programs in the United States, for example. And some neighborhoods, particularly neighborhoods with more renters or lower income tenants,  will resist planting trees, because they don’t have the means to take on that financial burden. And so it does need to be a partnership between the individuals and taking the time to make that relationship with trees and be stewards. And also the support of the city or the region to provide support for those who have a lower means in terms or especially financial means to maintain the trees on their street.

Metta Spencer  10:51

Yeah, it’s not just financial. I mean, I’m old, and I couldn’t do it. But, you know, I think high school kids would benefit a lot from it. In fact, I know, here in Toronto, some people who have a program with grade school kids, and they take them out into the ravines and show them how to collect seeds for trees, and they actually grow them in coffee cans and things. One woman told me whenever you want some trees, my kids have got a lot of them for you. So I look forward to planting some trees the children have raised from seeds. And I don’t know whether it be realistic to have some kids come in and water them every now and then.

Lorien Nesbitt  11:38

I think that’s very neat. The fact that they’re actually growing them from seed, and going through the entire process of producing and raising a tree, that’s beautiful.

Metta Spencer  11:49

Today in The Globe and Mail, there’s an article about the fact that apparently, Toronto plants, did it say something like 150,000 trees a year, which actually, I think we could do better than that. Because I imagine they have to take out a lot too with all this emerald ash borer disease and so on. But at any rate, they plant a lot of trees, but as I understand it, the mortality rate is very high. So it seems that there’s a lot of difficulty involved in maintaining trees in a healthy condition. So one of the things that they mentioned is a new organization, or maybe it’s not new, but and it’s certainly not an original idea. But they have as I understand it someplace, there’s a field at the size of a football field where they, they have put sections in with different kinds of soil, different species of trees, and they’re experimenting to see what the conditions are that are optimal for maintaining the health of various trees. And apparently, one can consult them before deciding what and where and how to plant. Is this a widespread? Is this a unique thing? Or is it something everybody’s been doing all along? That I didn’t know about?

Lorien Nesbitt  13:24

I don’t know. I mean, maybe others could comment. I don’t know that, that I’m not sure about whether there are sort of those types of programs everywhere. But I think urban ecologists and soil scientists, you know, could really talk about the conditions that are necessary for for trees to survive. And I know that there have been, there’s been a bit of work around sort of fungal inoculations, for example, to see whether mycorrhizal relationships could support urban trees, the way they support rural trees and less disturbed landscapes. But, so it does seem that there are multiple factors that are influencing tree decline. My understanding from speaking with some of my colleagues and maybe David or Stephen could jump in in a second on this, is that we certainly not setting up trees for success in many cases, because when we create a road, for example, or, or build a house, we remove a lot of the original soil, and then we replaced that with largely sand. And so the trees already don’t have the the nutrients or the soil quality that they need to survive in a I think in a vigorous way. So yeah, there’s lots of research that showing that it seems that in the first 10 to 15 years, as much as half of the trees that you plant can die.

Metta Spencer  14:42

Okay, is are you saying that it’s getting worse? Did I hear you or did I misunderstand you that tree survival is not as good as it used to be?

Lorien Nesbitt  14:56

I don’t know.

Metta Spencer  14:58

Okay, who can amplify her answer about where other people are doing this kind of experimental work?

Stephen Sheppard  15:11

Yeah, I’m not sure if I’ve got a lot of details on that matter. But there are certainly lots of research study is being done. I know Dr. Susan Day, from UBC, one of our colleagues has done a lot of work in that area. And she would be able to answer that question a lot better. I think I. So I think there’s a lot of research going on. I think the problem is, is there enough sort of systematic visible demonstrations of how these things can be done? And how do we spread the knowledge about things like soil conditions, and soil maintenance and tree type.? I mean, there’s lots in the literature about specific trees and specific sort of soil or shade conditions, things like that. But things are changing. I know, there’s a lot of new work on structured soils, for example, and this is the idea of giving the trees especially the root zone, a much healthier environment that not compacted and it’s not just sand like Lorien says, or clay. It’s a structured soil that can absorb water can hold and retain water, but it still has airspace as trees have to breathe. And one of the problems where you do see decline in trees, and it could be young trees, or old trees, is where they’re sort of wedged into these heavily compacted spaces. So even in a fairly leafy suburb that people some of us are lucky enough to live in. Those trees are really sort of stuck. They’re really their root balls are very often limited. They got a heavily compacted road about a meter away from them. And some of them are really urban old growth. I mean, we’ve got massive ELMS and beautiful maples. And we used to have lots of even native cedars and things like that in pretty urban environments, or sequoias, even. And sometimes, you know, they’re carpeted with asphalt right up to the root. But the ground space underneath which no one thinks about, is massively important. And so if a big tree falls over when it’s 100 years old, A you’ve had 100 years of benefits, but B probably should have given it a root space that would have let it grow for 300 years and stand up. So everything Lorien said is exactly right. We asked an awful lot of these trees that are giving us all these ecosystem benefits. And yeah, they do drop leaves, but leaves are fertilizer folks, that’s how you recycle the system. Don’t get me going on about leaf blowers.

Lorien Nesbitt  17:57

I’m with you on the blowers, Stephen.

Stephen Sheppard  17:59

Yeah, but but I think you asked a really important question matter. And I’m not sure I have really good numbers on this. But but in terms of decline, there are so many things happening now that weren’t happening 20 or 50 years ago. You know, a lot of trees, certainly in Canadian cities, were planted a good while ago, so some of them are aging out or becoming more susceptible due to age. Obviously, you have massive changes in climate. So that’s when strains that’s, you know, saturation of drought and heat, they clobber trees and again, if you don’t have the root space and the care and the stewardship, and watering during drought, we’re gonna lose them. And we’re seeing a lot of natives in BC dying out. And, and some of the older trees are being hastening in their demise. because of things like that. Melbourne is another great example of

Metta Spencer  19:01

The heat is getting worse. You’d say the mortality rate of trees is declining in over time in urban areas?

Stephen Sheppard  19:13

I wish I had better numbers to support that, but just my impression just from watching trees in in the parts of Vancouver that I frequent. I’ve certainly seen considerable die off of conifers, mature conifers, some of that is related to you know, root diseases as well that have become worse when they’re in drought conditions. And we’ve seen significant impacts from the heat down on trees young and old. And I think a lot of the numbers that you’re talking about and mortality rates are to do with young tree isn’t whether there will water enough and they may have gator bags, but there’s someone topping up and filling the gator bags.  And my sense is I’d be very surprised if trees were not declining in health, generally. And

Lorien Nesbitt  20:15

I just want to comment on that quickly before I have to run. We are seeing the decline of canopy. It’s not clear whether that’s due to increase tree mortality. But I think Stephen’s right; it is unlikely that we’re seeing more tree mortality. We are seeing loss of canopy as we build our cities out as we densify there’s just less space for trees. And we’re not thinking creatively about how to create that space. We’re doing business as usual approach. But we are seeing certainly, you know, more storms, more destruction from storms. As Stephen said, in BC there’s particular concern about western red cedar because they’re really failing in  our large forests as well as on our streets.

Metta Spencer  20:58

Cedars are particularly failing?

Lorien Nesbitt  21:01

Yeah, western red cedar. Well, maybe Stephen knows. But from talking to the urban foresters here, there’s some confusion about whether it’s just drought stress or whether there are other factors that are also stressing them.

Metta Spencer  21:15

Two things you rushed past. I don’t know what an alligator bag is and and the second thing Stephen mentioned — “don’t get me started on leaf blowers.” My sense is that leaf blowers are bad news, but I the only thing I heard was that they kill earthworms. And I understand that earthworms are not indigenous to Canada anyway, so maybe we don’t even want earthworms. Tell me what what your objections are — your fondness for alligator bags and your your aversion to leaf blowers.

Stephen Sheppard  21:52

Okay, well, David, jump in here, because I’m sure you could contribute a lot to some of these discussions here. But well, just quickly answer your question. So Gator bags — I suppose it’s a short abbreviation of alligator. They’re these sort of very often green or gray plastic bags that you see around the base of usually small trees. And they’re designed to help small newly planted trees over the first two or three years to  give them a sort of a longer duration watering. It’s like a bag with a zip and you put it around the tree, it’s got an open button, and you fill it up with water. And rather than sprinkling or irrigating which can be less,

Metta Spencer  22:41

Wait a minute. You mean the the root ball is inside this bag?

Stephen Sheppard  22:47

Not the root ball, it sits on the ground. So you plant the tree, hopefully in a in a nice good wide space with good soil and soil amendments, etc. And then you would water it conventionally. But then in order to maintain regular watering over over the whole summer, you create a sort of an extra container of water that sits around the tree around the tree stem, and right sitting on the ground where the tree is planted. And, it could be me 18 inches in diameter, sometimes more, and maybe two or three feet high, something like that. And, and you know, truck will come along or local people will water just by putting a hose in the top, fill it up and gradually that water will soak in. But it doesn’t go all at once. So if you put that much water on all at once, three quarters of it’s going to run off right away, right and usually

Metta Spencer  23:44

There’s something in the bag.

Stephen Sheppard  23:49

Now another bag is waterproof, but you put it has an open top, put water in the top of the bag holders like a tube. And the only place the water can get out is through the ground by slowly soaking in. And what it doesn’t do is run off into the gutters or onto the sidewalks. It holds it in place while the water column gradually gets absorbed.

David Price  24:11

It makes all the water available. So the roots of that tree rather than being left spread around and around it.

Stephen Sheppard  24:18

Yes, it’s much more efficient.

Metta Spencer  24:20

Those are beneficial?

Stephen Sheppard  24:21

Yes, it does help a lot as long as when they dry out, within  a couple of days somebody goes back in and tops them off, especially in hot summer dry conditions.

Metta Spencer  24:36

Okay. All right, that sounds good. Now what’s the problem with leaf blowers?

Stephen Sheppard  24:42

Well, let’s, how do we count the ways? So people often complain about leaflets because they’re noisy and they produce dust etc. And they’re also quite problematic for the operator because they tend to blow up in this dust and, and usually there’s a road nearby, and roads are repositories of many toxic substances that come from tires and oil and vehicles, generally, all of that stuff gets mixed up and put into the air and breathe in. But the biggest problem, I would say is, is my own opinion here is that you have a leaf blower and you hire and spend money on leaf on people with a leaf blower to do a couple of things, they’re gonna clean up a usually a green lawn or a sidewalk to make it clean, neat, you know that that was what we consider to be good citizenship. This is clean, neat, orderly, orderly frames that this kind of thing, right? It’s an aesthetic. But what it’s doing is removing the very nutrients in front, at least the ones that come from the trees, the leaves that are meant to go back into the ground as a part of the nutrient cycle. And you’re you’re removing from the tree, its own way of fertilizing itself, and improving soil, moisture, soil organic matter, soil water retention, so that soil underneath has been deprived of those nutrients. And if we end the bigger problem usually is lawns you can see why people obviously need to clean or sweep or somehow clean Street, roads and streets and and pathways. But the lawns are there to protect the war to protect the soil. And if we don’t have a way of cycling that somehow, so that what we really need to do is reduce the amount of lawn area and have enough nearby area within the root zone of these trees, where leaves can be stored and allowed to compost and rot and return into soil. And we designed to like Lorien said earlier, we don’t design many of our streets or front yards or backyards to do that. And so we spend money and time whisking up these leaves and put them in bags and a big truck comes and takes them away. And the landscape is crying out for leaves leaves these leaves leave the leaves, you know.

Metta Spencer  27:24

Okay, well, I’m so glad you said that. Because I have I have been on a real campaign against lawns. But that’s not the argument that I’ve used. I just don’t like them for 1000 Other reasons, I think there is so many detrimental things about lawns, you know, they absorb water, they require pesticides, they. And I would much rather have trees. I mean, I have been I’m looking right out my window at a big lawn surrounding my high rise condo. And I’ve been running a campaign to try to get them to plant Miyawaki forests out in the backyard. But there’s a little problem there because my underground parking is under that lawn and the soil. It’s only about a foot deep now. So I have had some difficulty finding the kinds of trees and plant all over the lawn. And I think some people would go along with it if if I could just put a little more energy into lobbying them. Anyway. Thank you for that. David, you concurred with his opinion about leaf blowers, right?

David Price  28:44

Never thought about it that much. I’ve never had one. Hey, I mean, I think they’re dirty, smelly things. But I suppose if I were to buy one, it would have to be one with rechargeable batteries.

Metta Spencer  28:56

Anyway, yeah, there’s a little controversy about the value of urban trees for the project that we have in mind, which is our rationale is that we’re trying to find things that enabled us to reduce climate change. And what we’re finding is that urban forestry is not going to help at least for 20 years or so, in a sequestering carbon. It takes too long to do but there are other ways in which the trees cool the cities in indirect ways that you that make the cities more livable. So I’d like your thoughts about why. Give us some good arguments since Lorien has not given us very compelling reasons why we should hate trees. Perhaps I can ask you for some reasons why urban dwellers should want more trees.

Stephen Sheppard  30:02

David, did you want to take a first shot at that?

David Price  30:06

Well, I’ve had a few thoughts listening to you and Lorien. And one response to matters, sort of idyllic view of the government or dining that we will dig up concrete and plant have armies of school kids planting trees. I think there’s a problem with this. And I have a maybe a better  suggestion, which is that many of the problems of urban trees that you have explained, Stephen,  relate to the fact that we’re planting them alone or planning to plant them along roadsides, and maybe in people’s backyards, where the space available for them is really quite limited. And there’s going to be compaction and other factors that contribute to not creating the best growing environment for those trees. So I would suggest that would be better for cities in particular, to think not so much about planting their streets as maybe digging up entire city blocks, and turning the occasional block into a small urban forest area where the trees themselves are basically able to, they can mutually shade, they can have better water absorption, they can do all the things that we like about trees. You don’t need leaf blowers to clean up the leaves decomposing on site, people can use this as a public space for amenity, recreation, all the all those good things. And you’ve got a sort of self contained little tree community, it will support the life and everything else. So many of those problems with urban trees go away, and many of the benefits are retained. And in fact, the climatic and carbon benefits limited, though they may be will still accrue because the cooling effect from those trees is basically due to  transpiration. And doesn’t matter whether they’re spread along the street or condensed into an occasional patch. If the leaf area is the same, under the same weather conditions, they will transfer just as much and produce just as much cooling at a local scale. And the carbon benefits are pretty darn small. But you’ll still get the carbon benefits accruing in those trees.

Metta Spencer  32:23

The benefit of the trees, you had to be walking under the tree. And certainly in a hot summer day, you would prefer to walk under trees. But even if you don’t have that right next door,  if you have a bunch of trees in the next city block you’re gonna get a lot of benefit from that too?

David Price  32:44

Absolutely. And let’s face it, the reason why you feel cooler is not because the air temperature is lower, it’s because you’re getting shaded from the direct rays of the sun. If you really want to go in cool down, you go and sit under a bunch of trees and you will get more shade and more radiative cooling than you would get from a single tree in your backyard. Not to say that a single tree in your backyard is a bad thing. It’s just that people don’t walk along the street to stay under the shade of the trees, they walk along the street because they’re going from A to B and and basically they’re going to suffer the same air temperature, even if they are shaded from from the sun. If we think about towns in France, where I’ve seen many, many situations where you have trees all around on the street. It’s great. Now don’t get me wrong. But to go to the step of taking a Canadian cityscape and basically ripping up the sidewalks, ripping up the street, maybe ripping up people’s yards to make room for a row of trees. That seems to me like a fool’s errand. Why not have these public spaces where people everyone can meet and do all these things that that we appreciate? We just need more urban parks.

Metta Spencer  34:04

But you know, I’m thinking of a video that I watched about Stockholm. Apparently a few years ago, they put about a billion trees in their city. I don’t know how many but they didn’t have the neighborhood go out and do it by hand. They had crews of people with jackhammers and whatever. But they but they showed they had a specific design for how they were going to plant trees. And they all have the same depth of the hole. And then they put something like rocks at the bottom so the tree roots could get through but nevertheless there was rock and I think there’s biochar in there and then they’d fill it up with the tree would be there and then  if they were going to be in sidewalk, they’d have a metal grille around it so the water could get into the hole. And I haven’t seen a follow up of whether or not those new trees in Stockholm are thriving or not. But they were pretty optimistic in their description of how they thought it would work. DDo you think that that is not the most promising way of reforesting?

David Price  35:27

Well, I’m sure you can do all of those things. And I’m sure the cost per tree is going to be extortionate. If you were to think of doing similar treatments for an entire city block, let’s take, I don’t know an area where, for whatever reason, most of the houses are rundown, and it’s either going to be redevelopment to a high rise, or maybe turn it into something different. You could do all of those ameliorate amendments to the soil, you could do the rock and the nice drainage and the quality soil, just cut it all in and lay it out and plant the trees, the cost per tree is going to be way lower, because of the economies of scale. Steven might have more ideas or opinions on this.

Stephen Sheppard  36:12

Well, I think you raise a really important point, which is that it’s possible to do neighborhood level cooling, for sure. And if you have enough trees, then that will cool the air which is really worried talking about by several degrees. And that will improve mortality rates and all this kind of thing. And you make the point about if we could aggregate these areas and create new parks, I agree that would be a fabulous thing. It would improve access to nature, and many great things, but I think it’s a bit more complicated than that. Because really, there is a sort of three domains of the urban forest. But there’s like you said, the sort of the park where you could pack in trees and get those big sort of cooling effects in that immediate neighborhood and have a sort of more natural cycling, maybe rewilding, those kinds of things would be terrific. But there’s also the streetscape, which we’ve talked about quite a bit, which may or may not cool the whole neighborhood. But if you can start to get the canopy up to overall something like 40% as sort of an optimal for these sort of neighborhood cooling effects. And you probably aren’t going to get enough Park situations to get to the overall 40% canopy cover levels. So you’re going to need some of those street trees which are still under public control. But the third domain is the private yard or the backyards the front yards, and to some extent the alleys. And we’re unfortunately that can be  up to 50% of the canopy covers more sometimes in some communities in Canada and beyond. So, I feel like we do need to put more emphasis on better management of those private spaces. And that really can only be done. Apart from strengthening tree protection bylaws, which are very weak in many places that are trumped by rezoning. We’ve done studies on this recently looking at Canadian policies across Canada. And that’s what we’re being told is when there’s redevelopment the trees come out. But even in existing neighborhoods, there’s lots of potential for more trees if we can overcome these cultural barriers that we talked about earlier. And I think that is still a significant piece of the puzzle we have to address and our approach and a lot of our recent research has been going to how can we get regular citizens Canadians residents, even apartment dwellers, to Yes, plant more trees. And yes, reduce maybe not eliminate, but reduce lawns. So we have this more pervious area more green space. Some of that can definitely be used to shade buildings on South and Southwest and West aspects where they get the most heat that can reduce energy bills, air conditioning bills, and improve prove health. And plus, if you can get a certain amount of trees around buildings, the science also shows that those reduce energy use in winter through sheltering against winds especially the less efficient the houses are. So there are these multiple benefits to private trees and most people are not really aware of what we’ve been trying to do in programs like the citizens toolkit program in Vancouver and the Oakbay Coolkit program, on Vancouver Island is work with a whole neighborhoods of people. So they kind of collectively can begin to not only improve the urban forest conditions in their own  space where they have some agency. Here’s something they can actually do, and they don’t have to wait for the city to do it. But they can also, have those stewardship programs on the public trees; they fill up those Gator bags for the young trees, they water, I’ve been watering our old growth trees or road elms, with a hose with a drip hose in the drought, so try and keep them alive. Because if we can keep those old trees alive and healthy, longer, that makes room for the young trees that are going to take 20 to 30 years to really develop the full ecosystem to come up. If we lose the old trees prematurely, you know, we’re gonna have this gap, and things are gonna get a lot worse. So I feel like it has to be a multifaceted thing. I don’t think Canada has a very good system of and maybe other countries too, of integrating these urban forest plans and strategies, and bringing the community into that in a more serious way. There are some good examples, the Toronto Leaf program, the stuff we’re doing in Oak Bay, right now we’re getting people working on de-paving their yards, and working with the cities on the public roadway to take back some parts of the street, the paving parts of the street, but still allowing access all these kinds of things. So I think the solutions are there. But we did much better engagement of whole communities and stakeholders, Whole neighborhoods. And I’ve just put in a pitch for a new extension program that we’ve started up at UBC, called Climate Action and community engagement. It’s a micro certificate for professionals and community leaders to learn these skills on how could we bring in green infrastructure and other climate solutions through active local climate champions. And it’s trying to get towards shifting culture away from negativity to trees and pave your backyard. So you can park your cars and leaf blowing everything away, and get them into a much more harmonious relationship with nature in a way that reduces the downsides and improves their health. And these can be fun, positive collective action, things to do. And the research shows that people are looking for those kinds of solutions. people worried about climate change. This is something you can do.

Metta Spencer  42:54

Great. Listen, you just said something. You said the magic word “parking.” You know what’s going to happen. We’re going to have electric taxis within five years. And you are not going to want to own a car because it’ll, it’ll be $5,000 a year less expensive to go everywhere with a driverless taxi than to own a car. So there won’t be any need for parking spaces. Because these cars, these taxis, they come in, they pick you up and they drop you off and then they go away. They don’t park. So all of those parking spots along the street, and even people’s garages are these parking pads that they make in their front yards. They can be planted with trees. How about it? i This is my big cause now. Sure, we try to tell people in advance. Look, get ready for it, folks, because there are going to be a lot of empty parking places, even in shopping malls.  And so let’s get ready to plant trees.

Stephen Sheppard  44:14

Well, I do think  if there was sort of formal systematic strategies of maybe this is part of what the Pugwash initiative could do is to is to ask governments — local, provincial and federal — to do the sort of forward projections. I agree with you I think the the space requirements for what we call habitat will will shrink over time and should and we want to expand where we call worm habitat and squirrel habitat, trees pervious area for multiple benefits, not just for biodiversity. So I think that we just need to clear a vision of what this might look like, what the core benefits might be, to see if we how much we can shift. It’s still going to be a lot of sort of public funding, and that comes with all those difficulties and timing and cetera. But the more these things can be systematized and normalized as this is what’s happening in COVID. We had pop up parks all over the place, right. And it demonstrated that you could rapidly take back road space, we put in cafes, we put in little pop up parks, some of those are being made permanent, not enough, in my opinion. But I think this discussion should be pushed forward with and some visioning, I’m really keen on visualization and showing people pictures of their own own block, their own city, their own neighborhood. Here’s how it could look, how do you want it to look?

Metta Spencer  45:54

What is a parking space for parallel parking? It’s, what about 15 feet long? And maybe eight? feet wide? Is that but

Stephen Sheppard  46:06

maybe it’s slightly less? But yeah, something

Metta Spencer  46:08

A little less. But you could put a couple of trees in a space like that, couldn’t you? I mean, I think what David said is, he’s skeptical, but couldn’t you put a tree or two?

Stephen Sheppard  46:20

Well, what they often do these bulge outs, and they do that for traffic calming. And again, there’s lots of benefits have been been shown from doing that in the streets or the middle of the street. And that can expand the space. It may not be enough for a big deciduous tree, which is probably the thing you most want in the long run. But it will certainly improve it over what it is now. And there’s no reason why we couldn’t have small groves, or medium sized trees to expand the canopy beyond the big trees that are already there.

Robin Collins  47:06

I’ve got three questions. Two of them are related to each other. One is on species selection. And I raised this because we had another massive tree drop in our backyard this week from  ice storm. And this is like the sixth tree we’ve lost. And they were all I think 50 to 60  or 70  year old trees. My assumption is that they weren’t necessarily all that unhealthy, per se. But they had reached their life limit. And so maybe they were the wrong trees to have been planted there in the first place. So that first question is on what species should we be looking at, for urban trees? Related to that is the question about and David kind of raised this when he spoke about transpiration? What’s the relative impact of trees from transpiration? That is, the release of water or the circulation of water, let’s say more generally, as compared to canopy that is shading cooling? Because transpiration is cooling effect. Canopy has, do we know what the relative impacts of those?  And the third is this issue of if we think that urban trees are really too minuscule to have a carbon dioxide footprint reduction impact directly that is by absorption of carbon dioxide? To what extent is this reduction of electricity for air conditioning or for heating, if you look at it from any other point of view, reduction of air conditioning usage, by cooling going to have an impact?  I know there’s some suggesting that it’s a relatively small number, but it’s not an irrelevant number. And I circulated the article as a very frightening suggestion that we would expect air conditioning to at least triple  by 2050 because of expected global warming. This would be the equivalent of the combined electricity capacity currently of the United States, the EU and Japan today. — massive amount. So the cooling, if it’s miniscule now, it may actually become more and more significant as we as the world crumbles in front of us.

Stephen Sheppard  49:57

 While I’ll quickly answer the first one, David, maybe you should want to tackle the transpiration one. And then we could jump on to the third one, but just on species selection makes huge difference. And so people in the more advanced municipalities, because this is mostly local government’s responsibility so far in Canada, they’re developing these sort of tree guidelines for selection based partly on sort of soil suitability, certainly on longevity, which is, I think, really your question. But also, drought tolerance is the biggest threat, and the biggest limiter. There’s also sort of heat resilience and cold resilience. But drought is probably the biggest killer. And so there are certain trees that you want to avoid in those kinds of situations where tree fall is a likely hazard in the future. And again, the big caveat is give it the root space and the care and the watering that it needs to maintain that longevity. So, certain trees are just short lived, birches are very short lived. Some of the some of the poplars can be very short lived, alders, things like that. The the bigger deciduous trees that tend to give you the most ecosystem benefits in the long run, shade in summer, and sunlight and heat gain in the winter, tend to have longer life. Life expectancies,  and the more advanced cities are doing life expectancy mapping of different trees and different growth. But it also depends on the pruning. And whether it falls over has to do with the conditions with the roots. Very often there’s a curb, as I said, a meter away from the tree. And that can very often just mean, you’ve got half a root ball rather than one. But it can also be the shape of the trees. Surrey is going to single leader trees, to avoid that splitting of multiple multi-stem trees that are more prone to big branches falling off. It’s good arboriculture to keep those trees in a good form, will also lengthen that that time period. But the certainly species selection guidelines are coming out for trees that are well adapted to those conditions.  David, do you want to tackle the transpiration cooling?

David Price  53:05

Maybe think about the obvious experiment, if you stand under a tree, and you measure the air temperature under the tree, you’ll get a number — let’s say it’s 20 degrees. If you walk 10 meters away, and now you’re in the open and you measure the air temperature, what do you think you’re going to get? It is going to be 20 degrees plus or minus point one of a degree. People who tell you it’s 10 degrees warmer in the open are not measuring the air temperature properly. If after transpiration essentially is the production of latent heat causing cooling to occur. And that’s going to happen and it basically does lower the temperature of the air in the surrounding in the surroundings. So it’s mainly about transpiration and much less about shade. The only caveat is that if you have extensive tree cover, the surface temperature on the soil is going to be lower because the trees have prevented the ground from warming up. So in that regard, under an extensive canopy, yes, the air temperature may well be a degree or two cooler, a lot depends on the wind, if there’s any wind at all, you can pretty much guarantee that warm air is going to feel cooler, but it’s not actually any cooler and come out the other side. It’s a lot of mixing going on all the time. So the one other point I want to make because I realized we’re running out of time. Most of the conversation Stephen has come about you’ve been talking about Vancouver and Victoria. And of course these are very humid zones. And we have you have a whole set of conditions on the West Coast, which are relatively unique within Canada. And we will talk about things like ripping out lawns and putting in trees. That’s great. But now think about what conditions are like in a city on the other side of the Rocky Mountains. especially in Alberta, where you’ve got a fraction of the annual rainfall, the big issue there is drought. And, and in fact, there’s a lot of incentives to not just take out the golf,  the lawn, you take out everything and you Zeroscape the entire yard. Now, that’s going to be a really hot yard on a hot day, because there’s no evaporation really going on at all. But water is such a limited resource, that you really cannot afford to be irrigating your gardens, and you need the water for human consumption and other purposes. So we really have to think about the role of tree cover in urban environments and relate it to the climate and conditions in which those cities are proud. So again, on the east coast, Toronto, maybe the issues are different, I don’t know much about those cities, though I do know in the summer, they can be extremely humid. And when you have a lot of humidity in the air, the trees really can’t transpire that much. Because they need that that vapor pressure gradient in order for the water vapor to be released to the atmosphere. So really, they’re not going to transpire very much. They’re not going to create much additional cooling. And if anything they’re going to add to the local humidity, which will actually make things feel worse under some circumstances.

Metta Spencer  56:24

Peter Meincke has had his hand up.

Peter Meincke  56:26

I just came across two research reports which fascinated me. And I wonder if you’d comment on them. One claims that the city of New York and the forestry and the coverage in New York actually absorbs as much co2 as the city produces. And the other one is a report by the Rocky Mountain Institute, which claims now that the cost benefit, doing a very, very general benefit analysis, including health, of planting trees and absorbing co2 is very high. It’s a very good benefit, you get a lot of benefit for very little cost.

Stephen Sheppard  57:16

I haven’t seen those papers. So be very interesting. Yes, see. And Dave’s probably much more of an expert on carbon sequestration than I am. I’d be surprised by that ratio for New York, I think you’d have to look and see exactly what emissions they’re measuring that are being balanced.

Peter Meincke  57:39

 I was gobsmacked by it. Yes. So what’s the reference?  I’ll send it. I’m sorry. I haven’t got it right here. I can send it to her. She could send it on or something.

Metta Spencer  57:52

I think I’ve seen it. Yeah. Amazing. Okay, well, so we’re informed, but now at least made curious enough to look it up. If you have further thoughts, on our website to save the world.ca — that’s where I post these videos after I’ve edited them. And underneath there, there’s a public comments column. So if you have further thoughts, or you want to dispute something somebody else said, you can always go there and have a discussion. Post your thoughts even, you know, a year from now, I’ll be looking at it a year from now, but you can post it any time there. So if I find any more out about the New York’s co2 ratio, maybe I can post it there. Or we can put some further information about both of those studies because I think they’re both very informative. Okay, you’re right, we have run out of time. And I wish we had another hour or so with you guys. Because I’ve enjoyed every minute of it. And I feel much wiser and smarter than I did an hour ago. So thank you both all of you. Above both of you experts, and and my dear Pugwash friends for an enjoyable afternoon. Take care. Thank you. 



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