NYC’s Trees: A Natural Defense Against Heat, But Not Equally Shared

IRA FLATOW: This is Science Friday. I’m Ira Flatow. It’s time to check in on the state of science.

SPEAKER 2: This is KERN.

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IRA FLATOW: Local science stories of national significance. New York city’s skyline is dominated by tall skyscrapers. But even though the city is known as a concrete jungle, there is a forest among that jungle. Tree canopy covers about 20% of the city. And woodlands are one of the few natural resources this city has.

Clarissa Diaz reports how New York City is using green space, large and small, to create an urban forest ecosystem in the face of climate change.

CLARISSA DIAZ: In the heart of the Prospect Park Woodlands, the coolness of the breeze is a relief from the heat on a summer’s day. But it’s not just any summer, it’s a pandemic summer.

SPEAKER 4: I feel like Prospect Park is usually packed. But I have seen an influx of groups of people here for sure more since COVID.

CLARISSA DIAZ: Tyena and her friend Cherise have come to the park almost every weekend since the summer began. And for Tienna, it’s quite a trek.

SPEAKER 4: I live in Bushwick, and it takes me a while to get all the way across Brooklyn. We don’t have nature like this around the city. So you need a place that feels local. That feels accessible that you can come to.

CLARISSA DIAZ: For New York City residents cooped up in their apartments during the middle of summer, it’s literal cabin fever.

SPEAKER 5: We live in a pre-war building, and it traps the heat in during the summer months. You can find a space that can call your own, or at least just escape.

SPEAKER 6: It’s cooler than hanging out by the concrete area.

SPEAKER 7: Concrete retains a lot of heat.

CLARISSA DIAZ: On average, urban forests in the city are about five degrees cooler than their surrounding areas according to the US forest service.

DAVID NOWAK: The average tree cover is around 21% across the whole city. And the city contains, from field samples, we estimate about seven million trees.

CLARISSA DIAZ: That’s David Nowak, senior scientist and iTree Team Leader at the US Forest Service. His team has developed an air temperature model, which looks at the weather conditions of New York every hour along with the surface parameters around each site of trees in the city.

DAVID NOWAK: As you move more towards impervious service and less away from trees and natural systems, you tend to get this greater heat island effect.

CLARISSA DIAZ: The urban heat island effect makes swaths of the city with fewer trees hotter in the summer as temperatures. Become more extreme, residents that do not have adequate means of cooling down may become more vulnerable.

DAVID NOWAK: I mean, the cities are hundreds of years old. And they have this infrastructure and this history of being built. They are what they are. It’s pretty probably easier to design parks as you develop a city in the beginning than try to retrofit parks in afterwards. Not that it can’t be done, but there are a lot of problems in doing that.

CLARISSA DIAZ: According to Nowak and other scientists, while more cooling can be achieved around a specific site with a grouping of trees, the spreading out of trees could lower temperatures across the city. Though not as cool as a large park would, but could provide some relief where city dwellers live.

DAVID NOWAK: You have two types of things going on creating these respites of cool air temperatures in the parks people can go to. But also, if you don’t have access to those parks, you want to create cooler air temperatures in and around the spaces where people are living, and individual trees can do that.

CLARISSA DIAZ: One key to making sure trees can thrive in urban environments is maintenance. Street trees, for example, need to be insured enough space for their roots to grow in proper soil conditions. The conditions for growing urban trees are specific to the site and the type of tree being planted.

DAVID NOWAK: You’ve got to look at it from the forest perspective and not necessarily the single tree perspective that you could have many trees of different sizes throughout an area that can create these cooler pockets. Like in a forested stand, we have all sorts of tree sizes that create that cooling environment.

CLARISSA DIAZ: The idea of planting a single tree in a larger system for reforesting the city through time makes the forest perspective unique from typical urban street landscaping, which may only take into account a particular neighborhood or site. What if tree plantings could be planned so that the cooling effects in adjoining neighborhoods could be combined? John Jordan is the director of landscape management at Prospect Park Alliance.

JOHN JORDAN: We’re really one of the first parks in the city that started doing organized forest management. And the Prospect Park Alliance, even at its outset, was really oriented all around saving the forest and forest restoration.

CLARISSA DIAZ: Jordan takes us on a tour of the park’s restoration efforts starting at the edge of the parks 526 acres.

JOHN JORDAN: Those are what we call perimeter woodlands. They’re impacted in a really different way. They have more invasive species, more human impacts, things like that.

CLARISSA DIAZ: They kind of serve as a buffer.

JOHN JORDAN: They serve as a buffer. And from a cooling perspective, they’re super important.

CLARISSA DIAZ: As we walk along the perimeter of the park, there’s more and more shrubbery.

JOHN JORDAN: Once you get a little further, you’ll see there’s a fence and a bunch of green behind those trees. And that’s going to be here the start of our reforestation or aforest station project where, basically, we took a trees and turf area that was having a lot of negative human impacts, and we’re starting to try to convert that back to an actual woodland.

CLARISSA DIAZ: The landscape gradually turns into a more mature urban forest showing distinct growth from one to four years of planting and maintaining trees. Those tulip trees are really doing well. They’re filling in the space. Now, we’re looking at several years of growth of a really fast growing tree. This is meant to be kind of like a meadow planting that would fill in and become forest over time.

CLARISSA DIAZ: Balancing the plantings of slow growing trees and fast growing trees is a delicate craft. Take the oak tree.

JOHN JORDAN: They’re more like long term strata. They’re going to grow slower, but they’re going to live longer. So the tulips probably will die 100 years. Maybe one of these tulips will fall down or get blown over. And the oak will take that opportunity to push up through the next layer.

CLARISSA DIAZ: But as temperatures warm and New York City’s climate zone shifts, some native trees, such as the sugar maple, are becoming more sparse. Species from the south are creeping in. And native trees that have a large range may become more dominant.

JOHN JORDAN: For example, tulip tree that we were looking at earlier grows all the way down to– through North Carolina, South Carolina, and beyond. And also, species like red maple or sweet gum, both native trees of ours, there’s red maple in Florida. And sweet gum grows all the way down to central Mexico.

Species like that will probably play a bigger role. The species that we know aren’t at the end of their range here.

CLARISSA DIAZ: And as for the sugar maple?

JOHN JORDAN: Essentially, we’re still planting them, but not as much as we used to because, I think, we’re a little worried about their longevity.

CLARISSA DIAZ: In an average planting season, Jordan estimates the Alliance plants 300 to 1,000 trees in the woodlands with another 500 to 2,000 shrubs.

JOHN JORDAN: We think that this area was forested pretty much since the park was created, maybe even before. The bigger trees have been here the whole time. What’s really different is the understory.

CLARISSA DIAZ: The Prospect Park Alliance works tirelessly to continue the growth of the woodlands that, in parts, has existed longer than the city itself. For Science Friday, I’m Clarissa Diaz.

IRA FLATOW: Clarissa Diaz is a reporter and designer at New York’s WNYC. She produced that story in partnership with John Upton at Climate Central and the Pulitzer Center on crisis reporting.

Now, we’re going to talk about the science behind planting an urban forest and how you figure out the benefit of a single tree. Let me introduce my guest. Dr. David Nowak, who you heard in that report, is a senior scientist and leader of the iTree Team with the Forest Service. We’ll talk about what that is a little bit later. He is based in Syracuse, New York. Welcome back to Science Friday.

DAVID NOWAK: Thanks, Ira. Glad to be here.

IRA FLATOW: In terms of trees, New York City is known for Central Park and other parks with lots of trees in one place. You talked about this in the piece. But the idea is thinking about a city in terms of an urban forest. Is this a new way of thinking, David?

DAVID NOWAK: It’s not necessarily new. Urban forestry, as a profession, has probably been around through the ’70s and ’80s starting in that time. But it’s becoming more and more popular now in that we have more and more data over the last 10 or 20 years. And we’re learning more about what these trees and our cities do.

IRA FLATOW: So it’s the idea of trying to get an additive effect of trees spread around a city.

DAVID NOWAK: Correct. There’s trees, not only in Central Park and where they’re concentrated and forest stands, but they’re spread out through backyards, along streets, throughout the whole city of New York and other cities. And these trees provide multiple benefits related to cooler air temperatures, removing air pollution, affecting water, affecting our human health as we view that.

So they have multiple benefits and there’s a high diversity of different types of species in different locations throughout cities.

IRA FLATOW: I think a few city dwellers think about their urban forest as having any other benefit besides a nice place to go eat lunch.

DAVID NOWAK: Well, that’s a common one. People appreciate various things. I think they appreciate that, the view, the aesthetics of the forest. I think they can understand cooler air temperatures because, naturally, as the city heats up, we tend to walk in the shade into cooler areas. So we can sense those.

But there’s many other benefits that people may not perceive such as the air being cleaned and water reductions going down, absorption of uv radiation, changes and sound. So just having these leaves out there, the biology of the– and the chemistry of the leaves and the physics of these elements of nature throughout the city affect our lives in many ways.

IRA FLATOW: Let’s talk about how you balance clustering versus spreading trees. Let’s say you have 100 trees. How would you plant them the best way in a city?

DAVID NOWAK: Well, that’s an excellent question. It would depend on, first of all, the space that you have. So to get a greater effect for area, it is best to cluster them if you have the space. So temperatures would probably drop more as you put those 100 trees together so we drop the cooling effect. But then, those 100 trees are limited to just that space that you planted in. So fewer people would probably receive that benefit, but the benefit would be greater at that local scale.

On the other side, if you spread those 100 trees out across the landscape, each individual tree will probably be a lesser impact per tree. But more people might receive those lesser impacts. So it’s a question of, if you have the space, clustering together is good for many reasons. But in many cases, we don’t have the space. So it doesn’t diminish the capacity of the forest to provide services, even if they’re spread apart.

IRA FLATOW: If you plant the trees together, do they sort of help each other out to grow?

DAVID NOWAK: Well, yes and no. In some ways, the services– because they’re planted together– will tend to get greater temperature reduction. But you also tend to block winds more with the greater conglomerations of trees. But on the other side, if you plant them together, they tend to grow slower because trees need light and water to grow. If you have trees in a forest stand, they’re all competing for that limited resource of light and water.

So we tend to have more competition and maybe more mortality, in some ways, in these forested stands. Because it’s hard for a small tree– a seedling that’s established in a forest stand to make it to maturity because it has to compete with not only animals that might eat it, but all the light and water that it needs to grow.

Where singular trees in cities have less competition for light and water. Often people water them. So in some ways, they’re better off and may be healthier– in some cases– being in open growing situations. But often, we put them in narrow pits near roadways, which limits rooting space, which cause problems. There can be vandalism, damage to the trees, pollution. So the other factors that could diminish the health.

There are benefits of having trees together. But there also tends to be a competitive factor of having that forest stand by the trees next to.

IRA FLATOW: I know anybody who goes into a stand of trees knows there’s a cooling benefit that comes with the trees. And it’s more than just shade, right?

DAVID NOWAK: Yeah, it’s definitely more than just shade. There’s a lot of evaporative cooling. So the trees are basically pumping water from the soil then evaporating at the leaf surfaces. And evaporation is a cooling process. You’re feeling two things when you go into the interior stands or stand in the shade of a tree. You’re getting less radiation hitting you. So you’re not heating up as much.

But also, you’re getting that evaporative cooling that’s blowing from the leaves down to you. So you’re getting cooler air temperatures refreshing you as the air passes over you.

IRA FLATOW: So if you took out all the trees in a city or a space, you would radically change the urban feeling there.

DAVID NOWAK: In so many ways you would. Visually, definitely, you would see the change. And that would have a big impact, I think, on people. But the physical environment would change. Temperatures would go up. Wind speeds would go up. It would become– sorry, they would become warmer.

But there’d be more pollution in the atmosphere because the plants are actually absorbing pollution. As it’s transpiring those gases through the leaf surfaces, it’s also taking in air pollution and the gases and removing it at the leaf surface. So many attributes of the environment would change. The acoustics of the environment would change.

There’s so many ways that– again, we maybe not appreciate all them, but once you– you’ll appreciate it once they’re gone.

IRA FLATOW: If you’re an urban planner or a tree specialist, like you are, how do you decide what tree would work best in what city?

DAVID NOWAK: Partly it’s based on local expertise of experience of knowing what trees will survive. But also, we’ve built a tool called iTree. We’re trying to help people match up the trees to not only surviving in their area, but what services they provide. When you plant a tree, you want the tree to survive. So you want the right tree in the right space.

So big trees do better than small trees. Long-lived trees do better than short-lived trees. You have to fit it to the space and the conditions of the soil in the environment around that. Like bigger trees produce more leaves. They’re better at removing air pollution. They’re better at reducing temperatures. Some store carbon better. So when you design your sites, look at what problems you might trying to solve. And then, pick from your palette for which trees do best on that.

We have a series of tools. They can use the tools and do it themselves. That’s the concept of iTree to build this data to let people easily collect data and make their own local assessments and their own local decisions. But in the process of doing this, we like working with the cities to learn what they need to do so we can customize these tools to meet their needs.

IRA FLATOW: So anyone can download this iTree app and figure out the value of their trees.

DAVID NOWAK: It’s for anybody. We try to make it as simple as possible. The myTree app is a single tree app that works on your phone where you can record where you’re at. You put the tree species and size, and it’ll tell you the value of that tree at that location. Very simple tools. We designed that for homeowners and kids. There’s a tool called Design where you can sketch your house and put trees around it. So it’s very site specific for a few trees.

If you start getting into a large number of trees, like the city of New York where we had the sample, anybody could do it. But it tends to be more people that are professionals and managers that want to know about larger systems.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC studios. We’re talking about the urban forest with David Nowak, a senior scientist and leader of the iTree Team with the Forest Service.

When you say the value of a tree, what is the metric that you’re using to measure the value? In terms of what? What is the value?

DAVID NOWAK: From the structure of the forests locally, we try to assess what the tree does, the services provided. We’ll never know the true value because we’ll never be able to assess all of the true values, such as aesthetics is very difficult. What we focus on is pollution removal, changes in water flows and water quality, air temperatures, carbon storage, effects on building energy use.

And many of those we can quantify. Building energy use, if we quantify how many kilowatt hours are being saved due to the evaporative cooling and shading of buildings, we can use local utility costs to determine what the monetary value of that is.

For things like air pollution, we look at changes in pollution concentration and how many people in the area. Then, we use a program from EPA called BenMAP to look at metrics of how changes in concentration affect human health metrics, such as mortality, asthma, and other things– heart attacks. So we use various methods to try and get out how it affects people or their, say, energy use. And then, put a dollar value on those services.

IRA FLATOW: So you can actually measure how much money a tree saves for a city.

DAVID NOWAK: Yes and no. Saving energy is a direct saving. Saving health effects is indirect. The city doesn’t directly see that. But people will see that through better human health, lower cost to health care because they won’t have to go to the doctor as much.

Some are more direct that you can see out of your pocketbook. Some are more indirect savings. And again, they’re all conservative. So there are many things that we haven’t been able to quantify yet.

IRA FLATOW: Tell me what it’s like being a forestry person working in a city or working with cities. What would you want people to know about their urban forests?

DAVID NOWAK: I think working in cities is one of the best places to work because the trees have a direct impact on the lives because the trees are very local. Where in more rural forest stand, there are a lot of trees and very few people. In the urban forest, there are a lot of trees and a lot of people. So you have a direct interaction day to day with these trees. And many people love the trees and are attached to trees and just enjoy being in the forest or the attributes of the forest such as wildlife.

So it’s exciting to be in cities just because of the interaction with people and their environment around them.

IRA FLATOW: Terrific. Thank you very much for taking time to be with us today.

DAVID NOWAK: Thanks, Ira. Much appreciate it.

IRA FLATOW: Dr. David Nowak is a senior scientist and leader of the iTree Team with the Forest Service, and he is based in Syracuse, New York. Also thanks to reporter Clarissa Diaz for bringing us that story.

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