How Trees Keep D.C. And Baltimore Cool

12:06 minutes

Three people on stage with a photo of two people planting a tree projected above them.
Ira Flatow with Ryan Alston and Dr. Mike Alonzo in Washington, D.C. Credit: The George Washington University

Springtime is a great reminder of just how beautiful trees can be. Cherry blossoms and magnolias put on a gorgeous show, but trees aren’t just there to look good. They play an important role in absorbing heat, sequestering carbon dioxide, and preventing soil erosion.

Dr. Mike Alonzo, assistant professor of environmental science at American University, is using satellites to determine just how effective urban trees are at keeping neighborhoods cool. He’s been able to track changes to the tree canopy over time, and identify when during the day trees do their best cooling work.

In Baltimore, Ryan Alston with the Baltimore Tree Trust has been working with the community to help residents understand the importance of planting trees. The city has a history of redlining, which affected the number of big trees in historically Black neighborhoods, leading to major differences in how hot certain neighborhoods get in the summer.

Alonzo and Alston join Ira Flatow live on stage at George Washington University to discuss the power of urban trees.

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Segment Guests

Mike Alonzo

Dr. Mike Alonzo is an assistant professor of environmental science at American University in Washington, DC.

Ryan Alston

Ryan Alston is Communications and Outreach Manager for the Baltimore Tree Trust in Baltimore, Maryland.

Segment Transcript

IRA FLATOW: This is “Science Friday.” I’m Ira Flatow, live from the George Washington University in Washington, DC.


Coming up, how food production and conservation intersect in the Chesapeake Bay, and what we can learn when an animal dies at the National Zoo. But first, we really came down to DC at the right time, right? Weather-wise this week, the famous cherry blossoms are full bloom around the Tidal Basin, as are the magnolias, the other flowering trees. It was just (EAST COAST ACCENT) gorgeous, as we say.

Of course, when the trees bloom, they’re beautiful. But the trees do a lot more in our cities than just look good. They play an important role in absorbing heat, sequestering carbon dioxide, preventing soil erosion. And here with me to talk about the importance of trees in our cities are my guests, Dr. Mike Alonzo, Assistant Professor of Environmental Science at American University, and Ryan Alston, Communications Director at the Baltimore Tree Trust. Welcome to “Science Friday.”


RYAN ALSTON: Thank you.

MIKE ALONZO: Thank you.

IRA FLATOW: Ryan, let’s start with the types of trees we’re likely to see when we walk around cities, Baltimore or other places. What kind of trees?

RYAN ALSTON: So our organization plants a very wide selection of tree species. I’m really focused on native species that are really proven to grow and sustain in city areas. And so we’re planting a wide palette. We have redbuds. We have magnolias. I was actually at the nursery, our tree nursery this week, and saw a wonderful stock of all of the different species that we plant. So it’s a very wide selection.

IRA FLATOW: But you choose those that are native to the region? That’s important, right?

RYAN ALSTON: Yes, yeah, very important. As we think about climate change and the different weather changes and patterns, we want to make sure that the trees that we’re planting are going to last specifically in all of the different changes that we’re seeing in the climate.

IRA FLATOW: Mike, what about in DC? What would you see walking around?

MIKE ALONZO: So a lot of the same stuff in Baltimore, for sure. We share the same climate zone, and in fact, overlap a lot in discussions of tree planting strategies. So there’s plenty of oaks, red maples, pin oaks, willow oaks. There’s the redbuds. There’s all the cherries, of course, the people saw this weekend. I could go on.

IRA FLATOW: Yeah, well, Ryan, tell us about the work you mentioned the Baltimore Tree Trust does. Why is it so important?

RYAN ALSTON: Yeah, so we are a local nonprofit focused on increasing the tree canopy across Baltimore City specifically. We understand as an organization that unfortunately, Baltimore City has some of those long-lasting effects of redlining and some of the disenfranchisement. And it’s illustrated in the lack of tree canopy and the lack of green spaces and trees that we see across these neighborhoods.

IRA FLATOW: What is tree canopy? Is that the top parts?

RYAN ALSTON: Yeah, so the tree canopy, if you’re looking at a city from a bird’s-eye view, the tree canopy number is kind of the space of the area of the city that is covered by trees.

IRA FLATOW: And it’s important to people’s health to have these trees?

RYAN ALSTON: Very much. There is, of course, the physical aspects. They give us clean air, clean water, which I’m sure we’ll get into a little later. But yeah, the mental health around having access to green spaces, being able to go to a park and see birds and natural wildlife, all of those benefits is really what we’re trying to provide to residents.

IRA FLATOW: There was a report last year that found redlining, which is a pervasive racist practice in American cities, that prevented Black people from also getting mortgages also had an effect on the tree canopies in different Baltimore neighborhoods. Tell me about that connection.

RYAN ALSTON: Yeah, so in a lot of the talks and presentations that we’re doing, we are able to show the visual of the history of the redlining map, and overlay that with what the current tree canopy map looks like. And a lot of the areas still are affected by those long-standing practices of the disinvestment. And so our job and my job, as communicating the mission of our organization, is to really get residents to understand those practices. Although they’ve been there, they’re what the history of Baltimore is, we have the opportunity to change that specifically through trees.

IRA FLATOW: And Mike, I understand that you use satellites to track the DC trees? What do you learn from that?

MIKE ALONZO: So you can learn all sorts of things from satellites. So just recently, because we can launch these toaster-sized CubeSats into orbit now, we have the opportunity to look at anywhere on Earth every single day. And once you can do that from a satellite, you can pinpoint when every single tree is putting its leaves on, when the cherry trees are blooming, for example, and when the leaves fall off at the end of the season. And so from that, you can start to see which species are sensitive to changes in spring temperature, and which species are kind of freaking out at the end of the year because they’ve run out of water or they get too stressed, and they drop their leaves too early. So there’s a lot of information you can get from these high resolution satellites.

IRA FLATOW: Can you actually count the trees, how many trees–

MIKE ALONZO: This is the big joke– my friends think that the only thing that I do is count trees. And so I’m a little bit triggered by that statement. But the short answer is yes, you can definitely count the trees, particularly in an urban setting where they’re kind of spaced out from each other.

IRA FLATOW: And how many trees are there in DC?

MIKE ALONZO: Oh, that was going to be the follow-up question.

IRA FLATOW: Well, you know I had to ask that question.

MIKE ALONZO: So I’m just going to throw this back to the great data we get from DC’s Urban Forestry Division that has somewhere around 160,000 or 190,000 just street and park trees. Those are laboriously captured by them going out and measuring them. So I think there’s a Forest Service estimate of maybe around six million trees in DC. But that’s a document I haven’t looked at in a little while.

IRA FLATOW: That’s a big number from 160,000.

MIKE ALONZO: Yeah, street trees, though, those are the ones that need the TLC. The ones in the middle of Rock Creek Park can kind of handle their business.

IRA FLATOW: [LAUGHS] And do they actually cool off places where they’re growing? Can you tell from that satellite?

MIKE ALONZO: Yeah, sure. And this is an important one, kind of a place where Ryan and I would intersect some here. So from a satellite, you can infer, based on their size and kind of the shadow they’re casting at different times of day, what sort of cooling benefit they’re going to give the city. There’s also thermal imaging, which is a little bit newer and harder to get this individual tree-level data.

But one thing we do do is we go and we fly drones with a thermal camera. And you can actually see how cool every tree canopy is that you’re monitoring and start to say things like which trees are doing well under the hot sun in the middle of the day, and which ones are shutting down and might not be good to plant under future climate conditions.

IRA FLATOW: Can you both tell from climate change how it’s affected the rate of growth or the seasons changing of tree growth, Mike?

MIKE ALONZO: Yeah, for sure. So what I was talking about before is called phenology. That’s the timing of tree life cycle events, like when the leaves come on. And so for instance, the cherry blossom trees in DC are leafing out– or having peak bloom, I should say, about a week earlier now than they were when they first started recording, which was 70 or 100 years ago. There’s still a lot of inter-annual, year-by-year variability in that. But that’s one clear climate signal.

RYAN ALSTON: Yeah, we have been seeing that– and similar to what you spoke about is maintaining the trees. It’s a lot more difficult to maintain the trees. Just last summer, when we had the long, I think it was 49 or 50 days without rain, it really caused us to make a pivot in our planting season, to stop planting trees and really focus on the care and maintenance, and making sure that those trees are watered because unfortunately, we just weren’t getting that naturally.

MIKE ALONZO: One thing that is happening that’s kind of positive is we’re seeing that trees in urban settings and more open settings in general are growing faster right now than trees in closed forests. So they’re getting these elevated CO2 levels and more direct sunlight– has some positive benefits. And kind of the take-home message is these kind of forest fragments and woodlots are worth preserving even if they’re not the beautiful middle of Rock Creek that you’re hoping for.

IRA FLATOW: Right. And Ryan, are you able to actually feel a change in temperature?

RYAN ALSTON: Very much. The trees that we’re planting– I know there were some pictures earlier– are very large, 200, 300-pound trees, ball and burlap trees. And so when we’re planting them, we’re able to really see an instant impact, especially when they’re nice and leafy. Residents come out and can really feel that instant temperature decline from those trees.

IRA FLATOW: And how do you decide where to plant these trees in Baltimore?

RYAN ALSTON: Yeah, so we use a lot of the data that’s coming out around surface temperature, really focused on some of those minority and disenfranchised communities to inform our planting plans. And so we have a really great team of site assessors who are out walking the streets, plotting points in our GIS system, and really informing our plans.

IRA FLATOW: Do you ever get calls from residents who say, I want a tree on my block?

RYAN ALSTON: Yes, we do. We get calls from residents who want trees. And unfortunately, we get calls from residents who don’t want trees.

IRA FLATOW: Who doesn’t want a tree?

RYAN ALSTON: Who doesn’t want a tree? I know, but it’s really our job to help clear up a lot of those misconceptions that residents unfortunately have about tree maintenance and tree care, tree species. It’s really on us to clear that up and make sure they understand the benefits of why we’re planting these trees.

IRA FLATOW: Is there sort of a critical mass of a canopy, when you got to reach a certain percentage of a canopy where you say, hey, now I know the cooling’s going to happen?

MIKE ALONZO: This is a open research question, for sure. There’s some studies that use surface temperature data that show that the incremental cooling, meaning the cooling that you get from every additional tree, is the highest where there are the least number of trees. And that’s good news. That’s what you hope for because that means it’s not a rich get richer story. It’s the people who need the trees get the trees.

What we found in the midafternoon with air temperature, though, is you need, like, 40% canopy to begin with before you get a drop-off in temperature for additional trees. So that’s not as great. But it depends on the time of day. It depends where you are. It depends on the wind conditions. There are a lot of factors to study.

IRA FLATOW: Ryan, you’re nodding your head on this–

RYAN ALSTON: Yeah, I was going to say I know there are a lot of studies that show that 40% number is a target healthy tree canopy. I know right now, Baltimore is floating around 26%, 27%. So all of the different organizations and city funding organizations are working towards that 40%.

IRA FLATOW: If you identify a part of the city that doesn’t have many trees, what’s your biggest challenge into getting trees planted there?

RYAN ALSTON: Oh, great question. It goes back to the residents. We are very much opt out organization. We give residents the opportunity to say, hey, I see the work you’re doing. I understand the work you’re doing. But ultimately, I don’t want a tree. And so when we get into those communities where there’s a lot of the misconception and the mistrust, it’s hard to battle that and just give trees when we want to give free trees, but understanding that some residents just don’t want that.

IRA FLATOW: Thank you both for taking time to be with us today. Mike Alonzo, Assistant Professor of Environmental Science at American University. Ryan Alston, Communications Director at Baltimore Tree Trust.

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