Science Friday - How Trees Keep D.C. And Baltimore Cool

Episode Date: April 9, 2024

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.The transcript for this segment will be available the week after the show airs on sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Trees are beautiful in the spring, but they do so much more than just look nice. 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. It's Tuesday, April 9th, and let me just take a look at my calendar. Yep, today is Science Friday. I'm sci-fi producer Kathleen Davis. If you live in a city, the shade of a tree can be hugely important. important during a hot day. There are a lot of functions that trees do that result in a better

quality of life for us people. But trees aren't equitably distributed in urban areas, and that has consequences. Ira spoke with two experts about urban trees live at the George Washington University in Washington, D.C. We really came down to D.C. 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 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 hear with me to talk about the importance of trees in our cities are my guests. Dr. Michael Lonzo, Assistant Professor of Environmental Science at American University, and Ryan Alston, communications director at the Baltimore Tree Trust. Welcome to Science Friday. Thank you.

Thank you. Ryan, let's start with the types of trees we're likely to see when we walk around cities, Baltimore, other places. What kind of trees? 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 red buds. We have magnolia.

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. But you choose those that are native to the region. Yes. That's important, right? 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. Mike, what about in D.C.? What would you see walking around? So a lot of the same stuff in Baltimore, for sure. You know, we share the same climate zone and, in fact, overlap a lot in discussions of tree planting strategies. So there's plenty of oaks, you know, red maples, pin oaks, willow oaks. There's the red buds.

There's all the cherries, of course, that people saw this weekend. I could go on. Yeah, well, Ryan, tell us about the work. mentioned the Baltimore Tree Trust does. Why is it so important? 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. It's illustrated in the lack of tree canopy and the lack of green spaces and trees that we see across these neighborhoods. What is tree canopy? Is that the top

parts? Yeah, so the tree canopy, thinking about it, 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 cities that is covered by trees. And it's important to people's health to have these trees? Very much. There's, of course, the physical aspects that 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. There was a report last year that came out 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 kind of piece in different Baltimore neighborhoods. Tell me about that connection.

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 reds, lining map and overlay that with what the current tree canopy map looks like. And a lot of the areas still are affected by those longstanding 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, you know, those practices, although they've been there, what the history of Baltimore is, we have the opportunity to change that specifically through trees. And Mike, I understand

that you use satellites to track the D.C. trees? What do you learn from that? So you can learn all sorts of things from satellites. So just recently, because we can launch these toaster-sized cube sats 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, you know, 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. Can you actually count the trees? How many trees? This is the big joke.

This is 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. And how many trees are there in D.C.? Oh, that was going to be the follow-up question. You know, I had to ask that question. So I'm just going to throw this back to the great data we get from D.C.'s Urban Forestry

division that has somewhere around 160 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 6 million trees in D.C., but that's a document I haven't looked at in a little while. That's a big number from 160,000. Yeah, you know, street trees, though, those are the ones that need the TLC, the ones in the middle of Rock Creek Park and kind of handle their business. And do they actually cool off places where they're growing?

Can you tell from that satellite? 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, like, based on their size and their, the 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

actually see how cool every tree canopy is that you're monitoring and start to say things like you know 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. Can you both tell from climate change how it's affected the rate of growth or the seasons changing of tree growth, Mike? Yeah for sure. So I mean what I was talking about before is called phonology, that's the timing of tree life cycle events, like when the leaves come on. And that, so for instance, the cherry blossom trees in D.C. are leafing out, or having pleak 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, you know, inter-annual, like year-by-year variability in that, but that's one clear climate signal. Yeah, we have been seeing that in kind of 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 like 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.

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 forest. 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 like the beautiful middle of Rock Creek that you're hoping for. Right. And Ryan, are you able to actually feel a change in temperature? Very much. The trees that we're planting, I know there are 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. And how do you decide where to plant these trees in Baltimore?

Yeah, so we use a lot of the data that's coming out around surface temperature, really focused on some. 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. Do you ever get calls from residents who say, I want a tree on my block? Yes, we do. We get calls from residents who want trees, and unfortunately we get calls from residents who don't want trees. They're not, it's super, it sounds crazy, right?

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 kind of clear that up and make sure they understand the benefits of why we're planting these trees. Is there sort of a critical mass of a canopy when you've got to reach certain percentage of a canopy where you say, hey, now I know the cooling is going to happen? This is an 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 mid-afternoon with air temperature, though, is you need like 40% canopy to begin with before you get a drop off and temperature for additional trees. So that's not as great. But it's, you know, 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. Ryan, you're nodding. Your head on. 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 percent. So all of the different organizations and city funding organizations are working towards that 40 percent. If you go in, if you identify a part of the city that doesn't have many trees, what's your biggest challenge in getting trees planted there? 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, 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. Thank you both for taking time to be with us today. Michael Alonzo, Assistant Professor of Environmental Science at American University, Ryan Alston Communications Director at Baltimore Tree Trust. And that's all the time that we have for now. Tomorrow, we'll talk about how aquaculture in the Chesapeake Bay has changed over the years. But for now, I'm SciFri producer Kathleen Davis. We'll catch you then.