How Redlining Shaped Baltimore’s Tree Canopy

12:15 minutes

Three men planting a tree sapling into a hole in a park.
Tree planting in Druid Hill Park, Baltimore. Credit: Baltimore Tree Trust

Redlining was pervasive in American cities from the 1930s through the late 1960s. Maps were drawn specifically to ensure that Black people were denied mortgages. These discriminatory practices created a lasting legacy of economic and racial inequality which persists today. 

Less obvious is how redlining has shaped nature and the urban ecosystem. A recent study found that previously redlined neighborhoods in Baltimore have fewer big old trees and lower tree diversity than other parts of the city. These findings are part of the Baltimore Ecosystem Study, a collaborative research project which has tracked the city’s changing urban environment for the past 25 years. 

A man wearing a "Baltimore Tree Trust" vest planting a tree with two young children helping.
Tree planting with students from Yorkwood Elementary. Credit: Baltimore Tree Trust

But it’s not all bad news. The city has a comprehensive tree replanting initiative and is now working to restore its tree canopy. In 2007, Baltimore set a goal to increase the tree cover from 20% to 40% by 2037. Since then, officials have been working closely with non-profit community organizations to plant trees all over the city—especially in previously redlined and otherwise under-served neighborhoods. 

Ira talks with Karin Burghardt, assistant professor of entomology at the University of Maryland about her latest research into the effect of redlining on Baltimore’s tree ecosystem. And later, Ira speaks with Ryan Alston, communications and outreach manager for Baltimore Tree Trust, which has planted over 16,000 trees in the city to date.

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Segment Guests

Karin Burghardt

Karin Burghardt is an assistant professor of Entomology at the University of Maryland in College Park, Maryland.

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. Later in the hour, a Florida team looks for clues to how gophers feed themselves. It is the mystery of the underground root farm. You’re not going to want to miss that. But first, redlining. The practice was pervasive in American cities beginning in the 1930s through the late ’60s. Maps were drawn specifically to ensure that Black people were denied mortgages. And these discriminatory practices created a lasting legacy of economic and racial inequality, which still persists today.

Less obvious, though, is how redlining has shaped nature, the urban ecosystem. A recent study found that previously redlined neighborhoods in Baltimore have fewer big old trees and less tree diversity. These findings are part of the Baltimore Ecosystem Study, a collaborative research project which has tracked the city’s changing urban environment for the last 25 years.

But it’s not all bad news. The city of Baltimore has a comprehensive tree replanting initiative and is working with the community to restore the tree canopy. Joining me now to talk more about the trees and her research is Karin Burghardt, assistant professor of entomology at the University of Maryland, based in College Park, Maryland. Welcome to Science Friday.

KARIN BURGHARDT: Yeah, thanks for having me on.

IRA FLATOW: You’re welcome. Let’s talk about your research. Tell me how the trees in redlined neighborhoods compared to trees in other neighborhoods in Baltimore.

KARIN BURGHARDT: Yeah, so first, if we’re looking at the overall number of trees, we find 25% to 35% fewer species of trees for the same number of trees that we’re looking across. So, basically, you are getting about a third fewer species in neighborhoods that were previously redlined.

IRA FLATOW: Now, is that an important difference?

KARIN BURGHARDT: It is important. So these trees are doing a lot for folks in the city. They are providing shade. They are providing habitat for all kinds of critters that use these– birds, insects. And they also can provide the ability to mitigate storm runoff.

And when we have fewer species in a given area, we are more likely to lose those particular individuals, kind of like a mutual fund. If you have a lot of different species that are in a particular community, then when we have pests that come through or other diseases– think about emerald ash borer or Dutch Elm disease– if everything in a given neighborhood is the same species, then we’re going to lose all of those individuals all at once.

IRA FLATOW: So why has this disparity in the number and the types of trees between the previously redlined neighborhoods and other parts of the city, why has that continued for so many decades?

KARIN BURGHARDT: Well, it’s really hard to say for sure. This is a correlational study, so we’re not able to say that the reason we’re seeing fewer trees is explicitly because of this policy. But there’s all kinds of things that these policies represent. Most importantly, they represent the investment of time and resources on the part of a city.

So when we have policy decisions that are made that decrease home values in a particular area, then it’s probably more likely that there will be less investment in maintaining street trees, which represent a lot of time and effort to keep healthy. Then, over time, you could see this impact on the overall diversity of trees and the number of trees. So another thing we found is that you’re nine times less likely to find a large old tree in a potential location in one of these previously redlined neighborhoods.

IRA FLATOW: Wow, and big trees also help the young trees to survive, too, don’t they?

KARIN BURGHARDT: It’s much harder to establish trees in an area that has very low canopy cover. And in these redlined neighborhoods, we tend to have lower tree canopy cover on average. And when that happens, you have higher temperatures. And those higher temperatures can make the conditions much more challenging for a young tree that’s planted to kind of get their leg up and grow to maturity.

IRA FLATOW: Now the good news, the city has a campaign working with local community groups to replant trees, right? Which we’re going to talk about in a bit. How will this research help the city reach its tree planting goals in the future?

KARIN BURGHARDT: We saw this really interesting increase in the number of small trees that were being planted in previously redlined areas. So we get this really clear signature that the city is reinvesting. But we do have the overuse or dominance of a single species. We’re getting a lot of planting red maple specifically. That kind of raises a little bit of a red flag that maybe we need to use studies like this to make sure that we are targeting our diversity goals in these previously redlined areas so that they are going to be able to withstand pests and other diseases in the future and grow these canopies that can provide services for residents.

IRA FLATOW: Very interesting. And what can other cities learn from your research, cities besides Baltimore?

KARIN BURGHARDT: So in Baltimore, we have red maple that is kind of the dominant species. But a lot of other researchers have found kind of similar reliance on single species in other cities. I also want to point out that 239 other cities in the US were redlined as well. And so this happened in many different cities. And so, as they’re planning these replanting campaigns, thinking about the legacy of these policies, how that might affect the potential survival of trees in these different neighborhoods, could be useful for them as a framework for making decisions about replantings there.

IRA FLATOW: Well, Dr. Burghardt, thank you for taking time to talk with us today.


IRA FLATOW: Karin Burghardt, assistant professor of entomology at the University of Maryland based in College Park. Now that we’ve identified some of the problems facing Baltimore’s tree canopy, I want to turn to some solutions. In 2007, the city of Baltimore set a goal to increase the tree cover from 20% to 40% by 2037. And since then, the city has been working closely with nonprofit community organizations to plant trees all over the city, especially in previously redlined and otherwise underserved neighborhoods.

The nonprofit Baltimore Tree Trust has the biggest footprint in the city. They’ve planted over 16,000 trees so far. Joining me now to tell us more about the organization’s work is my guest, Ryan Alston, communications and outreach manager for Baltimore Tree Trust based in Baltimore. Welcome to Science Friday.

RYAN ALSTON: Thank you so much for having me.

IRA FLATOW: You’re welcome. This ambitious effort to restore Baltimore’s trees as a collaboration with community members, how do you work with communities to identify where and when the trees will be planted?

RYAN ALSTON: Absolutely. So kind of what you spoke about earlier with Baltimore being the kind of blueprint for redlining, we here at the tree trust really acknowledge that. And we have the opportunity to really flip that and give communities and residents a voice when, historically, they’ve kind of been forgotten about and haven’t had that input. And so when we’re planning for our trees, we’re meeting with residents in their neighborhoods, in their communities, and giving them a chance to let us know what their needs really are.

IRA FLATOW: That’s interesting because there was a study that came out in 2019 showing that Detroit residents didn’t want these free trees provided by the city, because they were not consulted in the process. There was a deep mistrust between the city’s Black residents and the mostly white nonprofits overseeing the tree planting. Is there some trust that needs to be rebuilt between local nonprofits and community members in Baltimore, too?

RYAN ALSTON: Absolutely. I mean, we all have a running joke at Baltimore Tree Trust that building tree trust is really in our name and is kind of the foreground of a lot of the work that we’re doing when we’re meeting with residents and having those conversations, definitely understanding that every experience is valid and everyone is going to have a different experience with trees and with city workers and city development. But it’s really our job to connect with residents on that one-on-one basis and really listen.

IRA FLATOW: How do you do that? Do you go right out there into the neighborhoods and talk with people?

RYAN ALSTON: Right before our season starts, we’re working to identify community leaders. These are residents who’ve been in their homes. They’ve seen the changes that has occurred throughout the years. And so we’re using that as a launchpad to start these conversations. These stakeholders are able to connect us to different neighborhood associations or friends of groups or school leaders. We’re able to really branch off of these connections from these longstanding community members and people who are really invested in seeing good change in their neighborhoods.

IRA FLATOW: When you talk to people, do they understand how important trees are to their neighborhoods? I understand that summer is the best time to sell the benefits of trees to residents because of the heat, right?

RYAN ALSTON: Absolutely, that’s definitely one of the selling points. We’re walking with community leaders and residents around their neighborhoods. And having that ability to say this block is 12 to 15 degrees warmer in the summertime based on communities that have a lot more trees, that’s also part of our job is to get them to understand why trees, specifically in urban spaces and urban neighborhoods, are so important.

We’re planting shade. We’re planting trees that are going to help clear the air quality. And that always gets residents to start thinking about trees as something more than just arbitrary planting, but something that can really be part of the neighborhood infrastructure and this city infrastructure.

IRA FLATOW: I understand that one of the issues facing Baltimore’s tree canopy is that there are too many red maples and the lack of tree species diversity. How do you select which types of trees you’re going to plant?

RYAN ALSTON: Yeah, so one of our founding statements, I think, at the Tree Trust is right tree, right place, right? We’re understanding that there are different tree species that are going to do well in urban areas. So we work off of a palette of a few species that are proven to survive in urban settings, and specifically, their native species.

But of course, with climate change, and we’re seeing changes in temperatures and precipitation, that’s causing us to think a little bit more and switch our gears about the trees that we’re planting. So switching more to some Southern species that are able to survive in more of the warmer temperatures in a bit more of a drought and really keeping in mind that tree diversity is important.

IRA FLATOW: Hm. What advice would you give other cities who are working to replant trees?

RYAN ALSTON: I think definitely one piece of advice is that community outreach piece and engaging residents at every level of tree planting. So we’re giving residents the ability to have some input in the planting plans. We’re giving them input in allowing them to come join us for a planting day. And we’re also educating them on how to maintain and care for these trees long after we’re gone.

IRA FLATOW: Well, I want to thank you for taking time to be with us today. And good luck to you to that great city of Baltimore.

RYAN ALSTON: Thank you so much.

IRA FLATOW: Ryan Alston, communications and outreach manager for Baltimore Tree Trust, based, of course, in Baltimore, Maryland.

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