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The Inflation Reduction Act included $1.5 billion for the U.S. Forest Service’s Urban and Community Forestry Program; the first of those funds were allocated to U.S. states and territories in April of 2023, and another billion will be distributed in the coming months. Research suggests that adding trees in cities has benefits for the environment and for human health.
On Thursday, May 4, 2023, SciLine interviewed: Dr. Vivek Shandas, a professor and the director of the Sustaining Urban Places Research Lab at Portland State University. He spoke about topics including: the effects of trees on urban heat and air pollution; what research has shown about how urban forestry affects human health; evidence around planting urban trees as carbon sinks; racial and income disparities between neighborhoods with more and fewer trees; and emerging state and local policies around urban tree planting.
VIVEK SHANDAS: My name is Vivek Shandas. I’m a professor of climate adaptation at Portland State University. And I study the intersection of built environment, environmental health, and climate change in relation to historic planning policies in cities.
Interview with SciLine
How do trees affect urban heat?
VIVEK SHANDAS: Trees are one of the original air conditioning systems for the Earth. We know that. They, along with water and wind—and they can reduce temperatures in cities by upwards of 20 degrees Fahrenheit. In some cases, green spaces along a building wall, such as a green wall can reduce temperatures by upwards of 50 degrees Fahrenheit. So, it’s fair to say that green spaces through their shade, shading of sidewalks, or shading of walls can reduce temperatures. They’re also evapotranspiring lots of water by pulling it up through their roots and transpiring it through their leaves. And that also adds directly to the cooling of the surrounding environment.
How do trees affect air pollution in cities?
VIVEK SHANDAS: Trees really work in two primary ways: They offer a means by which particles in the air can deposit on the leaves or needles, and they’re also able to uptake gases through small little holes—holes called stomata—on the underside of leaves that actually bring in some of these gases that are produced, either from cars or other forms of combustion. And they’re really able to reduce air pollution through those two processes. I will note that some trees also can shade parking lots during hot days. And that reduces the evaporation of gas in gas tanks, which often creates what’s called volatile organic compounds. So, it really underscores the need for thinking about where the trees are in relation to what services that they’re providing us.
What is known about how city trees influence human health?
VIVEK SHANDAS: Trees are really helpful to human health in four different ways. The first is physical health. They’re really able to improve our cardiovascular as well as respiratory health through improving air quality, as well as shading communities from extreme heat. Second way is trees help social health through building social cohesion among communities. We see several studies that indicate that communities that live near forests, or parks, or even large number of street trees are much more connected socially. They spend more time together. They trust each other more. And that’s different than those communities that don’t live around a lot of trees. The third is around mental health. We’ve seen several studies come out that show that issues of anxiety, mood, depression are all alleviated through being around trees. In fact, there’s some cultures where studies have occurred, where we can find that people doing forest bathing—a practice that’s been common in many parts of Asia—actually really helps in addressing and improving the sympathetic nervous system, which really has to do with body repair and enabling our immune system to respond more effectively. And then finally, one area that has got a lot of evidence is around birth outcomes. We’re finding that trees, forests, and green spaces generally are really helpful for pregnant mothers because we see better birth outcomes when mothers are around trees and spending more time in green spaces—whether at their home or even outside of their home in parks and other green spaces in the community.
Do trees in cities absorb greenhouse gases?
VIVEK SHANDAS: Trees are really helpful in terms of sequestering carbon in their physical kind of trunk and their leaves and their roots. And they’re also amazing in terms of keeping carbon in the soil. So, they’re helpful in terms of greenhouse gases. If you compare it to a vast forest, it pales in comparison. Though trees in the city play an arguably smaller though important role in terms of keeping carbon in the soil as well as sequestering it in the trunks and leaves in different parts of the tree as well.
Are there racial, income, or other disparities among neighborhoods with more or fewer trees?
VIVEK SHANDAS: We’ve seen over the decades that the distribution of trees is very inequitable across our cities. And that’s in large part due to historic segregation policies that have really baked in these vast spaces of land that are sealed up by asphalt and concrete where it’s hard to get a tree into the ground. And those policies that are still casting a long shadow, in terms of being able to right some of those historic wrongs, is right at the center of a lot of the investments that are currently being made in cities to improve and expand the tree canopy in historically segregated and marginalized areas of cities. We know, for example, that in a place like L.A., 19% of the trees are in areas where only 1% of the population live. And we really need to be thinking about how do we bring green spaces with communities, for communities into places that haven’t historically had a lot of green space. It’s not an easy puzzle, but it’s something I think we’re all really eager to try to figure out.
What kinds of state or local policies exist for urban forestry? What do we know about their effects?
We do know that places that have a dedicated urban forestry division—ideally an urban forester—and resources to be able to do programming, in that places that have urban forestry management plans, places that have good data on where the trees are, and what condition those trees in—usually fare much better than those places that don’t have good data or don’t have dedicated governance structures to be able to manage all of the different green spaces in a city. So, there are many factors that go into ensuring that an urban forest is doing well. And the effectiveness is largely in part based on having these factors really align.
What roles can trees play in urban design as the climate changes?
VIVEK SHANDAS: We’ve often pitted the tree against the building. And this is a narrative that comes from a long history of city design and development. And what I’d really like to convey is that this is really a question about how we’re going about designing our cities. A lot of the ability to accommodate green space in our cities is about how we’re thinking about our homes, our buildings, our roads, and whether we would allow space for trees and for greenery. As the climate warms, we’re going to see a lot more hotter days, we’re going to see a lot wetter winters throughout the country, and being able to think about trees as a front-line defense for those hotter summers, wetter winters will be a really important point to underscore, I think, going forward.
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