The Cooling Effect of Urban Trees
BU URBAN's Ian Smith studies the ways Boston's forests regulate the warming climate
If you’ve ever been stuck outside on a sweltering summer day, you’ve probably ducked under a tree for a reprieve. But a tree’s cooling impact goes way beyond the shade from its canopy.
“Trees are constantly pumping water up from the ground and through their stems, and converting that liquid water into water vapor, which has a cooling effect on the surroundings,” says Ian Smith, a PhD student in BU’s Graduate Program in Urban Biogeoscience & Environmental Health (BU URBAN), who is studying the impact of urban trees on Boston’s temperature.
Smith (CAS’17, GRS’26), who majored in environmental science as an undergraduate at CAS, is fascinated by “the impacts of humans on the environment and the associated impacts of the environment on humans.” As temperatures rise—2021 marked Boston’s hottest year on record—his work is more important than ever.
“Heat waves are the number one cause of weather-related human mortality, so it is very important that cities find a way to cool their residents,” says Pamela Templer, a professor and chair of biology and director of BU URBAN. “In the US, more than 75 percent of people live in cities. We are creating a community of scholars and practitioners interested in using scientific knowledge to make cities a better place.”
Smith worked with his advisor, Lucy Hutyra, a professor of Earth and environment, and Patricia Fabian, an associate professor of environmental health, to quantify the cooling effect of trees in Boston. He created a model that identifies and compares their cooling power in different parts of the city—in Dorchester versus West Roxbury, for example. Smith then compared his data to a health index of communities’ heat vulnerability created by Fabian and research scientist Koen Tieskens.
“You can think of the model I constructed as a supply of cooling across Boston,” says Smith. “And then, on the public health side of things, the map indexing heat vulnerability portrays the demand for cooling.” By comparing the two, Smith pinpointed mismatches in the supply and demand of cooling across the city.
Next, he began identifying where trees get their water. Smith spent the summer of 2021 collecting irrigation water, as well as rain-, ground-, and wastewater. By comparing those samples to tissue samples—small clippings from the trees’ branches—he hopes to estimate what proportion of their water comes from each source.
“It will have important implications for how we manage the health of Boston’s urban forest,” Smith says. For example, if Boston trees are using primarily water from precipitation, trees—and their cooling effect—would be vulnerable to severe droughts brought on by climate change. “We might need to think about different ways of giving those trees water.”
Smith is also studying the impact that tree cover has on the temperature felt by residents across Boston. He’s comparing average summertime temperatures to high-resolution land cover maps that depict tree cover, grass cover, buildings, roads, and other paved surfaces. “We want to be able to say that if you’re able to increase your tree cover by X percent, you can expect an average decrease in the temperature felt by residents by Y percent,” he says.
“Oftentimes, when people try to quantify the services associated with vegetation in cities, they’ll use some metric of greenness,” says Smith. “But we’re going beyond that and identifying that grasses and shrubs, while providing other services, are not necessarily as powerful as trees are in providing that cooling effect.”