February is one of Charlotte’s coldest months, but researchers at UNC Charlotte have been investing a lot of time lately thinking about how hot it gets in the summer.
Veronica Westendorff, a landscape architect who’s been studying urban heat islands for six years, believes Charlotte’s tree canopy presents an opportunity to show leadership in a problem faced by cities throughout the United States.
Concrete, asphalt, buildings and other ‘hardscapes’ in cities trap heat and make urban environments far hotter than rural ones, and they prevent urban areas from cooling off at night. Although Charlotte’s tree canopy has taken losses within the last few years, Westendorff is optimistic. Her recent doctorate in infrastructure and environmental systems at UNC Charlotte is based on research on how Charlotte and other cities address urban heat islands.
“Right now we’re still just below 50% tree canopy,” Westendorff said recently. “And I think we could catch it now, and that’s what our leadership has tried to do. It dates back to 2008 when the city started this process, and then we got Sustain Charlotte and TreesCharlotte. They started early enough for us to have an impact, so I very much believe that Charlotte can be a leader in this.”
Deploying vegetation-based solutions to address heat, and strengthening Charlotte ordinances regarding tree cover could serve as a model for other cities with similar climates in North Carolina, including Winston-Salem and Greensboro, Westendorff said.
She surveyed planners in 15 cities throughout the United States, researched 50 cities through the American Council for Energy Efficient Economies, and did additional research on Charlotte. Survey responses from city planners indicated that vegetation was viewed as the most important method for reducing urban heat islands. Solutions could include ordinances regarding protection of trees and shrubs; climate action plans; and green infrastructures such as roof gardens.
Urban heat islands in Charlotte’s ‘wedge and crescent’
Existing data from The Trust for Public Lands and the National Oceanic and Atmospheric Administration show that tree canopy cover is higher in the “wedge” located in south Charlotte, with lower recorded temperatures in summer, and canopy is lower in the “crescent” throughout most of the rest of the city, with higher temperatures in summer. A 2020 report from The Trust indicates that nationwide, areas within a 10-minute walk of a park are as much as 6 degrees cooler in summer than areas beyond that range.
Researchers at UNC Charlotte are now gathering resources to track temperature data in Charlotte on a more granular level, said Katherine Idziorek, a professor of urban design and planning. They include people in geography, earth sciences, engineering, urban planning and meteorology.
“There are a lot of small actions folks can take,” Idziorek said. “They can maintain tree canopy in their yard, help maintain and plant vegetation. The more people can opt for multimodal transportation and reduce greenhouse gas emissions, the better,” Idziorek said. “However, it’s at the city level where policies need to incentivize the kind of development that helps mitigate urban heat islands.”
Surprising solutions
Westendorff’s research discovered several surprising solutions. One was that when homeowners take down a tree, simply replanting another can make a significant difference.
“The most surprising thing wasn’t the new development,” she said. “It was really homeowners and residential lots. That falls into the area of urban infill, but also homeowners taking a tree down because they are afraid a tree is going to fall on their roof or car. The growing number of tree loss on residential lots is the biggest cause of the decline of the tree canopy.”
Micro forests and mobile gardens
Mobile gardens already exist in Charlotte, Westendorff said, and they also make a difference. “If you walk around SouthPark, a lot of their trees are in planters. That’s a great future answer to bringing trees to places that have no room to plant.”
Micro forests are another solution. Beyond SouthPark, other Charlotte neighborhoods including Plaza Midwood and NoDa create micro forests by planting trees and vegetation on very small plots of land, including medians and parking lots, she said. Globally, they’re sometimes called “Miyawaki” forests, named for Japanese ecologist Akira Miyawaki, who developed a way to create fast-growing native forests.
Measure shade, not footage
While many cities have ordinances that require planting trees every 30 or 40 or 60 feet, Westendorff said shade is a more effective way to measure impact on heat. Forty percent coverage of tree canopy to surface area can reduce temperature by 2 to 4 degrees Celsius, she said. TreesCharlotte estimates that Charlotte has 47% canopy cover. But she explained that Charlotte tree ordinance planting requirements in new developments in public areas, including streets and parking, create only 10% coverage.
Trees for the next generation
Westendorff's earliest memory of trees is planting them with her father. “I think about my dad and planting trees with him all the time,” she said. “We were planting fruit trees, and I asked him how long until we would get apples. He would say, ‘We won’t get any, but the next people who buy this house will.’”
Casey Osiecki, Palmer Magri and Joe Wilwerding are students in the James L. Knight School of Communication at Queens University of Charlotte, which provides the news service in support of local community news.