by Veronica Westendorff, PLA, ASLA, SITES AP

Part 2: A Review of Policies and Programs Addressing UHI Across the US

To learn more about the impacts of climate change on our growing cities, I began to research some of the challenges that urban areas are experiencing as they grow. In addition to housing, offices, and shops for consumer goods and services, roads and other infrastructure are needed to support these communities. This brings more heat, and more consumption of energy, goods, and services in a way that is not sustainable. Last week, I took a look at urban trees as a means of reducing the urban heat island effect (UHI) within cities. Here, I’ll be exploring the question: what policies or programs are in place across the United States to reduce UHI in cities using trees?

The American Council for an Energy-Efficient Economy (ACEEE) created a list of cities in the United States with ordinances that address urban heat island and enhance cities’ energy efficiency, which is an integral part of reducing UHI. I reviewed the 50 cities below, looking at their programs and policies to see which were designed specifically to use trees to mitigate UHI.

Because there are so many variables involved in selecting the right programs and policies, I selected six cities in similar climates as Charlotte, NC. These are summarized below:

Atlanta, GA, has a unique method of gathering data on heat islands, using volunteers that carry sensors with them to collect data on location, temperature, and time of day that citizens upload to a city site. All street corridors are used to increase the tree canopy cover on publicly-maintained lands, with areas of low coverage given priority.

Dallas, TX, requires mitigation of any tree on their “saved” list with varying sizes and numbers of replacement trees. To bring the point home about how valuable these existing trees are, Dallas assigns a class and dollar value per caliper inch of the tree as replacement value. Historic trees shall be replaced at a 3:1 ration with a replacement value of $579 per caliper inch assessed. Less desirable trees have no cost assigned to their removal.

Hartford, CT, is among the top 10 cities experiencing the greatest increase in UHI temperatures. Because inequity in tree canopy cover and access to parks and green spaces has impacts on human health, they have established an Urban Forest Equity and Resilience Grant Program. The funds are used to target areas of priority and include planting, management, and tree care to improve survival rates for new and existing trees.

Louisville, KY, began with an urban tree assessment, posting the data as an interactive map on the city website. This allows citizens to retrieve data at the neighborhood level. From the assessment, Louisville created multiple heat management scenarios: one reviewing the status quo, one looking at using cool materials to reduce UHI, one using greening strategies, and then looking at a combination of both cool and green strategies to reduce UHI.

Raleigh, NC, has adopted a unified development ordinance that includes some strong measures to help with UHI. Tree conservation is required on all sites, with tree protection extended even to urban infill projects (under one acre). Street protective yards are established along road right-of-ways, and a “complete street” design is used for new construction. Within interior areas, one shade tree per 2,000 sf of parking is required. Goals include equity, resilience, sustainability, and accessibility.

Finally, Washington, D.C., uses multiple strategies to address UHI. First, the Sustainable DC Plan aims to reduce UHI through reduction of greenhouse gasses and efficiency. The Climate Ready DC plan aims to expand the tree canopy through preservation, planting goals, landscape ordinances, and focusing on at-risk areas.

Trees can be an effective means of reducing UHI and improving the sustainability of cities; however, tree canopy loss, unlike other urban ecosystem services, is non-transferable and site-specific.

5. Encourage use of sustainability certifications such as LEED, SITES, Energy Star, and other USGBC certifications. This increases efficiency, reduces climate impacts, and relieves city staff from additional work.

6. Encourage additional measures including green infrastructure, green roofs and walls, diversity of plant species, native plantings to reduce water consumption, and large planters whenever possible.

For information of specific cities please see the following websites:

Mitigation of Urban Heat Islands. American Council for an Energy-Efficient Economy (ACEEE).

Luederitz, C., Brink, E., Gralla, F., Hermelingmeier, V., Meyer, M., Niven, L., Panzer, L., Partelow, S., Rau, A.-L., Sasaki, R., Abson, D. J., Lang, D. J., Wamsler, C., & von Wehrden, H. (2015). A review of urban ecosystem services: six key challenges for future research. Ecosystem Services, 14, 98–112.

Morakinyo, T. E., Ouyang, W., Lau, K. K.-L., Ren, C., & Ng, E. (2020). Right tree, right place (urban canyon): Tree species selection approach for optimum urban heat mitigation – development and evaluation. Science of The Total Environment, 719, 137461.

Pataki, D. E., Alberti, M., Cadenasso, M. L., Felson, A. J., McDonnell, M. J., Pincetl, S., Pouyat, R. V., Setälä, H., & Whitlow, T. H. (2021). The Benefits and Limits of Urban Tree Planting for Environmental and Human Health. Frontiers in Ecology and Evolution, 9, 155.

Taha, H. (2000). Meteorological and air quality impacts of heat island mitigation measures in three U.S. cities.

Westendorff, V. E. (2020). Role of trees in mitigating urban heat island in Charlotte, North Carolina, USA. 73–83.

Veronica Westendorff, PLA, ASLA, SITES AP, is a registered landscape architect of over 20 years. Currently working on her dissertation at UNCC in the William Lee States College of Engineering program Infrastructure and Environmental Systems, she is researching urban green spaces and ecosystem services.

A tree is probably the best option for shade, air filtering and wildlife support everywhere, specially in high density urban areas.

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