The Field

by Veronica Westendorff, PLA, ASLA, SITES AP

Part 1: Urban Trees as a Means of Reducing UHI Within Cities

Charlotte, North Carolina, is one of the fastest growing areas in the U.S. The largest city in North Carolina, and 22nd largest in the country, Charlotte has an average of 44 new people moving into the metro area each day (Peterson, 2017). Construction within the city and in surrounding towns continues to put pressure on the existing land and ecosystems. This is not unique to Charlotte—all over the United States, development and growth are increasing the size and scale of urban areas, with both beneficial and detrimental effects.

While urbanization increases density, reduces the need for additional infrastructure, creates more efficiencies, and provides jobs, education, and resources, the exchange of land from forests or plains to built surfaces causes a loss of urban ecosystem services. One result is increased heat in urban areas, known as the urban heat island effect (UHI), caused by impervious areas that absorb heat during daylight hours and holds it into the night, releasing it slowly so that the next day starts with higher surface temperatures than the surrounding, less built-up areas. More built areas bring more heat, creating a positive feedback loop that is one of the great challenges cities face.

Why trees?

In terms of “bang for the buck,” urban trees provide the largest variety of ecosystem services and offer the most benefits while improving the resilience of the city. Trees provide aesthetic, cultural, and health benefits (Williams, 2017) and have an annual economic value estimated at $18.3 billion in the U.S. (Peterson et al. 2021). More broadly, three percent of urban forested land can sequester 14% of the carbon generated in the U.S. (Godwin, Chen & Singh, 2015).

However, even as we see the benefits trees provide, it’s essential to recognize that there are also negative impacts to consider (see the table below). With this in mind, it is clear there is no single approach that works for all locations, and this may well be part of the reason cities struggle with enacting landscape ordinances.

Threats to the Urban Tree Canopy

Besides the expansion of built surfaces within and along the edges of cities, trees in urban settings face other challenges. Age and declining health due to urban stress, poor understanding of plant physiology, and pests and diseases all contribute to the decline and loss of tree cover (see photo below). In Charlotte, NC, the large, cathedral ceiling created by the willow oaks has experienced damage, while canker worms and the emerald ash borer has decimated the ash tree population. City maintenance leaves mounded mulch against tree trunks instead of creating reservoirs for water and air at the tree flare, and this increases damage to root systems and allows the introduction of fungus and pests. Storms and ice cause broken limbs and weak trees to fall. These challenges to tree health and overall canopy cover creates fragmentation of important ecosystem services, such as increased heat absorption by bare soil as well as the introduction of new impervious surfaces that exacerbate heat stress on urban populations.

More heat requires more energy to cool homes and businesses which creates…more heat. Trees lost in these ways, and through new development as cities grow, are often mature specimens with broad canopies. While new developments and infrastructure may require new tree plantings and mitigation of lost trees, these are often planted at just 2-3” caliper and typically have a lag time of close to 10 years before the benefits of their ecological services can be felt (Elmqvist et al, 2015). Furthermore, lost trees may be mitigated off site, bringing no benefit to the areas that were affected by the tree canopy loss, including increased UHI. And that’s the big takeaway here: while water and air quality can be mitigated off site, heat reduction cannot. It is a non-transferable issue and has large consequences for human health and the environment. So what can be done and what has been done in the past?

Part 2 of this post reviews programs and policies that have been implemented to regulate tree canopy cover and UHI in the cities included in the American Council for an Energy-Efficient Economy (ACEEE)’s UHI Mitigation State and Local Policy Database.

Sources:

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

Bertram, C., & Rehdanz, K. (2015). The role of urban green space for human well-being. Ecological Economics, 120, 139–152.

Charlotte’s losing its green canopy, despite efforts to save trees. UNC Charlotte Urban Institute | UNC Charlotte.

Elmqvist, T., Setälä, H., Handel, S., van der Ploeg, S., Aronson, J., Blignaut, J., Gómez-Baggethun, E., Nowak, D., Kronenberg, J., & de Groot, R. (2015). Benefits of restoring ecosystem services in urban areas. Current Opinion in Environmental Sustainability, 14, 101–108.

Gómez-Baggethun, E., & Barton, D. N. (2013). Classifying and valuing ecosystem services for urban planning. Ecological Economics, 86, 235–245.

Miner, M. J., Taylor, R. A., Jones, C., & Phelan, P. E. (2017). Efficiency, economics, and the urban heat island. Environment and Urbanization, 29(1), 183–194.

Norton, B. A., Coutts, A. M., Livesley, S. J., Harris, R. J., Hunter, A. M., & Williams, N. S. G. (2015). Planning for cooler cities: A framework to prioritise green infrastructure to mitigate high temperatures in urban landscapes. Landscape and Urban Planning, 134, 127–138.

Peterson, Courtney, et al. Community Forests Prepare for Climate Change. Eos (Washington, D.C.), vol. 102, 2021.

Peterson, Miranda. Making Charlotte a Climate-Ready and Just City. Center for American Progress.

Salmond, Jennifer A., et al. “Health and climate related ecosystem services provided by street trees in the urban environment.” Environmental Health: A Global Access Science Source, vol. 15, no. 43, 8 Mar. 2016.

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

Williams, Florence. The Nature Fix: Why Nature Makes Us Happier, Healthier, and More Creative. First edition., W.W. Norton & Company, 2017.

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.