by Veronica Westendorff, RLA, ASLA
Preserving Urban Tree Canopies: A Cost-Effective Approach to Mitigating Urban Heat and Climate Change
It’s hot. End of July, early August, humid, southern hot. Not surprising really, but this year we are experiencing smog and air quality issues from more forest fires, heat waves are rolling across the globe, burn units are filling with second degree burn cases from touching the pavement. We build, we cut down, and then we are surprised…
I’ve been following biophilic design on LinkedIn, seeing the comparison of temperatures between solid pavement and permeable pavement with just turf—and a 12° difference in temperature from that single difference in the pavement material. I’ve been researching and writing about urban heat island, how we can use trees to mitigate the heat, and which policies have the greatest success in the opinion of city planners, in order to recommend policies and programs to reduce urban heat island.
Even without the support of research, we know through a lifetime of experiences that we prefer to sit, walk, run, drive, park, and keep our vehicles in the shade of trees. The denser the better, right? And denser shade comes from larger, healthier trees. Older trees. Trees that have space, have been cared for, have been selected to survive in the place that they were planted. Right plant in the right place and all that. I think it’s not difficult to convince people of the importance of saving trees when it’s the beginning of August and our homes are running the air-conditioning non-stop, but development is a complicated thing. After all, we live and work and exist in spaces that were once treed or greened, and we value our lifestyles, our economic growth. I don’t have an answer for that dilemma. I am, however, seeking solutions to these problems, and tree canopy cover is a solution to some of the challenges of increasing temperatures.
One way I have been exploring tree cover is using i-Tree programs to measure the tree cover in different areas across the United States. Specifically, I’ve been using i-Tree Canopy, although there are several options with different degrees of skill needed and detail available. One of my main goals is to use open source and free programs in my research so that they are accessible to anyone in the community, regardless of income or skill set, with access to the internet. With this program, an area can be categorized into multiple different land covers for comparison. Vegetative cover such as trees, shrubs, herbaceous materials, and grasses can be compared to other land covers, which include water, impervious buildings, impervious roads, impervious other, and bare earth. It’s been interesting to use this along with temperature data and look for correlation. I’ll write more about this in a future article, but there is indeed a correlation in most cases.
However, urban heat island involves more than shade from tree cover. Elevation, wind, rain events, and street canyons are just a few of the factors that influence heat, but vegetation is one of the most effective and least costly methods available to manage urban heat. There is enough data to support the importance of tree cover in reducing heat, which leads us to two main questions: how to get more tree cover, and how to save more trees.
In a survey I conducted of 50 cities listed by the American Council for an Energy Efficient Economy (ACEEE) for my dissertation research, with a response rate of 32% (16), I was able to see a pattern of responses that reflected which policies or programs were perceived by planning departments as most effective in addressing urban heat island. These cities used vegetation to impact heat directly or indirectly, 94% through trees and vegetation programs, and 62% used green infrastructure designated programs. Many cities find funding new planting and the subsequent care, watering, maintenance, and in some cases removal and replanting to be cost prohibitive. In these cases, funding through other sources—including public-private partnerships, sponsorships, and community groups—is needed. This is a small sampling of cities, but a good place to start.
There is a less costly way for cities to increase tree coverage, and that is through tree preservation and protection. Saving old, large trees is the best way to keep heat down. This also includes keeping these trees healthy and thriving. Tree protection policies preserve larger trees, heritage trees, and tree-save areas to manage heat on site. I stress this because mitigation of urban heat island through planting on a given site takes years. There is a limit to the size of tree that can be planted with the expectation of survival and growth. A single large maturing tree with a canopy radius of 20 feet can have canopy area of over 2,500 square feet while a three-inch caliper tree might only create light shade over a five-foot radius and 157 sf of canopy area.
So, saving one large tree is worth 15 new trees…at least for another 10-15 years. This is a powerful tool in the reduction of the impacts that growth and development can have on our climate, and very specifically on the heat health of a site. This becomes more difficult in areas of higher density, which are often areas that are trying to provide more affordable housing and development. Balancing the needs of people and ecology can be challenging, but they are not mutually exclusive.
A combination of saving as many large trees as possible on site, using creative design to develop around existing vegetation, and recognizing the value of saving one large tree is greater than replacing it with 10 small ones are your takeaways from this article—it’s a starting point for improving urban heat issues.
Veronica Westendorff, RLA, ASLA, SITES AP, is a registered landscape architect of over 20 years and a PhD candidate in Infrastructure and Environmental Systems, University of North Carolina at Charlotte, where she is researching urban green spaces and ecosystem services. Veronica also serves as co-chair for ASLA’s Sustainable Design & Development Professional Practice Network (PPN).