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The Coolest City…

By: Erin Hathaway

According to recent study estimating the number of deaths caused by high heat by the end of the century, Louisville, Kentucky ranked as the hottest and deadliest city. With an estimated 19,000 deaths by the year 2099 we need to think about ways to change this number.

Louisville and other urban areas are subject to the Urban Heat Island Effect (UHI) – higher temperatures in an urban area (as compared to surrounding suburban and rural areas) due to the high density and hard dry surfaces which trap the heat. However, the UHI can be reduced by increasing vegetation and tree canopies. Surprisingly, Louisville only has about a 10% tree canopy cover within the urban core, whereas New York City, the largest and most dense city in the country, has a 24% tree canopy cover. New York and other cities are adopting progressive policies to preserve and enhance their urban forests and as a result reducing the UHI. Louisville can adopt similar progressive tree restitution and valuation policies to help control the UHI and reduce the projected number of heat related deaths.

Striking difference between streets in NYC and Louisville. Without more tree cover Louisville will continue heating.  Image courtesy of Erin Hathaway.

Striking difference between streets in NYC and Louisville. Without more tree cover Louisville will continue heating.
Image courtesy of Erin Hathaway.

In New York, city planners recognize that trees are their most valuable environmental asset. In 2007 the City launched the MillionTreesNYC campaign to expand NYC’s urban forest by 20% by planting 1 million trees over the next 10 years. Million Trees NYC is one of the 132 PlaNYC initiatives to make the City more sustainable by 2030. This plan teaches citizens the importance of trees and gets the public involved in the planting and caring for their trees.

In the design and environmental profession, it is easy to recognize the value of trees in an urban environment. They reduce and help regulate temperatures, filter air and water, provide habitat to many creatures, provide flood control, reduce carbon, increase property values, and increase the overall user experience in the landscape. However, not everyone easily recognizes the value of urban trees. As a result, cities have created standard methods to evaluate the value and account for any damages. Tree valuation assigns a dollar value based on the individual characteristics and highlights the social, aesthetic, and ecological value of mature trees.

The New York City Department of Parks and Recreation (NYCDPR) uses tree valuation to quantify and qualify damaged or removed trees and restitution requires any tree under NYCDRP jurisdiction must be replaced if damaged. NYCDPR even assigns a value for invasive species such as the Tree of Heaven. Tree valuation, while valuing all tree species, places an significant emphasis on larger more mature trees as they  sequester carbon, reduce heat island effects, reduce run-off rates, and reduce airborne particulates at exponentially higher rates (than younger trees). For example, a 3-inch caliper cherry cannot replace one mature oak tree. If a tree is damaged or removed as the result of a construction project, then the responsible party must provide restitution, either in the form of money or replacement.

Given the value an urban tree can provide, many cities are adopting progressive tree policies. Programs such as itrees allow communities to inventory, assess, and value their existing trees. This is a starting point to create a policy to preserve and to enhance the urban forest. Trees are a critical piece of our urban environments and should be seen as more than just a component to beautify a city but a tool to combat the changing climate.

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