The Urban Tree Canopy and Why We Need It!

By: Camryn Fujita

The Popular Bryant Park in New York City. 
Source: Jean-Christophe Benoist, Wikimedia Commons

If you are someone who is lucky enough to have lots of trees in your neighborhood or in your city, have you ever considered what those trees mean to you? Do you see trees as vessels for reducing air pollution? As the solution to mitigating intense summer heat? Opportunities to improve public health? The goal of educating the public on the natural (and free) benefits of urban trees is to open people’s eyes to see trees as much more than urban beautification. As more and more Americans move into urban and suburban areas, improving our urban tree canopy and understanding the ways in which trees enhance our quality of life and protect us from climate change will only become more important.
 
The urban forest refers to trees and vegetation that grow in urban areas. According to the U.S. Forest Service, urban forests include “urban parks, street trees, landscaped boulevards, gardens, greenways, river corridors, wetlands, nature preserves, shelterbelts of trees, and working trees at former industrial sites.” In fact, the U.S. Forest Service estimatesthat 130 million acres of America’s forests are located in cities and towns. When people talk about the urban tree canopy (UTC), they are referring more specifically to the layer of coverage provided by trees and other vegetation when viewed from above. Measuring the amount of area covered by tree canopy is especially important because it indicates how many urban trees there are in an area and how much shade those trees are providing.

An Aerial View of Waikiki
Source: Michelle Maria

Research has consistently supported the idea that trees can improve people’s physical and mental health. Urban trees reduce depression, anxiety, and even improve the ability to focus in people with ADHD. Neighborhood trees also encourage people to spend more time outdoors and thus lead more active lives. Being near nature and even simply looking at trees has been known to lower blood pressure, stress, and make people feel happier and more generous. They also shade our cities and thus reduce the urban heat island effect and protect people from heat-related illness. Furthermore, trees are one of humanity’s best tools to reduce air pollution. A 2014 study in the journal Environmental Pollution used computer simulations and local environmental data to find just how much pollution trees have removed from the air. They found that trees in the United States “removed 17.4 million tonnes (t) of air pollution in 2010, with human health effects valued at 6.8 billion U.S. dollars. Health impacts included the avoidance of more than 850 incidences of human mortality and 670,000 incidences of acute respiratory symptoms.”
The 1999 USDA Forest Service RPA Assessment determined that urban areas had on average, 27-33% UTC coverage. In Honolulu, a study done by Smart Trees Pacific in 2015 found that over the previous four years, the UTC had declined about 5% and covered about 20% of the land. In the past, an UTC of about 40% or higher had been recommended universally. However newer research suggests that while setting UTC goals is important, specific areas and environments require more nuanced approaches to determining the optimal percentage of canopy coverage. Such approaches require measuring the current UTC and where trees are located, estimating the potential percentage of UTC and strategizing where trees might be most optimal to support “identified community priorities” such as improving water quality, providing shade, fostering diverse wildlife, etc., determining what species of trees are optimal, and finally with establishing UTC goals in local legislation or ordinances.
 
Considering the importance of urban trees to community and environmental health, it is important that urban forestry goals are strategic and planned carefully. The U.S. Forest Service recommends that studies of the current UTC in conjunction with other factors such as “impervious surfaces, socioeconomic information, traffic density, and heat island maps” can make the most out of an UTC study. Additionally, involving the citizenry in urban planning with respect to trees can play an important part in strengthening community relationships. UTC analysis, therefore, has the potential to serve as a vital tool for policymakers in helping a city reach its sustainability, social, and economic goals.