A Guide to Washington State’s Urban Tree Canopy

A Guide to Washington State’s Urban Tree Canopy

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Charles Brun, Regional Horticulture Specialist, Washington State University, Catherine Daniels, Pesticide Coordinator, Washington State University, Tim Kohlhauff, Urban Horticulture Extension Coordinator, Washington State University
More than 80% of the nation’s population lives in urban areas. The urban tree canopy (UTC) is an essential part of an urban areas’ infrastructure—an ecosystem characterized by trees and other vegetation in association with people and their developments. This publication outlines the many environmental benefits the UTC provides as well as recommendations for expanding it.
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This publication explores the most important environmental, economic, and social benefits of a healthy urban tree canopy across the entire state of Washington. It provides data on 33 different genera, species, and notable cultivars of both deciduous and coniferous trees. It includes a very extensive list of references that can be utilized for further study.

The urban canopy enhances the environment, increases community attractiveness and fosters community civic pride, while balancing economic growth with environmental quality and well-being (McPherson et al. 2002). Use this publication to select, preserve, and expand the urban tree canopy.

Define the Urban Tree Canopy

The urban tree canopy (UTC) consists of publicly and privately owned trees within an urban area, including those alongside streets, in home landscapes, in schools, parks, golf courses, and cemeteries, as well as any remaining rural forest (Safford et al. 2015). Simply stated, it is an ecosystem characterized by the presence of trees and other vegetation in association with people and their developments (Nowak et al. 2010). Urban foresters understand the art, science, and technology of managing trees and natural systems for the health and well-being of communities. As there are differences in ownership between all the different types of property (i.e., both public and private), there is no one way to manage the urban forest. Urban planners typically divide urban trees into three categories: yard trees, street trees, and trees for large public spaces. Street trees are often found in the public right of way (ROW), which is defined as the area between the curb and the sidewalk. In this publication, deciduous trees are classified into three sizes, following protocols used by the US Forest Service (Green Cities Research Alliance 2012) and based on their mature size: less than 30 feet, 30–50 feet, and over 50 feet.

Environmental Benefits

With more than 80% of the nation’s population living in urban areas (US Census Bureau 2010), it is imperative that the UTC is considered an essential part of the green infrastructure (Figure 1). In Washington State, a diverse UTC can serve to complement the broad acres of deep green forests (firs and pines) that lend themselves to the moniker the Evergreen State. In urban areas, people plant trees for their long lives, their architectural shapes, and their beauty.

Figure 1. Deciduous trees have been planted on the south side of these urban apartment buildings in Seattle.

Climate Change

The terms climate change and global warming are often used interchangeably, though the former is considered more accurate as there are other changes besides temperature increases (EPA 2013). Scientists refer to the increasing atmospheric levels of carbon dioxide as the leading greenhouse gas influencing climate change. Trees can take up carbon dioxide and release oxygen in the process of photosynthesis. Carbon storage and sequestration provided by trees has been addressed in a number of studies. Research foresters have collected data on urban forests, including tree species, diameter at breast height, tree height, crown width, and overall tree health. The data collected from urban forests can be compared with native forests. Cities produce 40–70% of all greenhouse gas emissions as they consume resources for energy, infrastructure, and transportation (Safford et al. 2015).

Carbon storage and sequestration

There are two different terms used in climate change studies examining carbon utilization by trees. The term carbon storage refers to the accumulation of carbon in both the aboveground and belowground biomass of the trees over many years (McPherson et al. 2013). Carbon storage is equal to one half of the biomass of each tree (Nowak et al. 2016). As trees increase in size, they store more carbon (Figure 2). The term carbon sequestration is used to describe the carbon stored aboveground and belowground over a single season. The rate of sequestration is typically expressed as the pounds of CO2 absorbed by a tree each year (McPherson et al. 2007). In a study conducted in Chicago’s urban forest, tree vegetation was found to store 7 million tons of atmospheric CO2 per year and sequester 155,000 tons of CO2 annually (Nowak 1994). In a



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