Why Do We Need City Trees? The Answer is Ecosystem Services!

by Kathleen L. Wolf, Ph.D.

In these challenging economic times, arborists and urban foresters working in both the public and private sectors must often explain why money should be spent on trees. Scientific evidence is one way to build a case to support the urban forest. In recent years a surge of research tells why trees are necessary in our communities.

How do we help connect the knowledge to everyday needs and issues in communities?

One way to share the research is to use the concept of ecosystem services. Ecosystem services are the conditions and processes through which natural systems sustain and fulfill human life, and contribute to human well-being. This article will explain more about ecosystem services and then describe some interesting applications in our region.

Ecosystem services defined

Ecosystem services represents a fundamental shift in how natural systems and resources are defined and valued by human society. Natural assets, such as forests, agricultural lands, shorelines, and seas, have been the sources of essential and economically valuable goods and services throughout human history. Ecosystem services include direct or indirect natural systems benefits and their economic consequences, such as flood protection, pollinator activity, natural filtering of potable water, and climate stability.[i] The term ecosystem services specifically signifies aspects of ecosystems that are valued by people.[ii]

Many ecosystem services descriptions focus on grander rural-to-wildland landscapes, assuming that services are generated beyond the city then ‘delivered’ to urban residents. Nonetheless, we know that ecosystem services are also provided by city trees and urban greening. Here are some examples.

Urban Forest Assessment and Values

The i-Tree suite of analytic tools has been developed by the USDA Forest Service to help communities quantitatively document the benefits and value of the urban forest in terms of energy conservation, carbon storage, stormwater retention, and other ecosystem services. i-Tree Eco is now being applied in the Seattle metro area.

This pilot study offers benefits to communities beyond Seattle. Study results will help us to understand the function and structure of regional forests, as well as to estimate monetary values for ecosystem services provided by regional trees. Ecosystem services results will help communicate urban forest values, and the Seattle experience may pave the way for more efficient use of i-Tree in other communities in the region.

Economic value of forested parks and open spaces

Metro Parks Tacoma is evaluating innovative and timely methods to assess the value of parks so it can more effectively steward park lands and serve users. In a unique application of ecosystem services valuation Metro Parks Tacoma has collaborated with the non-profit Earth Economics to provide a more complete assessment of park value in addition to a more traditional economic impact study.

The valuation report was released in January 2012, and claims that the total ecosystem services asset value of Metro Parks Tacoma parks is between $500 and $750 million.[iii]

Human health and well-being services

There are now accepted categories of ecosystem services – provisioning, regulating, supporting and cultural services. The cultural services include education, recreation, and spiritual experiences.

Nearly 40 years of research across the social sciences has confirmed our hunches about the importance of having nature in cities for livability and quality of life. The Green Cities: Good Health website is being developed at the University of Washington and is a catalog of the urban greening research that documents social, economic, and cultural benefits. The link is: www.greenhealth.washington.edu

The research evidence recognizes the cultural value of urban greening for recreation and learning. But contact with nature supports many other services, such as healing, therapy, and social cohesion. All of these human dimensions are ecosystem services, and have potential economic value. For instance, parks prescription is an emerging program where doctors add routine park walks to a patient’s treatment to improve health, reducing medications use, and providing cost savings to patients and communities.

Insightful individuals have long recognized the benefits provided by having nearby trees and nature. Science supports those intuitions and expands understanding. Ecosystem services is an idea that directly connects benefits knowledge to the needs and values of people.


[i] Daily, G. (ed.). 1997. Nature’s Services: Societal Dependence on Natural Ecosystems. Island Press, Washington, D.C.

[ii] Environmental Protection Agency (EPA). 2009. Valuing the Protection of Ecological Systems and Services, EPA-SAB-09-012. EPA Science Advisory Board, Washington, D.C., 122 pp.

[iii] Information about Metro Parks Tacoma studies of Economic Impact and Ecosystem Services Valuation

Kathy Wolf represents the University of Washington, College of Forest Resources on the Washington Community Forestry Council