UC Berkeley

This dissertation examined the role of municipal and nonprofit actors, scientific research, and local geography in the ecological characterization of street trees in the planning and policy arena of three northern California cities between 1980 and 2008. During this time period, the discourse of ecosystem services such as carbon sequestration, stormwater runoff management, criteria air pollutant reduction, avoidance of energy use and energy savings and thus reduction in power plant emissions, and wildlife habitat provision has been applied to street trees. Municipal agencies and nonprofit organizations have engaged in policies, programs, and activities that are increasingly characterizing street trees by the ecosystem functions they can provide; this is what I call the ecological street tree. Since trees have been planted along city streets, they have provided ecosystem services. Yet, over the last 30 years, two actions began occurring more systematically: (1) the ecosystem functions provided by street trees were incorporated into planning documents and activities and (2) researchers began publishing scientific evidence to support policy and advocacy claims about the environmental services provided by street trees.

To measure and analyze the emergence of the ecological street tree, a multiple-case study of three Northern California cities was conducted. One of the strengths of the case study, proposed Yin in 2003, is its "methodological versatility," i.e. multiple methods and sources of evidence can be incorporated into an overall strategy. Furthermore, the dissertation met Yin's criteria for using a case study strategy: (1) the study propositions were framed as "how" and "why" questions; (2) the phenomenon could not be manipulated by me; and (3) the ecological street tree phenomenon are contemporaneous. The study relied on multiple sources of evidence such as municipal and nonprofit reports and plans, nonprofit newsletters, newspaper articles, and interview transcripts. Computer-aided content analysis of nonprofit newsletters and document analysis of municipal and nonprofit reports and plans and newspaper articles were used to track the emergence of the ecological street tree. Transcripts from face-to-face qualitative interviews were also analyzed. Qualitative interviews were used in this project because I required in-depth information from the individuals involved in the conceptualization of the ecological street tree. Furthermore, there are few studies on how and why this conceptualization has taken place.

This dissertation asked six questions:

1. Has there been a rise in the ecological characterization of the street tree?

2. What is the role of the urban forest nonprofit?

3. Has the concept of the ecological street tree been mainstreamed through the nonprofit's newsletter?

4. How is the production of research evidence implicated in mainstreaming the ecological street tree?

5. If different services are salient to different cities, what factors account for this difference?

6. What strategies are used by different actors, in different cities to capture tree-based ecosystem services?

Based on a cross-case analysis of data from Sacramento, Palo Alto, and San Francisco, the main conclusions of this dissertation are as follows:

* Between 1980 and 2008, there was a rise in the ecological characterization of street trees in all three cities.

* The urban forest nonprofit has played a role in mainstreaming the ecological street tree, but this role varies in strength among the cities. Also, the nonprofit has not acted alone. Municipal agencies are part of the network of actors advancing the ecological value of street trees.

* The newsletter is not the nonprofit's primary mode of communicating the ecosystem benefits of street trees; it is one mode among a "landscape of communications".

* The production and dissemination of urban forest research was critical to legitimizing the ecological street tree.

* Different services were salient to different cities and contributing factors included climate, geography, infrastructure, culture, and the history of urban forestry development in each city.

* Different strategies such as policies and reports, program development and activities, funding streams, and research collaboration were used capture street tree-based ecosystem services.