Motivation and Goals
As our world becomes increasingly urbanized, an understanding of the context, mechanisms, and consequences of urban environments becomes more important. This issue involves both the climatic environment in and around cities and the remnants of the natural world in cities, or what I'll generally call “urban open space.”
Open space means exactly what it sounds like: forests, pastures, and fields in rural areas, and in urban environments, anything ranging from small gardens to large parks. Open space plays important roles in both vast forested watersheds and congested urban areas where even a single tree can brighten someone's day.
Service on a local citizen group, Durham's City and County Open Space and Trails Commission, was my first introduction to the issues behind urban open space. I spent the first couple of years waiting for the commission, as a whole, to be asked important questions by elected officials, but after a while I concluded that nobody was going to tell me or us our role. One part of the commission, the trails committee, had clear and present directions helping make citizen demands for trails throughout the region a reality, but the open space committee had much less citizen input or appreciation.
Our little open space committee clearly needed to figure out something to work towards. Taking stock of the situation, we could see that despite large-scale development, many parts of Durham County remain quite rural with plenty of privately owned open spaces, and dedicated staff seeking long-term preservation. Many local conservation groups protect natural areas quite forcefully, and there exist a few watershed protections (under continuous threat from development) given the recognized drinking water requirements throughout the region. Instead of focusing on rural areas, we started pushing the idea of Urban Open Space with the goal of protecting and developing nature and vegetation within the city. We boldly put (p.vi) forth a resolution asking for city and county support to direct the Planning Department to start work on an Urban Open Space plan with grand ideas of an even greener city.
After some time, I realized that I, for one, couldn't really provide a good cost-benefit argument for preserving and creating natural spaces in the city. Why should the city and county spend scarce money planting trees when schools need more resources, roads and sewers need maintenance, and people suffer harm at the hands of others? I couldn't really answer those concerns or give insight into those trade-offs beyond providing a vague idealistic image. Seeking an answer to that question turned out to be a wide-ranging study, and addressing it requires a broad synthesis of many environmental topics and human issues. Many of these issues involve the relative values of cleaner water versus smarter children versus calmer citizens versus higher taxes. These values lead to choices and compromises that science alone can't make, but science can and must inform elected officials and citizens making the choices. With this book, I intend to summarize that science and provide the synthesis needed to inform a broad range of interested students and citizens while they carry out these difficult compromises.
Although many people presently support planting trees in the name of carbon sequestration and sustainability, city natural spaces also involve economics, ecology, social aspects, and air and water quality. This book summarizes my study of urban environments, the role vegetation and trees play, and costs and benefits humans experience from urban open space.
There are a number of goods, but what resonates most with me concerns the of emotional health. Near the end of the book I show that low-income areas with rental housing have less vegetation, and it may be that by enhancing low-income citizens' lives through the planting of trees and shrubs—an active demonstration of a long-term investment in people's lives—a city can promote positive social interactions right where they're needed most. Of course, it's hard to believe that a lack of trees drives the conditions faced by low-income residents, but some evidence suggests that urban vegetation alleviates social ills. It makes sense. I truly enjoy gardening and love seeing plants regenerate every spring, a love probably instilled in me from growing up on a farm, watching my parents plant things for their future (and mine). As people of the cities become more accustomed to instant access and communication, and more detached from the delayed gratification needed to spend time planting something now with a payoff months (p.vii) or years off in the future, we face a greater challenge in making sure the cities we live in will have 50-year-old trees 60 years from now.
Another important function involves stormwater runoff, a topic I've only barely touched upon here. In my part of the country, rain falls on impervious surfaces and washes off pollutants. This stormwater runoff brings those substances with it into streams where organisms live and into reservoirs that serve people with drinking water: cleaner runoff, cleaner city water. Urban open space can play a role in purifying that water.
A few quick words about the book's approach and structure: When approaching any research question, one faces balancing depth and breadth. Each brief topic I examine warranted independent theses and books, evidenced by the cited primary literature, and the breadth covered here sacrifices discussing many known details. The plots and results I show often reflect just one small part of a greater whole, leaving out the various nuances about a particular topic that an entire publication or research discipline considers. I bring in just enough material to fill a several-hundred-page book that, I believe, introduces the many topics concerning this synthetic area of urban environments. Nevertheless, I hope my summary accurately reflects the author's relevant conclusions, or at least some small part of their study.
Graphs and plots provide the best summary of scientific results, and I use them liberally. Each of the plots I show comes from one or more peer-reviewed or agency-published publication,* and, on occasion, new and unpublished data. I've also shown a clear preference for the underlying data rather than statistical analyses. Some important studies present the results of formal and complicated statistics without showing a single plot of underlying data. It is certainly correct that scientists must perform statistical analyses, but I find the data even more appealing, with the statistical measures backing up and supplementing the data, not replacing them. Although that's my preference, showing only data is not a rule, and situations exist when data plots become unworkable.
Probably the most apparent structural feature I've introduced and used throughout is the “two-page” format, with plots, tables, and diagrams on left-hand, even-numbered pages, and the relevant explanatory text on the facing right-hand pages. As I stated above, my goal is covering the breadth of urban environment topics in a limited space. This format constrains how deeply I examine each topic, though to relax this constraint I use endnotes extremely liberally for citations, (p.viii) comments, and further details. In some ways the format puts concepts on an equal footing: Some topics have many studies, and others have very few, a variation that reflects differences between ease of study, commercial relevance, environmental importance, or human-health motivations. Those differences might not truly represent relative levels of importance to urban environments, and my hope was to cover more equally what I believe are important aspects of urban environments. Further, I think of the rigidly enforced two-page constraint as demanding a prioritization of information rather than expanding sections here and there as more content becomes available.
The format also reflects my visual nature, or my preference for data plots because the data speak for themselves much of the time. Why use squishy words to talk about a concept when the data show it more clearly? Granted, not everyone has such an inclination. For readers uncomfortable with reading graphs I've added an Appendix covering the basic concepts and ideas contained within.
In addition to plots, I provide various calculations throughout the book, many of them rough ones called order-of-magnitude calculations, designed primarily to see if something makes sense, an approach that checks whether different numbers from different concepts generally agree or disagree. This approach originates with my physics training and serves scientists well under many situations. Furthermore, I have struggled with the very odd combinations of measurement units, for example, the volume of stormwater from inches of precipitation per square meter. I've tried to make the units work with our U.S. system, so no matter what one does, that attempt becomes awkward.
In any event, I'll provide comments, clarifications, downloads, corrections, and updates accessible from my Web page, biology.duke.edu/wilson.
There are many people to thank for their help in many different ways: all my fellow Durham Open Space and Trails (DOST) commission members, in particular the open space committee members, and our hard-working staff, especially Jane Korest and Helen Youngblood. It was this connection that ultimately motivated my examination of urban environments.
I greatly appreciate Joe Sexton's efforts, who provided critically important and highly motivating data on the temperature and canopy profiles across Durham County. I'm also grateful to Rob Schick, who coupled canopy information and Census data, which prompted an understanding of environmental inequities.
(p.ix) Many thanks also to the many people I've corresponded with over these several years, including Bob Bornstein, Russ Bowen, John Cox, Grady Dixon, Chris Ellis, Theresa Fisher of Vaisala, Inc., Guido Franco, Chris Geron, Noor Gillani, Peter Groffman, Morgan Grove, Alex Guenther, Pat Halpin, Andy Hansen, Sharon Harlan, Nik Heynen, Alex Johnson, Sujay Kaushal, Rachel Kaplan, Chip Knappenberger, Frances Kuo, Gil Liu, Jim MacDonald, Dan McShea, Bill Morris, Jamie Pearce, Jennifer Peel, Tom Peterson, Chantal Reid, Carl Salk, Sandy Sillman, Jonathan Silverman, Tony Stallins, Will Stefanov, Duane Therriault of Durham GIS, Todd Twigg, Dan Walters, Jeff Wilson, Jennifer Wolch, and Jun Ying. Beyond a doubt I have forgotten important names in this list: my apologies. Numerous anonymous reviewers also put in greatly appreciated efforts that made the book better. I also benefited from the fortitude of my Spring 2007 Ecosystem Services seminar attendees, as well as students in my Cities and Trees classes. Finally, a book like this one could never have been written without the principles of academic freedom that the tenure system provides.
William G. Wilson
(*) In many cases I used a wonderful little open-source JAVA program, Plot Digitizer, freely available from plotdigitizer.sourceforge.net, to extract data from published graphs and replot them here.