Evaluating the impact of government land use policies on tree canopy coverage

Abstract

Many cities around the world are experiencing the negative effects associated with not sustaining a sufficient level of tree canopy coverage. Tree canopy provides environmental benefits such as clean water and air, erosion prevention, climate control, and native species habitat and provides economic benefits such as higher housing values and lower energy expenditures. We study local government policies in a large U.S. metropolitan area (the Atlanta Metropolitan Statistical Area) to find which policies perform the best at preserving or increasing urban forests. Empirical analysis reveals that a set of effective tree ordinance clauses, zoning ordinances, and having high quality smart growth projects in the community all help in preserving tree canopy in economically and environmentally meaningful amounts. Other actions, such as simply having a tree ordinance, designating a key management person in charge of tree programs, the presence of a tree board, and multiple communication channels were shown to be ineffective for our data set. Because benefits from tree canopy accrue to the local government's budget, to residents and to business owners, the entire community should gain from the passage of effective policies to preserve their local tree canopy. Estimated economic benefits from preserving tree canopy through an effective set of public policies are in the range of $10–15 million annually in an average county, mostly due to savings on stormwater management.

Introduction

Many cities around the world are experiencing the negative effects associated with not sustaining a sufficient level of tree canopy coverage. Trees and tree canopy play a crucial role in the environment, providing benefits such as clean water and air, erosion prevention, climate control, and sustained ecological resources and native species habitat (cf., Taha, 1996, Scott et al., 1999). Additionally, trees and tree canopy play an economic role by increasing housing values, alleviating expenditures related to erosion destruction, decreasing spending on sewer standards, increasing energy efficiency, and reducing medical costs related to health issues, such as asthma, that are associated with environmental degradation (Georgia Forestry Commission, 2006, McPherson et al., 2005).

In Europe, where realizing the importance of urban forests has lagged a little behind the U.S., increasing urban forests is now a high priority in Belgium, Denmark, Ireland, The Netherlands, and the United Kingdom (Konijnendijk, 2003). Also, a new ethos is emerging in Europe where urban forests are expected to serve societal needs rather than just produce lumber (Wiersum, 1999). However, one should note that this study examines tree canopy preservation in an American urban/suburban setting.¹ This would include urban forests, but also would include trees in the yards of private homes which would fall outside the common European concept of urban forestry (Ireland and the U.K. do include private trees under some policies²). In urban settings, public trees and private trees often offset each other. Planning codes which pertain to public spaces reference zoning codes with site design requirements (cf. the authors’ local Athens-Clarke County Planning Code, 2009). These cross references in code mean that public and private urban forest cover are highly intertwined and often one impacts the other (Zhu and Zhang, 2008). For these reasons we are interested in trees of all types – public trees and private trees, natural forests, planted forests, trees in private yards – and the effect of local government policies which preserve or encourage an increase in tree numbers and tree canopy.

To study which local government policies are the most successful in preserving tree canopy, an empirical study was performed using Greater Metropolitan Atlanta (the ninth largest metropolitan statistical area in the U.S., consisting of 28 counties each with a government, over 21,000 km² and approximately 5 million people) as the study area. Atlanta has experienced explosive population growth: 27% from 1970 to 1980, 33% from 1980 to 1990, and 39% from 1990 to 2000 (CensusScope, 2006). The result of this population growth has been considerable land conversion, with a gain of 12 ha per day of impervious surface from 1991 to 2001. This land conversion was accompanied by significant loss of tree canopy³ coverage, averaging a loss of 23 ha per day from 1991 to 2001 (Natural Resources Spatial Analysis Laboratory, 2007). These trends are troubling, given the environmental and economic benefits of tree canopy listed above.

A significant problem that may affect tree canopy loss is that many relatively rural counties in the Atlanta region are incurring high rates of population growth that they historically have not experienced. Excess housing demand has led homeowners to settle in neighboring counties where housing is both available and affordable. Although expanding land development due to population growth is inevitable, many of the urbanizing counties in which this growth is occurring face the same dilemma as metropolitan fringe counties all over the country: the need to address growth issues where the demand and/or resources to implement such practices previously did not exist (Daniels, 1999). For example, lenient land use policies such as low-density and single-use zoning, lack of impact fees on development, and minimum lot-size requirements are implemented in a number of counties in the Atlanta Metropolitan Statistical Area (Brookings, 2000, Giles et al., 1980, Heim, 2001). However, the complexity of administration through elected government officials, difficulty in changing administrative structures, and developer opposition to implementing more stringent standards are all challenges that impede county policy makers from instituting better conservation land use policies (Olsen, 2000).

In order for policy makers such as county commissioners to make the most efficient and effective policy choices, they must be supplied with information concerning both the benefits of tree canopy and the influence of policies that they can create to preserve it. An abundant amount of studies have examined the benefit of tree canopy (see Center for Urban Forestry Research, 2008, for a collection); however, to date, few studies have analyzed the influence of policies to preserve it.

Given this need, the central question of this study is: how much benefit does government intervention – through land use policies – have in reducing tree canopy loss (public and private combined) throughout the 28-county metropolitan Atlanta area? To answer this question, a variety of land use and tree protection policies are examined to see which can be empirically linked to the protection of tree canopy. The results of our empirical study should prove useful to policy makers and environmental advocates who wish to preserve tree canopy but need to know what policies are actually effective as opposed to those which look good on paper but do not actually produce results. Choosing better policies will be of benefit in the United States. In Europe, where very few cities outside of the United Kingdom and Ireland have comprehensive local urban forest policies, these recommendations could serve as the foundation for writing the first such policies (Konijnendijk, 1999).