ASSESSING THE VALUE OF URBAN FORESTS IN THE UNITED _,,_3 STATES

Transcription

1 FVUR }uai table ASSESSING THE VALUE OF URBAN FORESTS IN THE UNITED _,,_3 STATES David J. Nowak, John F. Dwyer, and Daniel E. Crane,, m,,2._.- ABSTRACT. Urban forests have value to society based upon the structure of the physical. <--. resource (e.g., compensation required due to the loss of trees) and tree functions (e.g., air.-. Rill-, pollution removal by trees). Structural and functional values were estimated for urban forests in the contiguous United States. The estimated structural value of U.S. urban forests is $2.4 trillion..o,_ The estimated national value for carbon storage is $14.3 billion, with an annual carbon <--. '- -" sequestration value of $460 million. _ =_,. g O O "1"1 _-. KEY WORDS. urban forestry, global climate change, carbon dioxide, urban forest value c_ -. O Urban areas in the contiguous United States doubled in area between 1969 and 1994, and._ =c -,..._" currently occupy 3.5% of the land base with an average tree cover of 27.1% (Dwyer et al. 2000; _ g m Nowak et al. 2001a). Although urban areas continue to expand and urban forests (all trees within _9-_m urban areas) play a significant role in environment quality and human health, little is known _o about this important resource or its value to society. Understanding the value of an urban forest can giye managers and planners a basis with which to develop and evaluate programs for managing urban trees. Urban forests can be valued on the basis of the structure of the physical resource. The structural or compensatory value is based in part on replacement costs and the compensation to owners for tree loss. This value provides an approximation of the structural value of a tree with a specific _ _> species, size, condition, and location..t7. m.. The urban forest also can be valued on the basis of tree functions (e.g., esthetics, pollution _" _7., -n o removal, temperature modification). Functional values are indirectly related to the structural value of the tree. Generally, the greater the structural value of the forest (e.g., increased numbers! of trees and tree sizes), the greater its ability to produce functional benefits. Structural value is._ based on the structure in place as an asset, while the functional value is an annual value based on c,_o the functions of the particular structure. -n-" = o 8 g O_. O To illustrate these types of tree value, consider a factory with a replacement cost of $1 million, _ that produces 10,000 widgets per year with an annual net profit of $100,000. The value of the k_-n o-v o physical structure of the factory is based on the cost to rebuild or replace the factory with a similar structure. However, the factory has an additional value based on the profits from factory -4 g _.-- outputs. The value of the factory structure ($1 million) is comparable to the structural value of =g the forest. The net profit ($100,000/yr) is analogous to the functional value of the forest. Trees _ o_ can have negative functional values (e.g., they can contribute to increases in annual building _ c? energy use in certain locations), which are analogous to monetary losses in factories (Nowak et _=" al. in review). _- 237

2 An example of one functional value is carbon sequestration by urban trees. As urban forests both sequester carbon dioxide (CO2), and affect the emission of this dominant greenhouse gas (e.g., through building energy conservation and consequent reduction of emissions from power plants), urban forests can play a critical role in helping combat increasing levels of atmospheric CO2. The purpose of this paper is to summarize results from national estimates of the structural value of urban forests 8 and the functional values associated with carbon storage and sequestration by urban trees (Nowak and Crane 2001). METHODS Field data were used to determine the urban forest structure (e.g., tree species composition and number of trees on all land uses) of eight cities: Atlanta, GA, Baltimore, MD, Boston, MA, Jersey City, NJ, New York, NY, Oakland, CA (Nowak 1993a, b), Philadelphia, PA, and Syracuse, NY. These data comprise the available data on urban forest structure, value, and carbon storage and sequestration by U.S. urban forests. Tree data (unpublished for seven of the eight cities) were collected from 1996 to 1999 and analyzed using the Urban Forest Effects (UFORE) model based on a stratified random sample of approximately 200 plots (0.04-ha) per city (Nowak and Crane 2000). Data were obtained on location, species, stem diameter at 1.37 m above the ground (dbh), tree and crown height, crown width, and canopy condition. Carbon storage and annual sequestration rates for trees were based on biomass, growth, and mortality estimates described in Nowak and Crane (2001). To estimate the monetary value associated with carbon storage and sequestration of urban trees, carbon values were multiplied by $20.3 per metric ton of carbon based on the estimated marginal social costs of CO2 emissions (Fankhauser 1994). The structural or compensatory value of urban trees was calculated based on procedures of the Council of Tree and Landscape Appraisers (CTLA) (1992). These procedures are used routinely in determining monetary settlement for damage or death of plants through litigation, insurance claims, loss of property value for income tax deductions, and real estate assessments. Compensatory value is based in part on the replacement cost of a similar tree; it is an estimate of the amount of money the tree owner should be compensated for tree loss. CTLA compensatory values are based on four tree and site characteristics: tree-trunk area (crosssectional area at dbh), species, condition, and location. Trunk area and species are used to determine the basic value, which is then multiplied by condition and location ratings (0-1) to obtain the final estimated compensatory value of the tree. Detailed valuation methods are given in Nowak et al. (2001 b)8. Data for individual trees in each city were used to determine total structural and carbon s NOWAK, D.J., D.E. CRANE, andj.f. DWYER. Compensatoryvalueof urban trees in the United States. In review in the Journal ofarboriculture. 238

3 sequestration values for the city. To estimate the structural and carbon value of urban forests nationally, total structural and carbon values of each city that had field data were divided by total city tree cover (m2) to determine the average structural and carbon value per unit tree cover (dollars per me). These median standardized values were multiplied by total urban tree cover in the contiguous United States (Dwyer et al. 2000; Nowak et al. 2001a) to obtain estimated structural and carbon values of urban trees nationwide. Estimates of tree cover were based on 1991 advanced very high-resolution radiometer data (Zhu 1994). The lowest and highest standardized values were used to estimate the potential range of national values. RESULTS Structural values of urban forests ranged from $101 million in Jersey City to $5.2 billion in New York City (Table 1). On the basis of individual city compensatory values per m 2 of tree cover, the estimated structural value of urban trees in the contiguous United States is $2.42 trillion (bounds: $1.75 to $4.85 trillion). The value of carbon storage for individual cities ranged from $0.4 million in Jersey City to $24.9 million in New York City (Table 1). The estimated carbon storage value of urban trees in the contiguous United States is $14.3 billion (bounds: $6.8 to $19.9 billion). Annual gross carbon sequestration values ranged from $16,000 in Jersey City to $855,000 in Atlanta (Table 1). National annual carbon sequestration by urban trees is valued at $460 million. Table 1. Estimated number of trees, structural value, and carbon storage and sequestration functional values for eight U.S. cities (from Nowak and Crane 2001 and Nowak et al. in reviews). City No. Structural Carbon Carbon Trees SE Value SE Storage SE Sequestration SE Thousands Millions of $ Millions of $ Thousands of $/yr Atlanta,GA 9, , New York, NY 5, , Baltimore, MD 2, , Philadelphia, PA 2, , Oakland, CA 1, na a na a Boston, MA 1, , Syracuse, NY _Jersey City, NJ I I 16 2 a Not analyzed. DISCUSSION The urban forests of the United States are a valuable resource with approximately 3.8 billion trees (Dwyer et al. 2000; Nowak et al. 2001a). Total tree values for a city will vary according to the total number and location of trees, species composition, diameter distribution, and tree 239

4 condition. Increased number of trees and a greater proportion of healthy, large-diameter trees, high-value species, and/or trees in more valuable locations (e.g., golf courses, institutional lands) will lead to greater structural values of urban forests. Urban forests also provide numerous functional benefits and values annually. Functional benefits generally increase with increased tree cover and health, and though proper design and management. Only two functional benefits - carbon storage and sequestration - were estimated in this paper. The value of carbon storage is based on the amount of carbon that has been accumulated by urban forests through the years. This storage value can be lost if the structure of the forest is not sustained (i.e., stored carbon can revert to atmospheric CO2 through tree death and decomposition). The value of carbon sequestration is based on the estimated gross amount of carbon removed annually through urban-forest growth. To more accurately estimate the functional values of urban forests, research is needed on how urban-forest structure affects functions (e.g., how different numbers, sizes, species, and locations of trees affect air pollution) and the value placed on these functions by society. Certain functional benefits and values of urban forests are being assessed (air pollution removal, energy conservation), but other values need to be quantified with respect to urban-forest structure (e.g., esthetic, social and community,'and wildlife values). CONCLUSION The urban forest resource of the United States is significant, not only in numbers of trees and extent of canopy, but also in the value of this resource and the numbers of people it affects. The physical resource of the urban forest nationwide has a structural value of more than $2 trillion. Each year, this resource also provides multi-billion dollar functional values that significantly affect human health and environmental quality in and around urban areas. LITERATURE CITED COUNCIL OF TREE AND LANDSCAPE APPRAISERS Guide for plant appraisal International Society of Arboriculture, Savoy, IL. DWYER, J.F., D.J. NOWAK, M.H. NOBLE, and S.M. SISINNI Connecting people with ecosystems in the 21st century: an assessment of our nation's urban forests. General Technical Report PNW-GTR-490, U.S. Department of Agriculture', Forest Service, Pacific Northwest Research Station, Portland, OR. FANKHAUSER, S The social costs of greenhouse gas emissions: an expected value approach. The Energy Journal. 15(2): NOWAK, D.J. 1993a. Atmospheric carbon reduction by urban trees. Journal of Environmental Management. 37(3): NOWAK, D.J. 1993b. Historical vegetation change in Oakland and its implications for urban forest management. Journal of Arboriculture. 19(5):

5 NOWAK, DJ. and D.E. CRANE The urban forest effects (UFORE) model: quantifying urban forest structure and functions. In: Hansen M. and T. Burk (Eds.), Proceedings: Integrated tools for natural resources inventories in the 21`t century, proceedings of the IUFRO conference, August 1998, Boise, ID. General Technical Report NC-212, U.S. Department of Agriculture, Forest Service, North Central Research Station, St. Paul, MN. p NOWAK, D.J. and D.E. CRANE Carbon storage and sequestration United States. Environmental Pollution. 116(3): by urban trees in the NOWAK, D.J., M.H. NOBLE, S.M. SISINNI, and J.F. DWYER. 2001a. Assessing the U.S. urban forest resource. Journal of Forestry. 99(3): NOWAK, D.J., J. PASEK, R. SEQUEIRA, D.E. CRANE, and V. MASTRO. 200lb. Potential effect ofanoplophora glabripennis (Coleoptera: Cerambycidae) on urban trees in the United States. Journal of Economic Entomology. 94(1): ZHU, Z Forest density mapping in the lower 48 states: a regression procedure. Research Paper SO-280. U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station Research Paper. New Orleans, LA. ABOUT THE AUTHORS David J. Nowak is a Research Forester and Project Leader with the USDA Forest Service, Northeastern Research Station, Syracuse, NY. John F. Dwyer is a Research Forester and Project Leader with the USDA Forest Service, North Central Research Station, Evanston, IL. Daniel E. Crane is a Computer Specialist with the USDA Forest Service, Northeastern Research Station, Syracuse, NY. ACKNOWLEDGMENTS We thank Chris Luley for assistance with field data collection and model inputs. This work was funded, in part, by the USDA Forest Service's RPA Assessment Staff, and State and Private Forestry, Cooperative Forestry's Urban and Community Forestry Program. Data collection in Baltimore, funded by the USDA Forest Service, is part of the National Science Foundation's Long-Term Ecosystem Research project. Data for Jersey City were collected and analyzed in cooperation with the State of New Jersey, Department of Environmental Protection and Energy, Division of Parks and Forestry. 241

6 forestry at the great divide 1 PROCEEDINGS SOCIETYOF AMERICAN FORESTERS 2001 NATIONAL CONVENTION Denver,Co[orado September 13-17, 2001

7 2002 Society of American Foresters 5400 Grosvenor Lane Bethesda, MD ' SAF Publication ISBN X