224 Nowak et al.: Oxygen Production by Urban Trees decomposition processes. If forest canopy cover is increased (more trees are added) and sustained through time, net carbon dioxide will be removed and oxygen produced. Given the carbon dioxide level in the atmosphere (≈0.03%) is much less than the oxygen level (21%), the relative impact of urban forests and their management are much more significant for carbon dioxide than for oxygen. Fossil fuel combustion also consumes oxygen and as a result of fossil fuel use, the oxygen content of the atmosphere is slowly declining. Between 1989 and 1994, the oxygen content of the atmosphere dropped at an annual rate of 2 ppm out of 210,000 ppm (Broecker 1996). Thus, reducing fossil fuel use in the management of urban forests not only reduces emission of carbon dioxide, but also the consumption of oxy- gen. If fossil fuels are consistently used to maintain an urban forest, the net effect of the forest and its management will be carbon dioxide production and oxygen consumption. The point in the future when carbon emissions from maintenance exceed carbon sequestration by the forest varies by tree spe- cies and intensity of fossil fuel-based management (Nowak et al. 2002b). Urban trees can improve air quality (e.g., Cardelino and Chameides 1990; Taha 1996; Nowak et al. 2000, 2006a). Because small changes in air pollution concentrations can have relatively considerable impacts on air quality and human health, the effects of urban forests on air pollution can be significant. Recently, the U.S. Environmental Protection Agency has introduced urban tree cover as a potential emerg- ing measure to help meet air quality standards (U.S. Envi- ronmental Protection Agency 2004; Nowak 2005). In general, tree effects on trace chemicals in the atmosphere (chemicals that are minor components of the total atmosphere) will have a much greater relative impact on environmental quality and human health than chemicals such as oxygen that comprise a large proportion of the atmosphere. Relatively minor changes in trace chemicals can have significant effects on environ- mental and human health (e.g., impacts of ozone, particulate matter, nitrogen, and sulfur oxides) and climate change (e.g., impact of carbon dioxide). Although the absolute magnitude of oxygen production by urban forests is over 2.5 times greater than for carbon se- questration and 85 times greater than for air pollution re- moval nationally, the relative impacts of carbon sequestration and air pollution removal are much more significant than oxygen production. Urban forest effects on trace chemicals can lead to significant improvements in environmental qual- ity and human health and well-being. CONCLUSION Urban forests produce large amounts of oxygen. However, with the large and relatively stable amount of oxygen in the atmosphere and extensive production by aquatic systems, this ©2007 International Society of Arboriculture tree benefit is relatively insignificant. Tree impacts on im- portant atmospheric trace chemicals such as carbon dioxide and U.S. Environmental Protection Agency criteria air pol- lutants (ozone, particulate matter, sulfur dioxide, nitrogen di- oxide, carbon monoxide, and lead) will have greater signifi- cant impacts on human health and environmental quality. Urban forest carbon sequestration and air pollution removal along with other environmental impacts of urban forests (e.g., water quality improvement, lower air temperatures, reduced ultraviolet radiation loads) need to be better incorporated within local and regional planning efforts to improve envi- ronmental quality and enhance the quality of urban life. Acknowledgments. This work was funded, in part, by the USDA Forest Service, RPA Assessment Staff, and State and Private For- estry, Cooperative Forestry’s Urban and Community Forestry Pro- gram. Data collection in Baltimore, funded by the USDA Forest Service, is part of the National Science Foundation’s Long-Term Ecosystem Research project. Data from cities in New Jersey were collected and analyzed in cooperation with Mike D’Errico and the State of New Jersey, Department of Environmental Protection and Energy, Division of Parks and Forestry; Calgary data collection was by Simon Wilkins, City of Calgary; Minneapolis data collection was by Davey Resource Group; San Francisco data collection was by Alexis Harte and the City of San Francisco; Toronto data collection was by Andy Kenney and the University of Toronto; Washington, DC, data collection was by Casey Trees Endowment Fund and the National Park Service; Morgantown data collection was by Jonathan Cumming and Sandhya Mohan (University of West Virginia); and Atlanta, Georgia, Boston, Massachusetts, New York, New York, and Philadelphia, Pennsylvania, data were collected by ACRT, Inc. LITERATURE CITED American Forests. 2006. American Forests Kicks Off the Holiday Season With Tips on How to Care for Your Christmas Trees. http://www.americanforests.org/news/ display.php?id125 (accessed 6/06). Broecker, W.S. 1970. Man’s oxygen reserve. Science 168: 1537–1538. ———. 1996. Et tu, O2? 21stC, The World of Research at Columbia University. Special Issue: Biospheres. http:// www.columbia.edu/cu/21stC/issue-2.1/broecker.htm (ac- cessed 6/06). Cairns, M.A., S. Brown, E.H. Helmer, and G.A. Baumgard- ner. 1997. Root biomass allocation in the world’s upland forests. Oecologia 111:1–11. Cardelino, C.A., and W.L. Chameides. 1990. Natural hydro- carbons, urbanization, and urban ozone. Journal of Geo- physical Research 95:13971–13979. deVries, R.E. 1987. A preliminary investigation of the growth and longevity of trees in Central Park. New Brunswick, NJ, Rutgers University, MS thesis.
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