Carbon storage and sequestration by urban trees in the USA

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Abstract

Based on field data from 10 USA cities and national urban tree cover data, it is estimated that urban trees in the coterminous USA currently store 700 million tonnes of carbon ($14,300 million value) with a gross carbon sequestration rate of 22.8 million tC/yr ($460 million/year). Carbon storage within cities ranges from 1.2 million tC in New York, NY, to 19,300 tC in Jersey City, NJ. Regions with the greatest proportion of urban land are the Northeast (8.5%) and the southeast (7.1%). Urban forests in the north central, northeast, south central and southeast regions of the USA store and sequester the most carbon, with average carbon storage per hectare greatest in southeast, north central, northeast and Pacific northwest regions, respectively. The national average urban forest carbon storage density is 25.1 tC/ha, compared with 53.5 tC/ha in forest stands. These data can be used to help assess the actual and potential role of urban forests in reducing atmospheric carbon dioxide, a dominant greenhouse gas.

Introduction

Increasing levels of atmospheric carbon dioxide (CO2) and other “greenhouse” gases [i.e. methane (CH4), chlorofluorocarbons, nitrous oxide (N2O), and tropospheric ozone (O3)] are thought by many to be contributing to an increase in atmospheric temperatures by the trapping of certain wavelengths of radiation in the atmosphere. Some chemicals though, may be reducing atmospheric temperatures (e.g. sulfur dioxide, particulate matter, stratospheric ozone; Graedel and Crutzen, 1989, Hamburg et al., 1997). Globally averaged air temperature at the Earth’s surface has increased between 0.3 and 0.6 °C since the late 1800s. A current estimate of the expected rise in average surface air temperature globally is between 1 and 3.5 °C by the year 2100 (Hamburg et al., 1997). Global warming is implicated in the recent discovery that floating ice over the Arctic Ocean has thinned from an average thickness of 10 feet in 1950 to <6 feet in the late 1990s, and a large expanse of ice-free water that has opened up at the North Pole in 2000 (Appenzeller, 2000, BBC News, 2000). As urban areas already exhibit climatic differences compared with rural environments, due in part to multiple artificial surfaces and high levels of fossil fuel combustion, climate change impacts may be exacerbated in these areas (Nowak, 2000).

Carbon dioxide is a dominant greenhouse gas. Increased atmospheric CO2 is attributable mostly to fossil fuel combustion (about 80–85%) and deforestation worldwide (Schneider, 1989, Hamburg et al., 1997). Atmospheric carbon is estimated to be increasing by approximately 2600 million metric tons annually (Sedjo, 1989).

Trees act as a sink for CO2 by fixing carbon during photosynthesis and storing excess carbon as biomass. The net long-term CO2 source/sink dynamics of forests change through time as trees grow, die, and decay. In addition, human influences on forests (e.g. management) can further affect CO2 source/sink dynamics of forests through such factors as fossil fuel emissions and harvesting/utilization of biomass. However, increasing the number of trees might potentially slow the accumulation of atmospheric carbon (e.g. Moulton and Richards, 1990).

Urban areas in the lower 48 United States have doubled in area between 1969 and 1994, and currently occupy 3.5% of the land base with an average tree cover of 27.1% (Dwyer et al., 2000, Nowak et al., 2001b). Though urban areas continue to expand, and urban forests play a significant role in environmental quality and human health, relatively little is known about this resource. As urban forests both sequester CO2, and affect the emission of CO2 from urban areas, urban forests can play a critical role in helping combat increasing levels of atmospheric carbon dioxide.

The first estimate of national carbon storage by urban trees (between 350 and 750 million tonnes; Nowak, 1993a) was based on an extrapolation of carbon data from one city (Oakland, CA) and tree cover data from various USA cities (e.g. Nowak et al., 1996). A later assessment, which included data from a second city (Chicago, IL), estimated national carbon storage by urban trees at between 600 and 900 million tonnes (Nowak, 1994). The purpose of this paper is to update the national urban tree carbon storage estimate based on data from eight new cities and national urban tree cover data. This paper will also include estimates of carbon storage and sequestration by urban trees at the national, regional and state level. These data can be used to help assess the actual and potential role of urban forests in reducing atmospheric CO2.

Section snippets

Field data

Field data were used to determine the entire urban forest structure (e.g. tree species composition and number of trees on all land uses) of 10 cities: Atlanta, GA; Baltimore, MD; Boston, MA; Chicago, IL (Nowak, 1994); Jersey City, NJ; New York, NY; Oakland, CA (Nowak, 1993a, Nowak, 1993b); Philadelphia, PA; Sacramento, CA (McPherson, 1998a); and Syracuse, NY. These cities were sampled based on methods developed by the USDA Forest Service for various urban forest research projects (e.g. Nowak

Results and discussion

Carbon storage in individual cities varies between 1.2 million tonnes in New York to 19,300 t in Jersey City (Table 1). Gross sequestration rates ranged from 42,100 tC/year in Atlanta to 800 tC/year in Jersey City. Total carbon storage and sequestration within cities generally increases with increased urban tree cover (city area multiplied by percent of tree cover) and increased proportion of large and/or healthy trees in the population. Large healthy trees greater than 77 cm in diameter

Conclusions

Urban forests can play a significant role in helping to reduce atmospheric carbon dioxide levels. Urban forests likely will have a greater impact per area of tree canopy cover than non-urban forests due to faster growth rates, increased proportions of large trees, and possible secondary effects of reduced building energy use and consequent carbon emissions from power plants. However, urban tree maintenance emissions can offset some of the carbon gains by urban forest systems.

The estimates given

Acknowledgements

We thank Chris Luley for assistance with field data collection, and Richard Birdsey and Richard Pouyat for review of this manuscript. Funding for this work and the assessment of national urban tree cover data was funded, in part, by the USDA Forest Service, 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

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