By David J. Nowak and Daniel E. Crane
Syracuse, NY (March 1, 2002)- 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 rnillion/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.
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.
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 con- sequent carbon emissions from power plants. However, urban tree maintenance emissions can offset some of the carbon gains by urban forest systems.
The estimates given in this paper are based on limited field data and become more uncertain as they refine from national to regional and state estimates. More field measurements are needed in urban areas to help improve carbon accounting and other functions of urban forest ecosystems. In particular, more field data are needed to assess regional variation in forest structure; long-term permanent plot data are needed to assess urban forest growth, regeneration, and mortality; and improved satellite monitoring of urban cover types is needed to more accurately assess changes in urban forest cover.
In addition, research needs to develop better urban tree biomass equations, improve estimates of tree decomposition and maintenance emissions, and investigate the effect of urban soils on carbon storage and flux in cities. A better understanding and accounting of urban ecosystems can be used to develop management plans and national policies that can significantly improve environmental quality and human health across the nation.
Carbon storage and sequestration by urban trees in the USA