By David J. Nowak, Eric J. Greenfield, Robert E. Hoehn, and Elizabeth Lapoint
Newtown Square, PA (April 15, 2013) – U.S. Forest Service researchers, led by David J. Nowak, quantified carbon storage and sequestration by urban trees in the U.S. to assess the magnitude and role of urban forests in relation to climate change. Their results: U.S. urban forests sequester estimated 25.6 million tonnes of carbon every year, at a value of $2 billion. In addition, they store an estimate 643 million tonnes, at a value of $50+ billion.
Urban trees and forests affect climate change, but are often disregarded because their ecosystem services are not well-understood or quantified. Trees act as a sink for carbon dioxide (CO2) by fixing carbon during photosynthesis and storing carbon as biomass.Trees in urban areas (i.e., urban forests) currently store carbon, which can be emitted back to the atmosphere after tree death, and sequester carbon as they grow.
Urban trees also influence air temperatures and building energy use, and consequently alter carbon emissions from numerous urban sources (e.g., power plants) (Nowak, 1993). Thus, urban trees influence local climate, carbon cycles, energy use and climate change.
The purpose of this paper was to update the national urban tree carbon storage and sequestration estimates using urban field data from 28 cities and 6 states and newer estimates of urban land area and urban tree cover. These data were applied to statewide urban tree cover measurements to determine total urban forest carbon storage and annual sequestration by state and nationally.
This new assessment produces more refined statistical estimates of the uncertainty of the national estimates and investigates the overlap between urban forest carbon estimates and U.S. forestland carbon estimates. These carbon storage and sequestration estimates provide better, more up-to-date information for national carbon estimates (e.g., IPCC, 2006) and can be used to help assess the actual and potential role of urban forests in reducing atmospheric CO2.
Findings show urban whole tree carbon storage densities average 7.69 kg C m−2 of tree cover and sequestration densities average 0.28 kg C m−2 of tree cover per year. Total tree carbon storage in U.S. urban areas (c. 2005) is estimated at 643 million tonnes ($50.5 billion value; 95% CI = 597 million and 690 million tonnes) and annual sequestration is estimated at 25.6 million tonnes ($2.0 billion value; 95% CI = 23.7 million to 27.4 million tonnes).