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At a scale greater than a few hectares, land restoration generally can be verified by remote sensing. A change in vegetation or in the density of vegetation is usually apparent; where it can be made quantitative, the effectiveness of the restoration can be assessed. Carbon gains associated with the new vegetation may be modeled with acceptable accuracy at this scale (except, perhaps, in very toxic soils). Confirmation requires field sampling of soil carbon and aboveground biomass.
Improving land management in urban and peri-urban areas can affect stocks of carbon through additions to aboveground and below-ground biomass in areas where grass and trees can be grown. Urban forests-in the form of trees and other greenery-constitute a major amenity feature of urban areas, but they are also elements of the urban ecosystem: They moderate air temperatures and water runoff, clean pollutants from low-level air, and provide wildlife habitat in addition to sequestering carbon (McPherson et al., 1997). In Tokyo, with its high population densities, the proportion of green coverage is 23 percent; in Vienna, a city known for its open areas, the proportion is 50 percent.
Rapid expansion of urban areas is occurring in the United States: Urban expansion of approximately 1.34 Mha yr-1 was recorded in the period 1992-1997, of which around 0.5 Mha was converted from forest (USDA-NRCS, 1999). Conversion of forest to urban development may remove only a portion of the forest cover (unless the development includes large buildings or solid hard surface), but there are few data to help estimate the amount of carbon loss that might be associated with the activity for activity-based carbon flux estimates.
Urban areas around the world generally exhibit a "heat island" effect that makes the center of the city warmer than its surrounding rural regions (Akbari et al., 1989, 1992). This effect increases the demand for energy for cooling, which typically is provided by fossil fuels. Urban vegetation, particularly urban trees, is an important factor in reducing urban heat build-up, reducing the energy demand and thus saving fossil fuels. Studies in the United States indicate that the daily electrical usage for air-conditioning could be reduced by 10-50 percent by properly locating trees and shrubs (Akbari et al., 1992). Although these fossil fuel savings will be accounted for elsewhere, the net carbon store in increased urban vegetation is not. A 10-year program aimed at increasing the canopy cover by 10 percent on residential lands and 5-20 percent on other urban lands in the United States could result in sequestration of 3-9 Mt C yr-1 in trees and soils and a 7-29 Mt C reduction in emissions as a result of energy conservation from improved shading, increased evapotranspiration, and reduction of the urban heat island, along with wintertime heat savings (Sampson et al., 1992).
Improving urban trees also contributes to the reduction of ozone formation through their effect on lowering urban heat island temperatures. Ozone in the lower atmosphere is an important GHG as well as a threat to human health. Mitigating the heat island effect is important in reducing the formation of ozone in the lower atmosphere. Rosenfeld et al. (1996) estimated that, for the city of Los Angeles, a program of tree planting and installing cooler roofing and pavement products could reduce by 12 percent the number of days when ozone levels exceed health standards, with half of the benefit coming from trees and the other half from increased reflection of incoming solar radiation during summer months.
Trees contribute to ozone formation by emitting volatile organic compounds (VOC) in the presence of NOX (largely from automobile exhaust); they also "scrub" ozone from city air. Because these VOC emissions-which generally constitute less than 10 percent of total VOC emissions in urban areas (Nowak, 1991)-are temperature-dependent, increased urban tree cover is believed to lower overall VOC and therefore reduce ozone formation (Cardelino and Chameides, 1990). Nowak (1994) found that Chicago's urban forest was removing 2,000 t O3 yr-1 from that city's atmosphere.
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