Some measures of air quality have demonstrated marked improvement since 1985, and in general, rural air quality over the United States has improved over the past decade. In both rural and urban areas, substantial progress has been made in reducing carbon monoxide and sulfur dioxide emissions and atmospheric concentrations. However, several major cities still have not complied with the ozone air quality standards of the U.S. Environmental Protection Agency. A major accomplishment has been the marked reduction in acid rain over forestlands of the northeastern United States (EPA 1994). However, reduced acidity, per se, might not accomplish as much as initially thought in terms of forest health. Chronic nitrogen additions might continue after SO4 concentrations (and hence pH) in rainfall have been reduced and might lead to nitrogen saturation of soils and increased cation leaching, conditions that emissions controls and sulfur reductions originally were designed to mitigate (Aber 1992).
Air quality definitely is affecting tree and forest health in many urban forests and in forests in airsheds surrounding large urban areas. These areas deserve special attention, because they vividly show the acute long-term effects of air pollution on trees and forests (for example, ozone damage to ponderosa pine trees in the San Bernadino Mountains outside Los Angeles; Miller and Evans 1974). These areas illustrate conditions that could become more common for forests in general. However, even chronic low-level exposure to ozone in rural ambient air may be negatively affecting forest productivity over much of the United States (Reich and Amundson 1985).
Urban forests are exposed to more altered environmental conditions than most rural forests. Concentrations of some agents, such as hydrocarbons, carbon dioxide, dust, or ozone, are higher in urban than in rural air. Major cities form what are known as "heat islands," where the temperature of the city center may be as much as 10° F higher than that of the surrounding countryside (Oke 1982, Lein 1989). The consequences of this temperature difference for tree health and forest dynamics are difficult to predict.
Trees and forests in urban areas can respond to changes in environmental conditions, but they also can contribute greatly to their amelioration. A study of metropolitan Atlanta showed that the urban forest has decreased by 65 percent since 1972. During the same period, average summer temperatures increased nine degrees more than those of the surrounding countryside (American Forests 1996). Increases in ambient temperatures might also contribute to Atlanta's ozone problem, and necessitate greater use of fossil-fuel energy to offset the increased environmental temperatures. In urban air, ozone is a highly reactive substance that breaks down on contact with most surfaces. Because trees have a higher surface area than other ground covers, provided primarily by leaves, they enhance the breakdown of ozone (Cavender and Allen 1991), even if they are damaged at high ozone concentrations. In addition, trees store carbon, filter particulates, absorb nitrogen from rain as NO3 in solution and from dry air as HNO3 vapor and NH4, and provide shade and other benefits,