Interactive effects of air pollution and climate change on forest ecosystems in the United States: current understanding and future scenarios
A review of the current status of air pollution and climate change (CC) in the United States from a perspective of their impacts on forest ecosystems is provided. Ambient ozone (O3) and nitrogen (N) deposition have important and widespread ecological impacts in U.S. forests. Effects of sulphurous (S) air pollutants and other trace pollutants have significant ecological importance only at much smaller geographic scales. Complex interactive effects of air pollution and CC for selected future CC scenarios are reviewed. In addition, simulations of past, present, and future hydrologic, nutrient, and growth changes caused by interactive effects of air pollution and CC are described for two U.S. forest ecosystems. Impacts of O3, N deposition, and CC on growth and hydrology of mixed conifer forests in the San Bernardino Mountains in southern California were projected with the DayCent model. Effects of N deposition, CO2 fertilization, N deposition, and CC on northern hardwood forests at the Hubbard Brook Experimental Forest in New Hampshire were simulated with the PnET-BGC model. Projected changes in these forests can influence the provision of ecosystem services such as C sequestration and water supply. The extent of these effects will vary depending on the future intensity and extent of CC, air pollutant emission levels, the distribution of air pollution, and other factors such as drought, pest outbreaks, fire, etc. Our chapter ends with research and management recommendations intended to increase our ability to cope with uncertainties related to the future interactive effects of multiple air pollutants, atmospheric deposition, CC, and other biotic and abiotic stressors.