Air Pollution Effects on Forests: A Guide to Species Ecology, Ecosystem Services, and Responses to Nitrogen and Sulfur Deposition, Trees (Volume 1)
Anthropogenic emissions of nitrogen (N) and sulfur (S) to the atmosphere from sources such as fossil fuel combustion and industrialized agriculture have increased atmospheric deposition of N and S several times over preindustrial levels in most developed nations. Recent efforts to reduce emissions in the United States, regulated under the Clean Air Act and Amendments, have succeeded in decreasing deposition, especially in the eastern United States. However, current deposition of N and S still remains well above preindustrial levels, and N deposition appears to be increasing or unchanged in much of the Midwest and western United States. N deposition, along with S deposition, is a major stressor to tree species because of its wide-ranging and multi-faceted impacts. N deposition can alter tree growth rates, contribute to soil acidification, alter below-ground mycorrhizal communities essential to tree health, and increase nitrate (NO3 - ) leaching that impairs downstream water quality. N deposition can also increase the vulnerability of tree species to secondary factors such as pest and disease outbreaks, drought, and freezing injury.