In the article the authors provide a formal definition of nitrogen saturation and set forth a series of testable hypotheses regarding the states of forest ecosystem response to chronic nitrogen deposition. These hypotheses are used to suggest early indicators of nitrogen saturation and to discuss the potential implications of nitrogen saturation of forest ecosystems over large geographic areas. Possible effects include elevated concentrations of nitrate, aluminum, and hydrogen in streams, which would decrease water quality; frost damage or other disruptions of physiological function that would reduce productivity in conifer stands; increase cation leaching from soils and nitrate losses that would lead to reduced soil fertility and increased acidity; and possible increases in emissions of trace gases such as nitrous oxide, which may alter atmospheric chemistry and contribute to the warming of Earth's atmosphere (the greenhouse effect).