Acid Rain Impacts on Calcium Nutrition and Forest Health

Forest ecosystems throughout the world are exposed to acid rain, a complex solution consisting largely of H+, SO42−, NH4+, and NO3− pollutant ions derived from sulfur and nitrogen oxides. Although the public in the United States may view acid deposition as a localized pollution issue specific to certain regions of North America and northern Europe, recent evidence of acidic deposition throughout much of Asia, including tropical forest regions, has demonstrated that acid deposition is actually a global phenomenon (Abate 1995). Furthermore, despite downward trends in sulfate concentrations associated with reductions in sulfur dioxide emissions mandated by the 1990 Clean Air Act Amendments, the acidity of precipitation in eastern North America does not seem to be decreasing (Scherbatskoy et al. in press). For example, over the course of 10 fog events on the summit of Mt. Mansfield, Vermont (elevation 1237 m), during summer 1998, the cloud water had an average pH of 2.8, with several events of pH 2.1–2.5 (Sean Lawson and Timothy Scherbatskoy, personal communication, data on file at the School of Natural Resources, The University of Vermont). The median and minimum cloud water pH values from Mt. Mansfield are more acidic than comparable summer measurements made on nearby Whiteface Mountain, New York (elevation 1483 m), from 1982 to 1987 (pH 3.4–3.8; Mohnen 1992). These findings collectively highlight the severity of cloud water acidity engulfing montane forests in the eastern United States and the ongoing nature of acid deposition. The prevalence and persistence of acid deposition has fueled public concern about the widespread impact of acid rain on forest health.