Air Pollution and Forest Ecosystems: A Regional to Global Perspective

Changes in the atmospheric concentrations of a number of air pollutants over the last century are hallmarks of the magnitude and extent of human impact on the environment. Some of these changes are important to ecologists because many pollutants, acting singly or in combination, affect ecological systems in general and forests in particular. The greatest concern lies with chronic levels of tropospheric ozone, cumulative deposition of hydrogen ion, nitrogen, and sulfur via wet and dry processes, a select number of airborne chemicals (e.g., mercury) that tend to bioaccumulate in continental landscapes, and ultraviolet—B radiation through the loss of stratospheric ozone. Because the atmospheric residence time of most pollutants of concern to ecologists is measured on time frames extending from a few weeks to decades, pollutant distribution and effects are regional to global in dimension. We present evidence that ambient levels of some air pollutants in North America are affecting managed and unmanaged forests, and that the two most important pollutants are tropospheric ozone and chronic nitrogen loading. Further evidence indicates that while concentrations of some air pollutants have been declining over the last decade in North America, others are expected to remain unchanged or increase, including tropospheric ozone. We conclude that air pollution is affecting many North American forests and some remote forests around the globe. In the immediate future, the concern for air pollution effects on forests and associated natural resources will broaden to include interactions with changes in climate and pollution effects in the world's developing countries. There has been a rapid evolution in air pollution studies in ecology, shifting away from the agricultural paradigm of single—factor experimentation toward new methodologies that are ecologically and multidisciplinarily based. This shift has been promoted by the recognition that air pollution is one of several factors influencing forest productivity, community dynamics, and biogeochemistry, and that effects arise through long—term exposures. This evolution in methodologies will become even more marked in the future as new ecological approaches are adopted and an understanding is developed of how air pollution interacts with changes in climate. One of the most promising methodologies is process level modeling, which utilizes the large base of data in tree physiology and forest ecology, watershed chemistry, and atmosphere— forest canopy meteorology to develop models of tree physiology and growth and to subsequently scale these investigations to the levels of forest stands and landscapes.