Potential Approaches to Developing Lichen-Based Critical Loads and Levels for Nitrogen, Sulfur and Metal-Containing Atmospheric Pollutants in North America

Abstract

“Critical load” and “critical level” (the deposition or concentration, respectively, of atmospheric pollutants above which negative ecosystem effects occur) concepts are reviewed for the North American lichenological community with the objective of stimulating interest and participation in this emerging field of research. We focus on how regional, lichen-based critical loads for nitrogen-, sulfur- and metal-containing pollutants could be developed for various terrestrial ecosystems in the U.S. utilizing existing data. Specifically we provide examples of how lichen community and element concentration responses to air pollution can be linked with measured or modeled estimates of depositional air pollutants. Preliminary critical loads of 0.26–0.33 kg ha−1 yr−1 for deposition of inorganic N in fine particulates and critical levels of 0.044–0.055 mg L−1 for wet deposition of ammonium ions are suggested for coniferous forests of western Oregon and Washington. These correspond to initial declines in presence and abundance of sensitive species and a % N concentration of 0.55–0.61 in Platismatia glauca. Indicator species distribution and abundance as well as physiological, morphological, growth or reproduction responses can also be used to set critical loads. Further research needs are identified.