Progress in Lichen Based Critical Loads for the Deposition of Fertilizing and Acidifying Air Pollutants in the United States

Linda Geiser1

Critical loads provide a science-based tool for land managers and regulators charged with protecting the environment from air pollution. Epiphytic lichens are very sensitive to air pollution and provide CLs that can be considered broadly protective of forest vegetation. Here I review the history and evolution of lichen-based critical loads and associated factors affecting their reliability. Over the past decade US Forest Service scientists have utilized national and regional scale lichen survey data to relate community composition to measured and modeled nitrogen and sulfur deposition, climate, and forest type.  Initially lichen CLs were established for specific ecoregions and critical loads were allowed to vary with climate variables or they did not account for other environmental variables. In recent work we recognize that at broad geographic scales, lichen communities are shaped by climate even in the absence of air pollution. If the response threshold (community composition) is allowed to vary with climate, single national critical loads for N and S can be selected independent of ecoregion.  Uncertainties surrounding critical loads estimates have been reduced by improvements in deposition and climate modeling; larger more representative response datasets; and new techniques in community analysis. While individual response curves (effect of deposition on species abundance) are intuitively easy to understand and relate to ecological roles; community response is a more reliable and robust indicator of deposition effects.


1USDA-Forest Service,