Selecting critical loads of atmospheric deposition when the response is continuous: a case study of risk assessment to lichens.

Linda Geiser1, Heather Root2 and Peter Nelson3

Empirical responses of terrestrial and aquatic organisms to the deposition of acidifying and eutrophying air pollutants are often continuous.  Whether the response metric be community composition, growth rate, seedling survival, or something else, the absence of a discontinuity signaling an obvious exceedance of a biological response threshold can make critical load selection annoyingly subjective.  How, by gum, does one evaluate harm—is even the smallest measurable amount of change harmful? Here we share some ideas for assessing risk associated with depositional loadings of 1.5, 2, 2.5, and 3 kg S and N ha-1 yr-1 to five lichen community indices: sites scores calculated as the abundance weighted mean sensitivity of species present; total species richness; forage lichen richness; cyanolichen richness; and sensitive species richness.  We use national-scale data to show how risk assessments can help managers select a geographical-, climate-relevant critical load and explain the potential harm associated with its exceedance.


1USDA Forest Service,
2Weber State University,
3University of Maine-Ft Kent,