Selecting sulfur and nitrogen critical loads of atmospheric deposition for lichen communities

Linda Geiser1, Peter Nelson2, Heather Root3 and Sarah Jovan4

Lichens site scores, based on the relative proportion and abundance of sensitive vs. tolerant species, correlate well to CMAQ modeled deposition of total S and N. NPMR models of site scores indicate critical loads (CLs) of 1.0 to 3 kg/ha/yr for N and for S. These CLs apply to all CONUS and SE/SC Alaskan forests, i.e. are independent of geographic location or climatic conditions.  Because sustaining biodiversity and ecological functionality are mission critical to federal land managers, we use 90% quantile regression to assess the relative risk to total species, forage lichen, and cyanolichen richness across the CL range.  We recommend that CLs retain between 80-90% of total species richness, and also of forage and cyanolichen richness if the unit is in habitat that is able to support these lichens.   For example, a 2.0 kg N critical load would be associated with a risk of 7% decline in total species richness, 14% decline in forage lichens, and a 25% decline in cyanolichens. A 2 kg S critical load would be associated with risk of 7% decrease in total species richness, 17% reduction in detections of forage lichens, and 21% reduction in detections of cyanolichens.  To protect cyanolichens to between 90 and 80% of maximum detection frequency would suggest nitrogen and sulfur critical loads of 1.0 to 1.5 and 1.0 to 2.0 kg/ha/yr respectively.


1USDA Forest Service, Washington, DC,
2University of Maine, Ft. Kent, ME
3Weber State University, Ogden, UY
4USDA Forest Service, Portland, OR