Lichen-Based Critical Loads for Nitrogen Deposition in Western Oregon and Washington Forests

Linda H. Geiser1, Doug A. Glavich2 and Matthew Porter3

Critical loads (CLs) define maximum atmospheric deposition levels that will not harm ecosystems. We calculated first CLs for nitrogen in the North American west coast marine forests ecological region. Using lichen survey data from grid-based sampling in western Oregon and Washington and multiple linear regression, we related epiphytic lichen community composition to wet deposition from the National Atmospheric Deposition Program; to wet, dry, and total deposition N deposition modeled by CMAQ/MM5; and to ambient particulate nitrogen measured by IMPROVE. Multiple deposition measures yielded consistent critical loads: scarcity of highly sensitive lichens was associated with 1-5 and 3-9 kg N ha-1 yr-1 in wet and total deposition, respectively. Annual precipitation was very important at the landscape level. Lichen-based CLs increased with precipitation, perhaps due to dilution or leaching of pollutants. Tight linear correlation between lichen and IMPROVE data suggests a rapid screening technique for CL exceedance.

1 US Forest Service Pacific Northwest Region Air Resource Management Program, PO Box 1148, Corvallis, OR 97339, USA; (541) 231-9452;
2 US Forest Service, Siuslaw National Forest Ecology and Air Program
3 Laboratory for Atmospheric Research, Washington State University