1. History of Critical Loads meetings – how have we gotten to this point? Andrzej Bytnerowicz 1, Rich Fisher 2 and Al Riebau 3 USDA Forest Service 1 Pacific Southwest Research Station, Riverside, CA 2 National Resources Research Center, Washington Office 3 Wildlife, Fish, Water and Air Research, Washington Office

2. Concept of Critical Loads is not new in the United States Fox et al., 1989, USDA FS GTR, considered possibility of using CL concept for in US forests Series of papers in Environmental Management (Strickland, Hunsaker, Hicks, Holdren, Driscoll, Cosby, Pardo, et al., 1993) examined utility of CL for evaluating atmospheric pollutants effects on sensitive ecosystems “Air Quality Criteria for Oxides of Nitrogen”, 1993, EPA questioned science and applicability of the CL concept in the United States

3. Concept of Critical Loads has been developed and refined in Europe Nilsson, 1986 Nilsson and Grennfeld et al, 1988 Schulze et al., 1989 Posch et al., 1995; 2001 Various ICP-Forest, ICP Modelling & Maps Reports

4. Recent turning points for the FS 2002 ICP Forests meeting in Lisbon, Portugal 2002 initiative by NFS managers to test a possibility of using CL concept on federal lands Ongoing CL research activities in New England

5. First Riverside Meeting, November 13 – 15, 2002 Considering using CL concept on US federal lands Presentation of CL methodologies and accomplishments in evaluating effects of atmospheric pollution on forests in Europe and North America Discussing data and methodological needs for evaluating effects of atmospheric N & S deposition on aquatic systems, soils and vegetation Exchange of ideas between the European and North American scientists and managers on how to develop effective collaboration Development of a concept of demonstration sites for evaluating feasibility of the European ICP methodologies on US federal lands

6. Second Riverside Meeting, November 4 - 6, 2003 Objectives: Develop a framework to develop and implement a methodology to assess N & S deposition effects on forests and protected areas in North America. Draft a report on approaches used in Europe & North America to calculate CL, include a summary of evidence linking S & N deposition with effects and include preliminary estimates of Sulfur and Nitrogen “Critical Loads”. Lay the groundwork for initiating a prototype demonstration-monitoring program to assess these effects in North America. Formulate a "path forward" for integrating science, monitoring, reporting and policy with regard to the concept of CL for S & N deposition.

7. Denver Meeting, March 31-April 1, 2004 (1) Need to specify: Management criteria – what to protect and to what degree An appropriate target load, in consultation with scientists How to use CLs in various air programs (e.g., PSD, NAAQS, SIPs) Need to know: That air pollution is linked to an ecosystem effect Ecosystem effects from a range of air pollution impacts The appropriate CL for each class I and II area Know the time to recovery or restoration

8. Denver Meeting, March 31-April 1, 2004 (2) Recommendations Common terms definitions have to be used No one method for estimate CL is possible – best approach depends on ecosystem Need a “tool box” of approaches to understand CLs (simple to complex) Need for integrating modeling & monitoring CLs should be established for total deposition Other stressors (e.g., drought, metals) need to be included in CLs

9. Denver Meeting, March 31-April 1, 2004 (3) Recommendations Aim to estimate CLs for the most sensitive part of a sensitive ecosystem Need sensitive indicators for excess N to each aquatic and terrestrial systems Need site-specific CLs For aquatics, the likely indicator is ANC For terrestrial, the likely metric is base saturation & Ca/Al ratio

10. Session “Monitoring and Mapping Critical Thresholds of Deposition on Ecosystems” at the Monitoring Science & Technology Symposium, Denver, September, 2004 USDA FS network of demonstration sites for testing ICP-Forests Methodologies (Bytnerowicz et al.) Collaborative monitoring strategies for assessing regional vulnerability to acidification in the Deleware River Basin (Murdoch et al.) What do we know about joint effects of N deposition, ambient ozone, and climatic changes in the western US? (Fenn et al.) Critical loads as applied to ozone for North American forests (Musselman) The IMPROVE network and the visibility information exchange Web system (VIEWS) – information source for national network on CL evaluation of N & S ecosystem effects (Fox et al.) The assessment of CL for N & S: a conceptual base for pollution control strategies in Germany (Nagel)