Pacific Southwest Research Station

Critical Loads (CL) approach as a tool to evaluate harmful effects of nitrogen and sulfur deposition to forests

There is a need for tighter air pollution controls and monitoring to protect sensitive the U.S. forests and other ecosystems (National Research Council, 2004). Standards based on air pollutant concentrations are inappropriate for biological resources at risk from nitrogen (N) and sulfur (S)deposition. Therefore the Critical Loads (CL) approach has been suggested as a tool for evaluating ecological effects of N & S deposition in the United States. Critical Loads for N and S deposition is a level of deposition of these two elements below which no harmful effects occur in an ecosystem.

The CL approach has been used in Europe since late 1980s by the International Cooperative Programs under the auspices of the UN Economic Commission for Europe (US and Canada area also members of those programs). The CL approach has allowed for developing risk probability maps from N and S deposition for forest and other ecosystems in entire Europe. A similar approach, at a smaller scale has been used in SE Canada and New England.

The National Research Council also recommends development of a secondary (biologically based) standard for ozone since negative effects on plants can develop before human effects occur. In that regard a concept of an effective flux of ozone on vegetations is being developed in Europe and in the future could also be used in the United States.

These new concepts may have important effects on how U.S. ecosystems are managed and protected from harmful effects of N & S deposition and ozone.