Critical Loads - Target Load Strategy

You have been directed to this Target Load Strategy because one or more critical loads have been exceeded on your forest.  This Strategy will guide you through a process to develop target loads (TL) for incorporation into the forest plan, the final step in the critical loads (CL) assessment.  Review all critical loads, deposition, and exceedance information developed under the CL Implementation Strategy before proceeding.

Critical loads are “quantitative estimates of exposure to one or more pollutants, below which significant harmful effects on specific sensitive elements (receptors) of the environment do not occur, according to present knowledge.”  Describing air pollution effects on ecosystems in terms of critical loads allows communication of this complex science to a broader audience.  The effects of sulfur and nitrogen deposition most often considered are acidification of surface waters or forested ecosystems, or nutrient nitrogen enrichment impacting various receptors (e.g., diatoms, lichens, mycorrhizal fungi, herbaceous vegetation, and forests).  Because receptors have varying sensitivities to air pollutant loads, multiple critical loads can be used to describe a continuum of impacts related to increasing deposition at a given location. 

When deposition exceeds the CL, there is increased risk for negative ecosystem effects including changes in aquatic biota, decline in forest health, and changes in biodiversity.  In Step 7 of the CL Strategy, areas where current deposition exceeds CLs were identified.  CLs for these areas will now be used to develop target loads.

A target load (TL) is a level of deposition set by policy makers to meet their objectives; for natural resources managers, the objective might be to protect sensitive ecosystem components.  The TL may be higher or lower than the CL based on the desired level of resource protection, economic considerations, “the life of the forest plan,” and stakeholder input.  Whereas the CL is a science-based threshold describing the amount of deposition an ecosystem can tolerate over the long term, the TL is a policy-based threshold that incorporates the concept of time (for example, the time required for a damaged ecosystem to recover if the total deposition does not exceed a specific load).  Management should be engaged in the process of setting target loads, especially when determining the level of protection desired for key sensitive receptors and the timeline for achieving this protection.

This strategy outlines a process that can be used to develop target loads.

Proceed to Step 1 of the Target Load Strategy: Review Available CL Exceedances to Determine how to Set Target Load.