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Critical Loads

Critical Loads Resources for Federal Land Managers

This website contains documents and information useful to Federal Land Managers and others for understanding calculations of critical loads for nitrogen and sulfur deposition to forest ecosystems in theory and practice.

Critical Loads for Nitrogen and Sulfur

The critical load is the level of deposition below which significant harmful ecological effects do not occur. 

Use of Critical Loads

The critical load is scientifically determined based on expected ecosystem response to a given deposition level.  The target load is set by policy makers, land managers, or air regulators to protect sensitive ecosystem components.  The target load may be higher or lower than the critical load, and is based on the economic cost of emissions reductions, timeframe, and other considerations.

[Image] Graph of N and S deposition.  X-Axis: N Deposition, Y-Axis: S Deposition.  flat line and then a 45 degree angle down[Image] Graph of Critical Load.  X Axis: Deposition, Y-Axis: Ecological Effects.  The Critical Load is the point on the line where the slope dramatically increases
Ecosystems are healthy when acidic deposition is beneath the critical value for both sulfur and nitrogen.


Critical Loads Overview

Projected emissions of both sulfur and nitrogen compounds are expected to have continuing negative impacts on forests, and to present serious long-term threats to forest health and productivity in the US, despite decreased sulfur emissions as a result of  SO2  abatement legislation.  Critical loads can be used to determine the level of deposition expected to cause harmful ecological effects.

The critical load is often defined as the level of deposition below which significant harmful ecological effects do not occur.  Critical loads can be determined by empirical observation, simple mass balance calculations, or dynamic models.  The method used is driven by data availability.  Empirical calculations are based on observed effects of deposition on a specific ecosystem, are fairly simple, and have low data requirements.  Simple mass balance calculations estimate the long-term effects of deposition through comparison of net gains and losses, and have moderate to intensive data requirements.  Dynamic models use a mass balance approach expanded by incorporating internal feedbacks, and allow the prediction of time to damage and time to recovery. Dynamic models are generally used at sites where extensive data are available.

Last Modified: 09/26/2008