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Individual Highlight

Air-Pollution Thresholds Protect Ecosystems

Composite critical load exceedance map for seven major vegetation types in California. The figure indicates areas where and by how much atmospheric nitrogen deposition is higher than the critical load (e.g., threshold) for risk of harmful ecological effects. Forest ServiceSnapshot : Research benefits air-quality specialists, land managers, scientists, and policymakers

Principal Investigators(s) :
Fenn, Mark E. 
Research Location : United States
Research Station : Pacific Southwest Research Station (PSW)
Year : 2012
Highlight ID : 96


In recent years in the United States, a recognition has grown for the need to define the critical loads of atmospheric pollution inputs, which put natural resources and ecosystem services at risk. Recent advances led by Forest Service scientists in defining thresholds for acceptable air-pollution exposure have greatly increased U.S. capacity to protect and restore at-risk ecosystems. The use of these thresholds applies to toxic, acidic, and bloom-inducing effects on aquatic and terrestrial ecosystems that are affected by pollutants such as mercury, sulfur, and excess nitrogen.

The research took place primarily during the past decade in all major regions of the United States. Air-pollution studies in California formed an important part of these efforts. The Forest Service, National Park Service, and other entities use the critical loads developed in this work for natural resource protection.

Another aspect of this work is a synthesis of empirical critical loads for atmospheric nitrogen deposition effects for major ecoregions of the United States. The synthesis included U.S. maps showing regions where ecosystems are at risk from the harmful effects of nitrogen deposition. A related high-profile report geared for a broad audience was published on the use of air-pollution thresholds in policy and to protect and restore U.S. ecosystems. Last, a federally mandated peer-reviewed report was submitted to Congress that provided an update on the status of pollutant emissions, atmospheric deposition, ecosystem effects, and projections of emissions necessary to prevent future adverse ecosystem effects in the United States.

Forest Service Partners

External Partners

  • National Park Service

Program Areas