USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Air Quality

Critical Loads

Smoke obscures the sun across a mountainous landscape with vegetation  shadowed in the foreground.
Critical loads can be determined for a range of ecosystem components or processes, including for elevated nitrate concentrations in watershed runoff. (U.S. Forest Service/Mark Fenn).

Air pollution emitted from a variety of sources is deposited from the air into ecosystems. These pollutants may cause ecological changes, such as long-term acidification of soils or surface waters, and soil nutrient imbalances affecting plant growth. A critical load is the highest level of one or more air pollutants that will not cause chemical changes associated with long-term harmful effects on an ecosystem.

Critical loads are based on scientific information about expected ecosystem responses to a given level of atmospheric deposition. For ecosystems that have already been damaged by air pollution, critical loads help determine how much improvement in air quality would be needed for ecosystem recovery to occur. In areas where critical loads have not been exceeded, critical loads can identify levels of air quality needed to maintain and protect ecosystems into the future.

Critical loads have been established for seven major vegetation types in California. A given ecosystem can have multiple critical loads, depending on what ecosystem component, process or species is to be protected. For example, in a forest ecosystem critical loads could be set for nitrate leaching (water quality), changes in epiphytic lichen communities, increased tree mortality, forest resilience, decreased tree growth (could be a different critical load for each species), understory biodiversity, impacts on mycorrhizal fungi or fine root biomass, or net tree biomass growth within the forest stand considering the nitrogen responses of all species in the stand.

Smoke obscures the sun across a mountainous landscape with vegetation  shadowed in the foreground.
Map showing critical load exceedance for seven vegatation types in California: mixed conifer forests, oak woodlands, chaparral, coastal sage scrub, grassland, desert scrub, pinyon-juniper.

The process of establishing critical loads provide a science-based and highly-useful tool that can be implemented in resource protection efforts. Critical load exceedance information and mapping also indicate to what degree air pollution emissions are still beyond acceptable levels for natural resource protection.

In recent years, extensive and ongoing collaborative efforts between governmental and university scientists and air quality specialists from the Forest Service (including PSW), National Park Service, and the Environmental Protection Agency has resulted in an exponential increase in critical load research and management application products in the U.S.

Recent efforts include work on developing links between critical loads and ecosystem services — a further application towards understanding the real-world impacts of nitrogen deposition on human uses of our natural resources. PSW has been an active participant in this process.

For further information, visit:

A Forest Service website on Critical Loads for Land Management Planning for more detailed information on critical loads and their use in land management.

A committee website link from the National Atmospheric Deposition Program.

  • Evaluation of atmospheric N deposition and nutrient N critical loads at Devils Postpile National Monument (DEPO). M. Buhler, A. Bytnerowicz, M. Fenn and S. Jovan. 2015-2016.
  • Nitrogen critical loads in chaparral catchments. M. Fenn and Ken Hanks. 2013-2016.
  • Forecasting forest response to N deposition: Integrating data from individual plant responses to soil chemistry with a continental scale gradient analysis. Gregory Lawrence, Linda Pardo, Erica Smithwick and R.Q. Thomas as PI’s. Mark Fenn is a Powell Center workshop participant and collaborator on this study. 2014-2016.
Last Modified: Nov 30, 2018 12:08:17 PM