Climate change impacts on forest soil critical acid loads and exceedances at a national scale

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Federal agencies are currently developing guidelines for forest soil critical acid loads across the United States. A critical acid load is defined as the amount of acid deposition (usually expressed on an annual basis) that an ecosystem can absorb. Traditionally, an ecosystem is considered to be at risk for health impairment when the critical acid load exceeds a level known to impair forest health. The excess over the critical acid load is termed the exceedance, and the larger the exceedance, the greater the risk of ecosystem damage. This definition of critical acid load applies to a single, long-term pollutant exposure. These guidelines are often used to establish regulations designed to maintain acidic deposition, e.g., nitrogen and sulfur, inputs below the level shown to exceed an ecosystem’s critical acid load. The traditional definition for a critical acid load generally assumes that the ecosystem is in a steady state condition, i.e., no major changes in the factors that regulate the ecosystem’s ability to absorb acids. Unfortunately, climate change is altering weather patterns and, thus, impacting the factors that regulate critical acid load limits. This chapter explores which factors associated with establishing forest soil critical acid load limits will most likely be influenced by climate change, and how these changes might impact forest soil critical acid load limits across the United States. In New England, for example, base cation weathering could increase with global warming, along with nitrogen uptake as a function of increased forest growth. Nationally, a moderate 20-percent increase in base cation weathering and nitrogen uptake would result in at least a 30.5-percent decrease in the amount of forest soil area that exceeded the critical acid load limit and at least a 64.4-percent decrease in the amount of high exceedance area. While these results are encouraging, they do not account for other negative potential forest health risks associated with climate change such as elevated fire, insect, or disease risk. Additional study is needed before the full impact of climate change on forest health can be assessed.

  • Citation: McNulty, Steven G.; Cohen, Erika C.; Moore Myers, Jennifer A. 2013. Climate change impacts on forest soil critical acid loads and exceedances at a national scale. In: Potter, Kevin M.; Conkling, Barbara L., eds. 2013. Forest Health Monitoring: national status, trends, and analysis 2010. Gen. Tech. Rep. SRS-GTR-176. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station. 95-108.

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