Systematic assessment of coarse scale, national level and fine scale, special interest area critical load assessments

PARTNERS: USDA Forest Service Southern Research Station and Rocky Mountain Research Station

SUMMARY: Numerous research studies have shown that nitrogen, sulfur, and ozone pollutants reduce forest growth, degrade soils, damage lake and stream water quality, alter biodiversity, and increase forest mortality. Despite this knowledge and significant investments to reduce or mitigate negative pollution effects, policy makers and regulators have no easy means by which to judge the sensitivity of U.S. ecosystems to important air pollutants. Critical load, defined as the level of a pollutant above which an ecosystem experiences detrimental effects, is a metric that can be used for this purpose. This study proposes to develop a national map of critical pollutant loads for nitrogen and sulfur, following the leads of Canada and European nations. Previous critical load mapping studies do not account for the combined influence of multiple pollutants (e.g., nitrogen and ozone) or the impact of non-critical load stresses (e.g., drought, insect, or disease) on forest ecosystem health. A systematic assessment of coarse scale national level and fine scale special interest area critical load projections is proposed in this five phase study plan. The five phases include: 1) a coarse resolution critical load map for the continental U.S. using an existing Simple Mass Balance (SMB) critical load model and existing synoptic-scale data for soils, geology, climate, and other pertinent factors; 2) site-specific critical load maps, also derived from the SMB, for special interest areas, including sites with long-term records such as Coweeta Hydrologic Laboratory, special use areas such as Clean Air Act mandatory federal class one areas, USDA Forest Service critical load prototype sites, and areas with high pollutant deposition such as the Fernow Experimental Forest; 3) an examination of the relationship between coarse and fine scale critical load maps at overlapping areas with an emphasis on spatial sensitivity analysis; 4) an uncertainty analysis aimed at identifying the variability in environmental factors exerting the greatest influence on critical load estimates and developing confidence intervals around critical load predictions; and 5) an evaluation of the SMB with regard to non-pollutant stress (e.g., climate, insects, fire, and disease) impacts. A review of previous accomplishments and discussion of future research will follow the completion of the final phase.

EFETAC'S ROLE: The project is being led by Eastern Threat Center scientists and supported by Eastern Threat Center funding.

STATUS: Ongoing

PROGRESS: This project has been expanded to examine changes to critical acid loading due to climate change, resulting in several assessments. The first assessment used fixed increases and decreases to simulate the impact of climate change on critical loads. The second assessment incorporated precipitation and temperature changes from several global circulation model projections. The results from the first assessment are included in the Forest Health Monitoring 2010 National Report. The National Forest System is using results from the conterminous critical loads work in a watershed condition class assessment; researchers presented this work at the 2011 National Atmospheric Deposition Program Annual Meeting.

Critical acid load exceedancesAverage annual exceedance of the critical acid load for forest soils expressed in eq ha-1 yr-1 for the conterminous US for the years 1994-2000 at a 1-km2 spatial resolution. Click image to enlarge.

McNulty et al., 2007

Critical acid loadsAverage annual critical acid load for forest soils expressed in eq ha-1 yr-1 for the conterminous US for the years 1994-2000 at a 1-km2 spatial resolution. Click image to enlarge.

McNulty et al., 2007

Potter, K.M. and B.L. Conkling, eds. 2011. 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. 164 p. (PDF)

McNulty, S.G. and J.L. Boggs. 2010. A conceptual framework: Redefining forest soil's critical acid loads under a changing climate. Environmental Pollution 158:2053-2058. (PDF)

Huber, C, L. Geiser, E. Cohen, W. Jackson, L. Pardo, and R. Graw. A national application of critical loads by the US Forest Service to assess atmospheric deposition effects on watershed condition (abstract). In: Volk, Lisa, editor. 2011 Annual Meeting and Scientific Symposium NADP at the Nexus: Cross System Connections; October 25-28, 2011; Providence, Rhode Island; 38.

Paoletti, E., A. Bytnerowicz, C. Andersen, A. Augustaitis, M. Ferretti, N. Grulke, M.S. Günthardt-Goerg, J. Innes, D. Johnson, D. Karnosky, J. Luangjame, R. Matyssek, S. McNulty, G. Müller-Starck, R. Musselman, and K. Percy. 2007. Impacts of Air Pollution and Climate Change on Forest Ecosystems: Emerging Research Needs. The Scientific World Journal 7(S1):1–8.

Pardo, L.H., P.H. Templer, C.L. Goodale, S. Duke, P.M. Groffman, M.B. Aadams, P. Boeckx, J. Boggs, J. Campbell, B. Colman, J. Compton, B. Emmett, P. Gundersen, J. Kjonaas, G. Lovett, M. Mack, A. Magill, M. Mbila, M.J. Mitchell, G. McGee, S. McNulty, K. Nadelhoffer, S. Ollinger, D. Ross, H. Rueth L. Rustad, P. Schaberg, S. Schiff, P. Schleppi, J. Spoelstra and W. Wessel. 2006. Regional assessment of N saturation using foliar and root δ15N. Biogeochemistry 80:143-171. (PDF)

Li, H. and S. McNulty. 2007. Uncertainty analysis on simple mass balance model to calculate critical loads for soil acidity. Environmental Pollution 149: 315-326. (PDF)

McNulty, S., E. Cohen, H. Li, and J. Moore Myers. 2007. Estimates of critical acid loads and exceedances for forest soils across the conterminous United States. Environmental Pollution 149: 281-292. (PDF)


"Air Pollution Could Worsen Water Shortages in a Changing Climate" (related article from CompassLive)

Steve McNulty, Eastern Threat Center Research Ecologist, or 919-549-4068

Updated June 2018

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