Ecological effects of atmospheric nitrogen (N) and sulfur (S) deposition on two hardwood forest sites in the eastern United States were simulated in the context of a changing climate using the dynamic coupled biogeochemical/ecological model chain ForSAFE-Veg. The sites are a mixed oak forest in Shenandoah National Park, Virginia (Piney River) and a mixed oak-sugar maple forest in Great Smoky Mountains National Park, Tennessee (Cosby Creek). The sites have received relatively high levels of both S and N deposition and the climate has warmed over the past half century or longer. The model was used to evaluate the composition of the understory plant communities, the alignment between plant species niche preferences and ambient conditions, and estimate changes in relative species abundances as reflected by plant cover under various scenarios of future atmospheric N and S deposition and climate change. The main driver of ecological effects was soil solution N concentration. Results of this research suggested that future climate change might compromise the capacity for the forests to sustain habitat suitability. However, vegetation results should be considered preliminary until further model validation can be performed. With expected future climate change, preliminary estimates suggest that sustained future N deposition above 7.4 and 5.0 kg N/ha/yr is expected to decrease contemporary habitat suitability for indicator plant species located at Piney River and Cosby Creek, respectively.

Abstract Human activity has significantly increased the deposition of nitrogen (N) on terrestrial ecosystems over pre‐industrial levels leading to a multitude of effects including losses of biodiversity, changes in ecosystem functioning, and impacts on human well‐being. It is challenging to explicitly link the level of deposition on an ecosystem to the cascade of ecological effects triggered and ecosystem services affected, because of the multitude of possible pathways in the N cascade. To address this challenge, we report on the activities of an expert workshop to synthesize information on N‐induced terrestrial eutrophication from the published literature and to link critical load exceedances with human beneficiaries by using the STressor–Ecological Production function–final ecosystem Services Framework and the Final Ecosystem Goods and Services Classification System (FEGS‐CS). We found 21 N critical loads were triggered by N deposition (ranging from 2 to 39 kg N·ha−1·yr−1), which cascaded to distinct beneficiary types through 582 individual pathways in the five ecoregions examined (Eastern Temperate Forests, Marine West Coast Forests, Northwestern Forested Mountains, North American Deserts, Mediterranean California). These exceedances ultimately affected 66 FEGS across a range of final ecosystem service categories (21 categories, e.g., changes in timber production, fire regimes, and native plant and animal communities) and 198 regional human beneficiaries of different types. Several different biological indicators were triggered in different ecosystems, including grasses and/or forbs (33% of all pathways), mycorrhizal communities (22%), tree species (21%), and lichen biodiversity (11%). Ecoregions with higher deposition rates for longer periods tended to have more numerous and varied ecological impacts (e.g., Eastern Temperate Forests, eight biological indicators) as opposed to other ecoregions (e.g., North American Deserts and Marine West Coast Forests each with one biological indicator). Nonetheless, although ecoregions differed by ecological effects from terrestrial eutrophication, the number of FEGS and beneficiaries impacted was similar across ecoregions. We found that terrestrial eutrophication affected all ecosystems examined, demonstrating the widespread nature of terrestrial eutrophication nationally. These results highlight which people and ecosystems are most affected according to present knowledge, and identify key uncertainties and knowledge gaps to be filled by future research.

Anthropogenic stressors such as climate change, increased fire frequency, and pollution drive shifts in ecosystem function and resilience. Scientists generally rely on biological indicators of these stressors to signal that ecosystem conditions have been altered. However, these biological indicators are not always capable of being directly related to ecosystem components that provide benefits to humans and/or can be used to evaluate the cost-benefit of a change in health of the component (ecosystem services). Therefore, we developed the STEPS (STressor - Ecological Production function - final ecosystem Services) Framework to link changes in a biological indicator of a stressor to final ecosystem services. The STEPS framework produces "chains" of ecological components that explore the breadth of impacts resulting from the change of a stressor. Chains are comprised of the biological indicator, the ecological production function (EPF; which uses ecological components to link the biological indicator to a final ecosystem service), and the user group who directly uses, appreciates, or values the component. The framework uses a qualitative score (High, Medium, Low) to describe the Strength of Science (SOS) for the relationship between each component in the EPF. We tested the STEPS Framework within a workshop setting using the exceedance of critical loads of air pollution as a model stressor and the Final Ecosystem Goods and Services Classification System (FEGS-CS) to describe final ecosystem services. We identified chains for four modes of ecological response to deposition: aquatic acidification, aquatic eutrophication, terrestrial acidification, and terrestrial eutrophication. The workshop participants identified 183 unique EPFs linking a change in a biological indicator to a FEGS; and when accounting for the multiple beneficiaries, we ended with 1104 chains. The SOS scores were effective in identifying chains with the highest confidence ranking as well as those where more research is needed. The STEPS framework could be adapted to any system in which a stressor is modifying a biological component. The results of the analysis can be used by the social science community to apply valuation measures to multiple or selected chains, providing a comprehensive analysis of the effects of anthropogenic stressors on measures of human well-being.

The biogeochemical model, PnET-BGC, has been used to evaluate the long-term acid-base response of surface waters to changes in atmospheric acid deposition. We propose a methodology to identify the input factors of greatest model sensitivity and propagate uncertainty of input factors to model outputs. The quantified model uncertainty enabled application of an “exceedance probability” approach to determine allowable atmospheric deposition in the form of Total Maximum Daily Loads (TMDLs) for twelve acid-impaired streams in Great Smoky Mountains National Park. Results indicate that acidification of surface water resulting from acidic deposition has been substantial. Even if current atmospheric deposition is reduced to pre-industrial levels, only one of the twelve impaired streams might be recovered to its site-specific standard by 2050. Our sensitivity analysis indicates that the model is most sensitive to precipitation quantity, air temperature and calcium weathering rate, and suggests further research to improve characterization of these inputs.

Current and historic atmospheric nitrogen (N) deposition has impacted aquatic ecosystems in the Greater Yellowstone Area (GYA). Understanding the spatial variation in total atmospheric deposition (wet + dry) of N is needed to estimate air pollution deposition critical loads for sensitive aquatic ecosystems. This is particularly important for areas that have an increasing contribution of ammonia dry deposition to total N (TN), such as the GYA. High resolution geostatistical models and maps of TN deposition (wet + dry) were developed using a variety of techniques including ordinary kriging in a geographic information system, to evaluate spatial variability and identify areas of elevated loading of pollutants for the GYA. TN deposition estimates in the GYA range from <1.4 to 7.5 kg N ha(-1) yr(-1) and show greater variability than wet inorganic N deposition. Critical loads of TN deposition (CLTNdep) for nutrient enrichment in aquatic ecosystems range from less than 1.5 +/- 1.0 kg N ha(-1) yr(-1) to over 4.0 +/- 1.0 kg N ha(-1) yr(-1) and variability is controlled by differences in basin characteristics. The lowest CLTNdep estimates occurred in high elevation basins within GYA Wilderness boundaries. TN deposition maps were used to identify critical load exceedances for aquatic ecosystems. Estimated CLTNdep exceedances for the GYA range from 17% to 48% depending on the surface water nitrate (NO3-) threshold. Based on a NO3- threshold of 1.0 mu mol L-1, TN deposition exceeds CLTNdep in approximately 30% of the GYA. These predictive models and maps can be used to help identify and protect sensitive ecosystems that may be impacted by excess atmospheric N deposition. (C) 2017 Elsevier Ltd. All rights reserved.

Acid deposition has impacted sensitive streams, reducing the amount of habitat available for fish survival in the Great Smoky Mountains National Park (GRSM) and portions of the surrounding Southern Appalachian Mountains by decreasing pH and acid neutralizing capacity (ANC) and mobilizing aluminum dissolved from soil. Land managers need to understand whether streams can recover from the elevated acid deposition and sustain the healthy aquatic biota, and if so, how long it would take to achieve this condition. We used a dynamic biogeochemical model, PnET-BGC, to evaluate past, current, and potential future changes in soil and water chemistry of watersheds of the GRSM in response to the projected changes in acid deposition. The model was parameterized with soil, vegetation, and stream observations for 30 stream watersheds in the GRSM. Using model results, the level of atmospheric deposition (known as a "critical load") above which harmful ecosystem effects (defined here as modeled stream ANC below a defined target) occur was determined for the 30 study watersheds. In spite of the recent marked decreases in atmospheric sulfur and nitrate deposition, our results suggest that stream recovery has been limited and delayed due to the high sulfate adsorption capacity of soils in the park resulting in a long lag time for recovery of soil chemistry to occur. Model simulations suggest that over the long term, increases in modeled stream ANC per unit decrease in NH4+ deposition are greater than unit decreases in SO42- or NO3- deposition, due to high SO42- adsorption capacity and the limited N retention of the watersheds. Watershed simulations were used to extrapolate the critical load results to 387 monitored stream sites throughout the park and depict the spatial pattern of atmospheric deposition exceedances. These types of model simulations inform park managers on the amount of air quality improvement needed to meet the stream restoration goals.

Terrestrial ecosystems provide important resources for many human uses, however, these resources are threatened by nitrogen from air pollution. Identifying reductions in air pollution that will protect ecosystems is the main focus of the mitigation strategies currently being developed in the United States.

The development and use of critical loads of air pollutant deposition in the U.S. is gaining momentum, and recent research efforts in the U.S. have produced valuable data for calculating critical loads. Critical loads are used to quantify the levels of air pollutants that are expected to impact forest health, soil fertility, aquatic biota condition, and other ecosystem responses. In addition, model refinements for improving critical loads estimates, and maps for illustrating critical loads for acidification and nitrogen saturation and eutrophication resulting from excess nutrient nitrogen, have been developed at various scales. However, prior to the effort described here, no cohesive process existed to provide a national-scale critical loads database and maps as a unified product representing all U.S. ecosystems. The FOCUS (Focal Center Utility Study) Project was initiated to coordinate the development and implementation of a clear, consistent, repeatable process for calculating and mapping critical loads within the U.S. In the FOCUS Phase I Pilot Study, empirical and calculated critical loads data for the U.S. were synthesized from dozens of regional and national-scale monitoring networks, research projects and publically available databases following an approach similar to that used in Europe. The United Nations Economic Commission for Europe (UNECE), through its International Cooperative Programme on Modelling and Mapping of Critical Levels & Loads and Air Pollution Effects, Risks and Trends (ICP-M&M) collects, analyzes and maps critical loads data. Countries participating in the Convention on Long-range Transboundary Air Pollution (CLRTAP) use a Critical Loads "Focal Center" in each country to serve as the point of contact for submitting regional and national-scale critical loads data to the ICP-M&M. One of the purposes of this study was to develop a foundation for interacting with other Focal Centers by assembling critical loads data, creating a database, establishing modeling protocols, and developing infrastructure within the U.S to report and update critical loads on a national scale. Because the U.S. does not currently have an officially designated Focal Center, critical loads data were provided as an informal, unofficial submission to the Coordination Center for Effects (CCE) of the ICP-M&M in March 2011, in the interest of international cooperation and exchange of information on the effects of atmospheric deposition of pollutants on ecosystems. We envision that these data will enable U.S. scientists, land managers, and environmental policymakers to enter into a productive and meaningful dialogue within the US, and also with the international scientific community on methods for estimating, calculating, mapping, interpreting, and refining critical loads for the effects of acidification and excess nutrient nitrogen on terrestrial and aquatic ecosystems. This paper describes the process used to develop national-scale critical loads in the U.S., summarizes the FOCUS Phase I approach and database development effort, and presents some initial national-scale critical loads mapping products. Published by Elsevier Ltd.

Remote national parks of the western U.S. and Alaska are not immune to contaminants of emerging concern. Semivolatile organic compounds ( SOC s) such as pesticides and PCB s can selectively deposit from the atmosphere at higher rates in cold, high‐elevation and high‐latitude sites, potentially increasing risk to these ecosystems. In the environment, SOC s magnify up food chains and are known to increase health risks such as cancer and reproductive impairment. One hundred twenty‐eight fish in 8 national parks in Alaska and the western U.S. were analyzed for contaminant concentrations, assessed by region, and compared to human and wildlife health thresholds. SOC concentrations from an additional 133 fish from a previous study were also included, for a total of 31 water bodies sampled. PCB s, endosulfan sulfate, and p , p ′‐ DDE were among the most frequently detected contaminants. Concentrations of historic‐use pesticides dieldrin, p , p ′‐ DDE , and/or chlordanes in fish exceeded USEPA guidelines for human subsistence fish consumers and wildlife (kingfisher) health thresholds at 13 of 14 parks. Average concentrations in fish ranged from 0.6‐280 ng/g lipid (0.02‐7.3 μg/g ww). Contaminant loading was highest in fish from Alaskan and Sierra Nevada parks. Historic compounds were highest in Alaskan parks, while current‐use pesticides were higher in the Rockies and Sierra Nevada . This study provides a rigorous analysis of CEC s in fish from national parks and identifies regions at potential risk.