To evaluate potential long-term effects of climate change and atmospheric nitrogen (N) deposition on subalpine ecosystems, the coupled biogeochemical and vegetation community competition model ForSAFE-Veg was applied to a site at the Loch Vale watershed of Rocky Mountain National Park, Colorado. Changes in climate and N deposition since 1900 resulted in pronounced changes in simulated plant species cover as compared with ambient and estimated future community composition. The estimated critical load (CL) of N deposition to protect against an average future (2010-2100) change in biodiversity of 10% was between 1.9 and 3.5 kg N ha(-1) yr(-1). Results suggest that the CL has been exceeded and vegetation at the study site has already undergone a change of more than 10% as a result of N deposition. Future increases in air temperature are forecast to cause further changes in plant community composition, exacerbating changes in response to N deposition alone. (C) 2013 Elsevier Ltd. All rights reserved.

National parks in the United States are protected areas wherein the natural habitat is to be conserved for future generations. Deposition of anthropogenic nitrogen (N) transported from areas of human activity (fuel combustion, agriculture) may affect these natural habitats if it exceeds an ecosystem-dependent critical load (CL). We quantify and interpret the deposition to Class I US national parks for present-day and future (2050) conditions using the GEOS-Chem global chemical transport model with 1/2° × 2/3° horizontal resolution over North America. We estimate CL values in the range 2.5–5 kg N ha−1 yr−1 for the different parks to protect the most sensitive ecosystem receptors. For present-day conditions, we find 24 out of 45 parks to be in CL exceedance and 14 more to be marginally so. Many of these are in remote areas of the West. Most (40–85%) of the deposition originates from NOx emissions (fuel combustion). We project future changes in N deposition using representative concentration pathway (RCP) anthropogenic emission scenarios for 2050. These feature 52–73% declines in US NOx emissions relative to present but 19–50% increases in US ammonia (NH3) emissions. Nitrogen deposition at US national parks then becomes dominated by domestic NH3 emissions. While deposition decreases in the East relative to present, there is little progress in the West and increases in some regions. We find that 17–25 US national parks will have CL exceedances in 2050 based on the RCP8.5 and RCP2.6 scenarios. Even in total absence of anthropogenic NOx emissions, 14–18 parks would still have a CL exceedance. Returning all parks to N deposition below CL by 2050 would require at least a 50% decrease in US anthropogenic NH3 emissions relative to RCP-projected 2050 levels.

The purpose of the research described here is to apply a new approach for generating aquatic critical load (CL) and exceedance calculations for an important acid-sensitive region of the eastern USA. A widespread problem in regional aquatic acidification CL modeling for US ecosystems has been the lack of site-specific weathering data needed to derive accurate model CL estimates. A modified version of the steady-state water chemistry CL model was applied here to estimate CL and exceedances for streams throughout acid-sensitive portions of Virginia and West Virginia. A novel approach for estimating weathering across the regional landscape was applied, based on weathering estimates extracted from a well-tested, process-based watershed model of drainage water acid–base chemistry and features of the landscape that are available as regional spatial data coverages. This process allowed extrapolation of site-specific weathering data from 92 stream watersheds to the regional context in three ecoregions for supporting CL calculations. Calculated CL values were frequently low, especially in the Blue Ridge ecoregion where one-third of the stream length had CL < 50 meq/m2/year to maintain stream ANC at 50 μeq/L under steady-state conditions. About half or more of the stream length in the study region was in exceedance of the CL for long-term aquatic resource protection under assumed nitrogen saturation at steady state. Land managers and air quality policy makers will need this information to better understand responses to air pollution emissions reductions and to develop ecoregion-specific air pollution targets.

Biodiversity has been described as the diversity of life on earth within species, among species, and among ecosystems. The rate of biodiversity loss due to human activity in the last 50 years has been more rapid than at any other time in human history, and many of the drivers of biodiversity loss are increasing, including habitat loss, overexploitation, invasive species, climate change, and pollution, including pollution from reactive nitrogen (Nr). Of these stressors, climate change and Nr from anthropogenic activities are causing some of the most rapid changes. Climate change is causing warming trends that result in poleward and elevational range shifts of flora and fauna, and changes in phenology, particularly the earlier onset of spring events and migration, and lengthening of the growing season. Nitrogen (N) enrichment can enhance plant growth, but has been shown to favor, fast-growing, sometimes invasive, species over native species adapted to low N conditions. Although there have been only a few controlled studies on climate change and N interactions, inferences can be drawn from various field observations. For example, in arid ecosystems of southern California, elevated N deposition and changing precipitation patterns have promoted the conversion of native shrub communities to communities dominated by annual non-native grasses. Both empirical studies and modeling indicate that N and climate change can interact to drive losses in biodiversity greater than those caused by either stressor alone. Reducing inputs of anthropogenic Nr may be an effective mitigation strategy for protecting biodiversity in the face of climate change.

We evaluated the ecological thresholds associated with vegetation and soil responses to nitrogen (N) deposition, by adding NH4NO3 in solution at rates of 5, 10 and 30 kg N ha(-1) yr(-1) to plots in a species rich dry meadow alpine community in Rocky Mountain National Park receiving ambient N deposition of 4 kg N ha(-1) yr(-1). To determine the levels of N input that elicited changes, we measured plant species composition annually, and performed one-time measurements of aboveground biomass and N concentrations, soil solution and resin bag inorganic N, soil pH, and soil extractable cations after 3 years of N additions. Our goal was to use these dose-response relationships to provide N critical loads for vegetation and soils for the alpine in Rocky Mountain National Park. Species richness and diversity did not change in response to the treatments, but one indicator species. Carex rupestris increased in cover from 34 to 125% in response to the treatments. Using the rate of change in cover for C. rupestris in the treatment and the ambient plots, and assuming the change in cover was due solely to N deposition, we estimated a N critical load for vegetation at 3 kg N ha(-1) yr(-1). Inorganic N concentrations in soil solution increased above ambient levels at input rates between 9 kg N ha(-1) yr(-1) (resin bags) and 14 kg N ha(-1) yr(-1) (lysimeters), indicating biotic and abiotic sinks for N deposition are exhausted at these levels. No changes in soil pH or extractable cations occurred in the treatment plots, indicating acidification had not occurred after 3 years. We conclude that N critical loads under 10 kg ha(-1) yr(-1) are needed to prevent future acidification of soils and surface waters, and recommend N critical loads for vegetation at 3 kg N ha(-1) yr(-1) as important for protecting natural plant communities and ecosystem services in Rocky Mountain National Park. (C) 2012 Elsevier Ltd. All rights reserved.

The ForSAFE-VEG model was used to estimate atmospheric nitrogen deposition and climate effects on soil chemistry and ground vegetation in alpine and subalpine zones of the northern and central Rocky Mountains region in the USA from 1750 to 2500. Model simulations for a generalized site illustrated how the critical load of atmospheric nitrogen deposition could be estimated to protect plant biodiversity. The results appear reasonable compared with past model applications in northern Europe. Atmospheric N deposition critical loads estimated to protect plant biodiversity were 1 to 2 kg N/ha/year. This range could be greater, depending on the values selected for critical site-specific parameters (precipitation, temperature, soil chemistry, plant nutrient uptake, and any eventual harvest of biomass) and the amount of biodiversity change allowed.

Concern over impacts of atmospheric nitrogen deposition to ecosystems in Rocky Mountain National Park, Colorado, has prompted the National Park Service, the State of Colorado Department of Public Health and Environment, the Environmental Protection Agency, and interested stakeholders to collaborate in the Rocky Mountain National Park Initiative, a process to address these impacts. The development of a nitrogen critical load for park aquatic resources has provided the basis for a deposition goal to achieve resource protection, and parties to the Initiative are now discussing strategies to meet that goal by reducing air pollutant emissions that contribute to nitrogen deposition in the Park. Issues being considered include the types and locations of emissions to be reduced, the timeline for emission reductions, and the impact of emission reductions from programs already in place. These strategies may serve as templates for addressing ecosystem impacts from deposition in other national parks.

Abstract Critical loads are a potentially important tool for protecting ecosystems from atmospheric deposition and for promoting recovery. Exceeding critical loads for nitrogen and sulfur can cause ecosystem acidification, nitrogen saturation, and biotic community changes. Critical loads are widely used to set policy for resource protection in Europe and Canada, yet the United States has no similar national strategy. We believe that ecosystem science and resource protection policies are sufficiently advanced in the United States to establish critical loads for federal lands. Communication and interaction between federal area managers and scientists will ensure that critical loads are useful for assessing ecosystem conditions, influencing land management decisions, and informing the public about the status of natural resources. Critical loads may also be used to inform air pollution policy in the United States, regardless of whether critical loads are directly linked to air quality regulations and emissions reductions agreements, as they are in Europe.