Mapping of nitrogen critical loads for selected national parks in the western United States
Sullivan TJ, McDonnell TC, Moore DL, Barna MG. 2016. Mapping of nitrogen critical loads for selected national parks in the western United States. Natural Resource Report. NPS/NRSS/ARD/NRR—2016/1286. National Park Service. Fort Collins, Colorado
Air pollution has long been recognized as a significant stressor to natural resources in national parks. Significant impacts to surface waters, soils, vegetation, wildlife, and visibility have been documented in a wide range of ecosystems across portions of the United States, including within the National Park system. Nitrogen (N) in human-caused air pollution has been found to affect natural plant communities, changing species composition, facilitating the spread of invasive species, altering fire cycles, and ultimately reducing native plant diversity. Nitrogen in air pollution results from emissions associated with energy, transportation, agriculture, and other sources. Airborne N is eventually deposited to the Earth’s surface through rain, snow, and dryfall. In recent years, N sources have increased in some regions, with concomitant increased risks to sensitive ecosystems, particularly those that evolved under naturally low N conditions. In the western U.S., the combination of increased N sources, together with the presence of N-sensitive receptors, has been responsible for a variety of chemical and biological changes in aquatic and terrestrial ecosystems. As documentation of these changes increases, scientists have been able to identify the levels of N addition at which ecosystems begin to change, a deposition loading known as the “critical load” (CL). Pardo et al. (2011a) compiled empirical data on N CLs in ecoregions throughout the United States for a variety of N-sensitive resources, including lichens, mycorrhizal fungi, herbaceous plants and shrubs, and forests. These CLs are generally reported by Pardo et al. (2011a) as ranges for a given ecoregion. Because ecosystems vary, sometimes substantially, within an ecoregion, the CL range for a given receptor may not apply to all plant communities or all portions of the ecoregion. When selecting appropriate CLs for a specific national park, it is important to examine the scientific basis of the CL, that is, whether the CL estimate was based on a specific vegetation type, and whether that vegetation type occurs in the park. Here, we evaluate empirical CL estimates for ecoregions in the western United States and analyze available vegetation data to specify appropriate CL ranges for selected national parks. We also examine total (oxidized and reduced) N deposition estimates in and near the parks. We compare deposition estimates for 2011 to park N empirical CLs to evaluate the likelihood that CLs are exceeded in NPS areas. Our focus is on 15 national park units located in the southwestern United States and in the Northern Great Plains (NGPN) and the Rocky Mountain (ROMN) National Park Service (NPS) Inventory and Monitoring (I&M) networks. The parks selected for study include many of those that have been identified as having N-sensitive ecosystems. Some of the parks have experienced in recent years increasing levels of nearby energy, agricultural, mineral extraction, and/or transportation development, with associated increasing N emissions. This report evaluates N CLs and their potential exceedances (extent to which ambient deposition exceeds the CL required for resource protection) at selected national parks throughout the intermountain western United States, in the context of potential new N emissions sources, including energy development and agriculture. The CL and exceedance estimates presented here were based on the lower limits of the ranges of CL, by ecoregion, reported by Pardo et al. (2011a). This was done to conform with the NPS mandate when considering CL to err on the side of resource protection.
- Type
- Published Report
- Authors
- Sullivan, Timothy; McDonnell, Todd; Moore, Deian; Barna, Michael
- Date of Issue
- 2016-08
- Publisher
- National Park Service
- Units
- ARCH , ARD , BADL , BLCA , CANY , CAVE , COLM , DINO , GRCA , HOVE , MEVE , NABR , NRSS , ROMO , SAGU , THRO , WICA
- Keywords
- agriculture, ammonia, atmospheric deposition, CAFO, CAMx, confined animal feeding operation, critical loads, nitrate, nitrogen, oil and gas
- Subjects
- Ecological Framework: Air and Climate | Air Quality | Wet and Dry Deposition , Ecological Framework: Air and Climate | Air Quality | Air Contaminants , Ecological Framework: Biological Integrity | Invasive Species | Invasive/Exotic plants , Ecological Framework: Biological Integrity | Focal Species or Communities | Desert Communities , Ecological Framework: Biological Integrity | Focal Species or Communities | Shrubland Communities , Ecological Framework: Biological Integrity | Focal Species or Communities | Vegetation Complex , Ecological Framework: Human Use | Point-Source Human Effects | Point-Source Human Effects , Ecological Framework: Human Use | Non-point Source Human Effects | Non-point Source Human Effects , Ecological Framework: Landscapes | Fire and Fuel Dynamics | Fire and Fuel Dynamics