Ecological Effects of Nitrogen Deposition in Rocky Mountain National Park

Extensive ecosystem research has documented significant effects from nitrogen deposition to sensitive aquatic and terrestrial ecosystems at Rocky Mountain National Park. High-elevation ecosystems at RMNP are more vulnerable to atmospheric nitrogen deposition than many ecosystems in the eastern U.S. or in Europe. This vulnerability is due to three key factors: 1) the granitic bedrock and shallow soils found in the Park do not provide much chemical buffering; 2) short growing seasons at high-elevation limit the amount of time plants have to absorb nitrogen during the year; and 3) these plants evolved under very low nitrogen conditions, so they are more adapted to nitrogen impoverishment rather than nitrogen enrichment. Monitoring of atmospheric deposition and research on its effects has been ongoing in the Park for over 20 years. A wealth of published studies demonstrate that high-elevation ecosystems in the Park on the east side of the Continental Divide show evidence of changes from nitrogen deposition. Nitrogen effects include changes in spruce forest soil and foliar chemistry, increased soil nitrogen mineralization rates, nitrogen saturation of alpine soils, elevated nitrate in streams and lakes, and shifts in phytoplankton communities. Manipulation experiments also suggest that alpine tundra vegetation communities may currently be at the threshold of change in the park. The National Park Service (NPS), Colorado Department of Public Health and Environment (CDPHE), and the U.S. Environmental Protection Agency have entered into a Memorandum of Understanding (MOU) with the to address harmful impacts to air quality and other natural resources occurring in Rocky Mountain National Park and to reverse the trend of increasing nitrogen deposition. Based on the ecological effects research in the park, and subsequent published critical loads for a variety of sensitive park resources, the NPS has established a critical load for wet nitrogen deposition of 1.5 kilograms per hectare per year for protecting high elevation aquatic ecosystems in the park.