Assessing Nitrogen Critical Loads at North Cascades National Park Service Complex

Abstract:

Anthropogenic nitrogen (N) deposition (N_dep) contributes globally to disruptions in nutrient cycling, ecosystem functioning, and shifts in community composition. National Park Service (NPS) lands, including the North Cascades National Park Service Complex (NOCA), contain ecosystems that are potentially sensitive to N_dep. Accurate measurements of N_dep are needed to determine N critical loads, levels of N_dep that can be sustained without adverse biological effects. However, model estimates have a high degree of uncertainty, especially in mountainous regions such as NOCA. The N concentration of epiphytic lichens can be utilized to monitor N_dep because their relationship can be modeled for a given region, and their N stable isotope composition can assess contributions of different N pollution sources. The goal of this study is to determine which ecosystems in NOCA are affected by N_dep and identify predominant sources of N emissions. We established 30 plots across NOCA to determine lichen community composition and N content and stable isotope composition in the summer of 2018 and 2019. We found levels of N_dep increased moving east in the northern portion of the park. There was not a strong relationship for lichen N content along an elevational gradient, but N stable isotope composition tended to be higher with increasing elevation. Nitrogen stable isotope composition was very negative, ranging from -10.6 to -0.7‰, and varied among lichen species. Results will be incorporated with lichen community composition and atmospheric chemistry models in a GIS framework to develop a powerful approach to evaluate N critical load exceedance in the North Cascades. Integration with U.S. Forest Service (USFS) lichen air quality datasets will allow for further assessment of patterns of N_dep across the region.