Assessing nitrogen deposition in mountainous regions: Nitrogen critical loads in the North Cascades

Background/Question/Methods

Anthropogenic nitrogen (N) deposition (Ndep) 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 Ndep. Accurate measurements of Ndep are needed to determine N critical loads, levels of Ndep that can be sustained without adverse biological effects. However, model estimates have a high degree of uncertainty, especially in mountainous regions such as the North Cascades. The N concentration of epiphytic lichens can be utilized to monitor Ndep because the relationship between lichen N concentration and Ndep can be determined for a given region, and their N stable isotope composition (δ15N) can assess contributions of different N emission chemistries. Lichen communities, comprised of species with varying tolerances to N, can be assessed as an indicator of levels of Ndep as well. This study is expected to increase our understanding of patterns of atmospheric Ndep in mountainous regions. Our focus is the North Cascades, and our goal is to determine the extent to which ecosystems in the North Cascades are being affected by Ndep. We established 30 plots across the North Cascades to determine lichen community composition and N content and δ15N.

Results/Conclusions

We found levels of Ndep increased moving east in the northern portion of the study area. There was not a strong relationship for lichen N content along an elevational gradient, but δ15N tended to be higher with increasing elevation. δ15N was very negative, ranging from -0.7 to -10.6 ‰, and varied among lichen species. This study will incorporate lichen community composition and atmospheric chemistry models in a GIS framework in the future.