Background/Question/Methods The western Sierra Nevada Mountains, California, have been exposed for several decades to high levels of air pollution resulting from various anthropogenic activities, such as agriculture, urban development, transportation, and industry in the San Joaquin Valley (SJV). The SJV is one of the most polluted areas in the nation, surpassed only by the Los Angeles Basin. People and ecosystems suffer from high concentrations of air pollutants including ozone (O₃), nitric acid (HNO₃) and ammonia (NH₃). Ozone is a well known phytotoxic pollutant, while HNO₃ and NH₃ are important contributors to dry nitrogen (N) deposition. In order to understand the effects of these pollutants on the sustainability of mixed conifer forests, a multidisciplinary study has been established as part of the Kings River Project, SW Sierra Nevada. A network of 18 sites was used for monitoring O₃, HNO₃ and NH₃ with passive samplers during the 2003-2005 summer seasons. Nitrogen deposition inputs were  measured as wet deposition at a newly installed NADP site and as throughfall.  Epiphytic lichens were used as sensitive indicators of changes caused by N deposition, and as a tool for determining critical loads and levels for protection of these forest ecosystems. Lichen responses to pollution gradients were characterized by correlating N ambient concentrations with lichen community composition changes and accumulation of N in the epiphytic macrolichen, Letharia vulpina (L.) Hue.
Results/Conclusions Highly elevated concentrations of the measured pollutants and a strong SW-NE gradient of their distribution were determined. These elevated concentrations of HNO₃ and NH₃ resulted in increased N deposition.  Increasing presence of N air pollution seemed to significantly altered lichen species composition leading to an apparent shift from N-sensitive lichens to the dominance by N-tolerant and N-“loving” species. High deposition of N was likewise reflected by enhanced nitrogen in L. vulpina. Simple models relating lichen variables to the N_HNO3+NH3  ambient concentrations were used to derive preliminary critical levels for the Kings River Project area. While atmospheric concentrations of N pollutants are elevated above background, there is no evidence that significant amounts of N are moving through the soils into the streams (data from Kings River Experimental Watershed).Our results are essential for evaluation of the effects of air pollution on forests and development of proper management strategies for maintaining clean air, clean water and healthy ecosystems on land managed by the US Forest Service.