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    Empirical critical loads (CL) for N deposition were determined from changes in epiphytic lichen communities, elevated NO3 leaching in streamwater, and reduced fine root biomass in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) at sites with varying N deposition. The CL for lichen community impacts of 3.1 kg ha-1 year-1 is expected to protect all components of the forest ecosystem from the adverse effects of N deposition. Much of the western Sierra Nevada is above the lichen-based CL, showing significant changes in lichen indicator groups. The empirical N deposition threshold and that simulated by the DayCent model for enhanced NO3 leaching were 17 kg N ha-1 year-1. DayCent estimated that elevated NO3 leaching in the San Bernardino Mountains began in the late 1950s. Critical values for litter C:N (34.1), ponderosa pine foliar N (1.1%), and N concentrations (1.0%) in the lichen Letharia vulpina ((L.) Hue) are indicative of CL exceedance.

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    Fenn, M.E.; Jovan, S.; Yuan, F.; Geiser, L.; Meixner, T.; Gimeno, B.S. 2008. Empirical and simulated critical loads for nitrogen deposition in California mixed conifer forests. Environ. Pollut. 155:492-511.


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    Ecosystem protection, Atmospheric deposition, Epiphytic lichens, Biomonitoring, Nitrogen saturation, Harmful effects

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