Empirical and simulated critical loads for nitrogen deposition in California mixed conifer forests

Abstract Empirical critical loads (CL) for N deposition were determined from changes in epiphytic lichen communities, elevated NO 3 − 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 NO 3 − leaching were 17 kg N ha −1 year −1 . DayCent estimated that elevated NO 3 − 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.