title: 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.
authors.affiliation: USDA Forest Service, Pacific Southwest Research Station, Riverside, CA 92507, USA. mfenn@fs.fed.us
authors.forename: M E
authors.initials: ME
authors.lastname: Fenn
authors.forename: S
authors.initials: S
authors.lastname: Jovan
authors.forename: F
authors.initials: F
authors.lastname: Yuan
authors.forename: L
authors.initials: L
authors.lastname: Geiser
authors.forename: T
authors.initials: T
authors.lastname: Meixner
authors.forename: B S
authors.initials: BS
authors.lastname: Gimeno
mesh_terms: D000393:Air Pollutants / Q000032:analysis / Q000633:toxicity*; D018533:Biomass; D002140:California; D017753:Ecosystem; D004784:Environmental Monitoring / Q000379:methods*; D016468:Forestry; D008011:Lichens / Q000254:growth & development; D009584:Nitrogen / Q000032:analysis / Q000633:toxicity*; D018517:Plant Roots / Q000254:growth & development; D064028:Tracheophyta
chemical_list: D000393:Air Pollutants; D009584:Nitrogen