Methodologies for Determining Empirical Nitrogen Critical Loads and Exceedances for California Ecosystems

M.E. Fenn*, E.B. Allen, S. Jovan, S.B. Weiss, L.H. Geiser, A. Bytnerowicz, G.S. Tonnesen and R.F. Johnson
USDA Forest Service
Pacific Southwest Research Station
4955 Canyon Crest Dr.
Riverside, CA 92507

In recent years it has become apparent that chronic N deposition is affecting various ecosystem types in California, from desert, grassland and shrub vegetation to mixed conifer forests. Because of the difficulty in measuring dry deposition to these ecosystems several approaches have been used to estimate N deposition. Broad scale estimates were made with the CMAQ simulation model and site-specific measurements were determined with ion exchange resin (passive) throughfall collectors or the inferential method using data from passive samplers of atmospheric pollutant concentrations. Likewise, a variety of biological or chemical indicators have been used to determine empirical critical load (CL) values for these Mediterranean ecosystems. CL in California ecosystems have also been estimated with various models, but empirical CL are important for evaluating how realistic the modeled CL values are. Prominent endpoints used to estimate empirical CL include lichen functional groups, streamwater nitrate concentrations, mycorrhizal community changes, and increases in invasive annual grasses. Preliminary empirical CL are also proposed for soil acidification, ponderosa pine fine root biomass and forest sustainability. At relatively low CL values (5-8 kg N/ha/yr) grassland and coastal sage scrub are susceptible to type conversion due to enhanced risk of invasion by exotic annual grasses. Similarly, in the desert, CL exceedance favors grass invasion which greatly increases the risk of fire in areas where fires are normally infrequent, again with high potential for catastrophic vegetation type change. Areas of potential N CL exceedance in California are indicated by overlaying vegetation and N deposition maps.

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