A Comparison of Empirical and Modelled Nitrogen Critical Loads for Mediterranean Forests and Shrublands in California

Nitrogen (N) deposition is impacting a number of ecosystem types in California. Critical loads (CLs) for N deposition determined for mixed conifer forests and chaparral/oak woodlands in the Sierra Nevada Mountains of California and the San Bernardino Mountains in southern California using empirical and various modelling approaches were compared. Models used included the Simple Mass Balance (SMB) model for nutrient N and acidification (both site-specific and regional approaches) and the Daycent process-based biogeochemical simulation model. Empirical CLs reported herein were based on responses across N deposition gradients of lichen community functional groups and streamwater nitrate (NO3) leaching. Broad scale CL mapping for the San Bernardino Mountains using the SMB model resulted in nutrient N CL values that were on average approximately 50 % lower than the empirical CL value for NO3 leaching (17 kg ha−1 year−1) in California mixed conifer forests. Over the range of elevations and vegetation types in the San Bernardino Mountains, SMB CL values ranged from 5.1 to 13.0 kg ha−1 year−1 for nutrient N. For both the empirical NO3 leaching CL and the SMB estimate, the CL was generally lower for chaparral vegetation than for forests. The estimated CL for NO3 leaching derived from the Daycent model was equal to the empirical CL (17 kg ha−1 year−1), but the severity and frequency of elevated NO3 leaching was underestimated by Daycent. Statewide empirical CL exceedance maps indicate that 3.3 and 4.5 % of the chaparral and forested areas in California are in excess of the NO3 leaching CL. Likewise, 23.4, 41.2 and 52.9 % of the mixed conifer forest, oak woodland and chaparral areas are in excess of the empirical N CL for epiphytic lichen community effects, respectively. Eutrophication effects in terrestrial ecosystems of California are widespread, while significant acidification effects are limited to the more polluted sites in southern California.