Throughfall deposition of nitrogen and sulfur in a Jeffrey pine forest in the San Gabriel Mountains, southern California

Deposition of N and S in bulk precipitation and throughfall, and dry deposition to surrogate surfaces were determined in 1994 in a high-elevation mixed conifer forest dominated by Jeffrey pine (Pinus jeffreyi Grev. & Balf.) in the San Gabriel Mountains northeast of Los Angeles, California. Three sites 2 km apart, with a range of N deposition inputs, were chosen for study. N deposition in throughfall and dry deposition fluxes of N varied among the three sites, presumably because of differing pollutant transport with varying air flow patterns as a function of ridge and canyon orientation and the prevailing northeasterly winds. Net throughfall, an estimate of dry deposition, was the dominant input source of NH4+ and NO3−, except at Vista where wet deposition of NH4+ was greater than dry deposition of NH4+. Wet deposition of SO42− was much greater than dry deposition of SO42− at all three sites. Higher SO42− deposition in bulk precipitation and throughfall in this study compared to montane sites further from the coast was presumably because of marine aerosols transported inland during storm events and possibly by the prevailing winds. The location of the study sites in relation to the ocean (65 km inland) and to the urban smog source, resulted in throughfall inputs which were higher in S (7.8–12.5 kg ha−1 year−1), but lower in N (6.0–11.5 kg ha−1 year−1) compared to montane sites in the lower San Gabriel Mountains (23 kg N ha−1 year−1) or in the western end of the San Bernardino Mountains further inland (19 kg N ha−1 year−1; 3 kg S ha−1 year−1). S concentrations in pine foliage (0.69–0.83 g kg−1) were also greater in the San Gabriel Mountain sites than in four sites located further inland in the San Bernardino Mountains (0.48–0.56 g kg−1). Results of this and other studies suggest that the threshold for throughfall N deposition levels which lead to symptoms of N saturation in southern California montane ecosystems is ca. 16–19 kg ha−1 year−1.