Summer throughfall and winter deposition in the San Bernardino Mountains in Southern California

Abstract

Summer throughfall and year-round precipitation chemistry were studied for three years at Barton Flats (BF), a low to moderate pollution site in the San Bernardino Mountains (SBM) in southern California. Winter fog plus dry deposition, and bulk deposition were also measured during one season at three sites traversing an atmospheric deposition gradient in the SBM. Throughfall N (NO 3 − + NH 4 +) inputs during the growing season in 1993 were 4.0 kg ha −1 compared to 0.4 kg ha −1 for S. Deposition of N and S in summer 1994 was 50% of that in 1993, and precipitation in summer 1994 was 31 % as high as in 1993. Summer throughfall concentrations were highest in 1994, the summer with the lowest precipitation. Ionic concentrations in rain and throughfall at BF during the summer, particularly NO 3 −, were generally as high or higher than values reported for forests in Europe and eastern North America with much higher atmospheric deposition inputs. The high throughfall concentrations at BF were apparently due to the washoff of accumulated dry pollutants from mature trees with large surface areas during infrequent low-volume rain events. White fir, the species with the greatest foliar surface area, had the highest throughfall concentrations and the lowest throughfall volumes. Canopy effects of ponderosa and Jeffrey pine on precipitation chemistry and volume were intermediate, while California black oak throughfall was less modified chemically than the throughfall of the conifer species. Ionic deposition (mg m −2 land area) was highest under fir and pine, intermediate under oak, and lowest in rainfall. During the winter, peaks in wet deposition inputs at BF were associated with high monthly rain volumes (11–44 cm), while similar wet deposition peaks in August occurred with only a few light rains (1.4–3.4 cm). Winter (13 December 1993 to 11 April 1994) bulk deposition of N and S at Camp Paivika, a western high-deposition site, was 7.7 and 1.7 kg ha −1, respectively, compared to 0.9 and 0.3 kg ha −1 of N and S at BF. Winter deposition of NO 3 −, NH 4 + and SO 4 2− to passive nylon line collectors (fog plus dry deposition) was 15-,18- and 6-times greater at Camp Paivika than at BF. Winter deposition in both wet and dry forms in the SBM was greater than previously estimated from precipitation data for other forest sites in southern California.