Current and future effects of ozone and atmospheric nitrogen deposition onCalifornia's mixed conifer forests

Mixed conifer forests in southern California are exposed to elevated levels of ozone (O-3) and atmospheric nitrogen (N) deposition. Growing season 12- h daily average O-3 levels are 0.06-0.09 ppm, and N deposition rates are 5- 45 kg N/ha per year. However, N deposition rates have high spatial variabil ity due to the heterogeneous and open-patchy characteristics of these stand s. Annual deposition fluxes are greatest to canopy-covered areas with high pollution exposure. The harmful effects of O-3 on sensitive pines are well documented, and selected areas in the San Bernardino and San Gabriel mounta ins have reached N-saturation as demonstrated by high nitrate (NO3-) export in streams, and a suite of plant and edaphic indicators. The adverse effec ts of O-3 culminate at the community level as a result of chronic effects o n pine needle structure, retention, and physiology. Alterations in whole-tr ee biomass occur after several years due to higher carbon retention in the shoot for O-3 detoxification, and lower carbon allocation to roots. as a re sult of O-3 and N deposition. As a consequence, tree susceptibility to drou ght, windthrow, and root diseases could be exacerbated. Changes in forest c ommunity structure may occur due to the death of O-3-sensitive pines and fi re suppression and pine replacement by faster-growing, O-3-tolerant cedar a nd fir species. The negative impact of atmospheric N deposition occurs at t he ecosystem-level as an alteration of biogeochemical nutrient cycling. For trees growing on N-deficient soils, increased supplies of N could moderate harmful O-3 effects on growth for several decades. Over time, levels of so il N rise due to sustained inputs from the atmosphere, and the accelerated production and senescence of N-rich foliage by O-3, The increase in soil N is expected to favor the proliferation of nitrophilous overstory and unders tory species relative to pines. At this stage, N-limitation of forest produ ctivity would be partially alleviated, and N emissions from soil and NO3- l eaching losses elevated as a result of high soil N availability. In the wes tern San Bernardino Mountains and in low-elevation chaparral watersheds in the San Gabriel Mountains, NO3- revels in streams are as high or higher tha n in any other undisturbed montane watersheds in North America, and NO3- co ntamination of domestic water supplies is of near-term concern. The effects of chronic O-3 exposure and N deposition are expected to become more preva lent in the southern Sierra Nevada as human populations and influences in a djacent areas increase in the coming years. (C) 2001 Elsevier Science B.V. Ah rights reserved.