Current and future effects of ozone and atmospheric nitrogen deposition on California’s mixed conifer forests

Abstract

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 variability due to the heterogeneous and open-patchy characteristics of these stands. 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 mountains have reached N-saturation as demonstrated by high nitrate (NO 3 −) export in streams, and a suite of plant and edaphic indicators. The adverse effects of O 3 culminate at the community level as a result of chronic effects on pine needle structure, retention, and physiology. Alterations in whole-tree 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 result of O 3 and N deposition. As a consequence, tree susceptibility to drought, windthrow, and root diseases could be exacerbated. Changes in forest community structure may occur due to the death of O 3-sensitive pines and fire suppression, and pine replacement by faster-growing, O 3-tolerant cedar and fir species. The negative impact of atmospheric N deposition occurs at the 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 soil 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 understory species relative to pines. At this stage, N-limitation of forest productivity would be partially alleviated, and N emissions from soil and NO 3 − leaching losses elevated as a result of high soil N availability. In the western San Bernardino Mountains and in low-elevation chaparral watersheds in the San Gabriel Mountains, NO 3 − levels in streams are as high or higher than in any other undisturbed montane watersheds in North America, and NO 3 − contamination of domestic water supplies is of near-term concern. The effects of chronic O 3 exposure and N deposition are expected to become more prevalent in the southern Sierra Nevada as human populations and influences in adjacent areas increase in the coming years.