Research Program Area: Ecosystem & Multimedia Effects
The following report describes the findings of the California Air Resources Board-sponsored study entitled, "Effects of Acid Fog and Ozone on Conifers." The objectives of the study were to evaluate the effects of acidic fog (pH 2.0, 3.0, or 4.0) on the physiological, biochemical, and growth responses of two coniferous tree species (Pinus ponderosa and Abies concolor), and to determine if exposure to acidic fog predisposed the tree seedlings to the phytotoxic effects of ozone (O3). The present study was conducted as two sequential experiments. The first experiment was conducted in the spring of 1987, and focused on determining the metabolic basis of acidic fog-induced alterations in seedling growth. The second experiment was conducted in the summer of 1987, and focused on determining whether the prior exposure to acidic fog altered the sensitivity of the seedlings to O3- induced effects on growth and injury. The criteria used to assess seedling metabolic responses to acidic fog and O3 were measure of gas exchange (i.e., net photosynthesis, transpiration, stomata1 conduc-tance of water vapor), metabolite levels (i.e., foliar pigment concentrations, needle starch content), membrane permeability and mycorrhizal establishment. Additionally, measures of growth (i.e., organ dry weights, stem dimensions), and foliar injury were also assessed. The spring fog experiment was conducted in open plots located on field 8C on the University of California at Riverside (UCR) Agricultural Field Experiment Station. Acidic fog was applied three times per week over a six week period (16 fog exposures in total). Injury symptoms developed on newly emerging needles in both tree species exposed to pH 2.0 fog, but did not occur in plants in the other fog treatment groups (pH 3.0 or 4.0, and no fog). For white fir, significant injury at pH 2.0 was also found for old needles. While white fir exhibited a greater amount of foliar injury than ponderosa pine, fog did not cause significant alterations in growth in either species. Membrane permeability, measured as changes in the conductivity of needle segment extracts and K+ leakage, were significantly increased in ponderosa pine following exposures to pH 2.0 fog. In contrast, fog treatments did not significantly alter membrane permeability responses in white fir. On selected dates gas exchange rates in ponderosa pine were reduced by pH 2.0, and in white fir by pH 2.0 and 3.0 compared to plants exposed to pH 4.0 fog, Despite these reductions, whole-study average photosynthesis rates were not significantly inhibited. Needle starch levels were significantly decreased in white fir by pH 2.0 fog. Ozone treatments were applied during the summer utilizing the open-top chambers in field 8C maintained by the California Air Resources Board (ARB). The seedlings were exposed to either charcoal-filtered (CF) or nonfiltered (NF) air from 22 May 1987 to 14 September 1987. The 12-hour average O3 concentration in NF and CF chambers was 73 and 18 mL L-1 ,respectively. After 115 days of exposure to NF air, there were no significant effects of O3 on injury development in either species, or on growth in ponderosa pine. On the other hand, white fir exposed to NF air exhibited higher stem and needle dry weights than trees grown in CF air. Conductivity and K+ leakage were increased by exposure to NF air in ponderosa pine. On a percentage basis, old ponderosa pine needles were affected more than young needles. In contrast, membrane permeability in young needles of white fir was decreased by exposure to NF air, but old needles were not affected. Air quality or fog pretreatment effects on foliar pigment concentrations were variable in both tree species. While significant treatment effects were sporadically observed during the study, carotenoid levels in young needles of ponderosa pine and chlorophyll levels in old needles of white fir were altered most often by exposure to O3. Mycorrhizal colonization responses were significantly higher in ponderosa pine seedlings exposed to NF air compared to plants in CF air. In white fir, no significant treatment effects were observed. The results of the present study provide evidence that the growth and metabolic responses of two coniferous tree species could be altered by multiple applications of acidic fog, and by exposure to ambient O3. In general, the alterations were slight to modest, which may be attributed to the low degree of stress severity, and the slow rate of tree growth. In the spring, significant treatment effects were found only in seedlings exposed to pH 2.0 fog. Affected seedlings in both species exhibited increased injury to young needles, and increased membrane permeability in old needles. In the summer, exposure to O3 had sporadic effects, and no clear trends were evident. The findings of the present study indicate that exposure to acidic fog followed by O3 does not cause detectable changes in conifer seedling growth within a single growing season. Nevertheless, it is clear that acidic fog and O3 cause temporal alterations in seedling physiology and biochemistry. Additional research is needed to determine if these data provide a reliable indication of air pollution effects on conifer seedling growth within a single growing season, and to provide baseline information for making accurate assessments of air pollution effects on forest ecosystems in the Sierra Nevada.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
Stay involved, sign up with CARB's Research Email Distribution List