Effects of O3 and acidic rain on photosynthesis and growth in sugar maple and northern red oak seedlings

Two-year-old sugar maple Acer saccharum and northern red oak Quercus rubra seedlings were exposed to all combinations of several levels each of ozone (O₃) and simulated acidic rain. Deposition rates and amounts of simulated rain were normal for eastern North America (12·5 mm of rain twice per week) and levels of acidity in the various treatments ranged between pH 5·6 and 3·0. Plants were exposed to O₃ for 7 h per day on 5 d per week. Concentrations of O₃ were constant and ranged between 0·02 and 0·12 μl litre⁻¹ in the various treatments. Ozone treatments caused significant declines in net photosynthesis in both species, with the largest reductions observed (30% in maple and 20% in oak) after two months in the highest O₃ treatment (0·12 μl litre⁻¹). Reductions in growth as a result of O₃ treatments occurred in sugar maple, but apparently due to the relatively short duration of the pollution treatments, growth reductions were not observed in red oak. Chlorophyll contents in sugar maple leaves increased as a result of O₃ exposure. Simulated acidic rain treatments had no effect on either net photosynthesis or growth in either species and no interactive effects of the two pollutants were observed. The results of this study suggest that sugar maple and red oak are relatively insensitive to acidic rain over the course of a single growing season, but potential long-term effects are unknown. These two species were sensitive to relatively low concentrations of O₃, and ambient levels of O₃ in eastern North America could be having significant deleterious effects on sugar maple and red oak in the field.