Growth response of green and white ash seedlings to ozone, sulfur dioxide, and simulated acid rain

Nine-week-old green (Fraxinus pennsylvanica Marsh.) and white (F. americana L.) ash were exposed to O₃ and/or SO₂ (control, 0.10 ppm O₃, 0.08 ppm SO₂, or 0.10 ppm O₃ + 0.08 ppm SO₂) for 4 h d^-1, 5 d wk^-1 in combination with simulated rain (pH 3.0, 4.3 or 5.6, 1 h d^-1, 2 d wk^-1 at 0.75 cm h^-1) for 6 weeks, under controlled laboratory conditions, with rain applied either just before or after fumigation. Across all rain treatments, with ash biomass was suppressed by the application of O₃ and cumulative shoot elongation of green ash exposed to O₃ and/or SO₂ was less than controls. The combination of O₃ + SO₂ did not affect the growth of either species more than the pollutants applied alone. Leaf area ratio (LAR) and root to shoot ratio (RSR) exhibited quadratic responses to rain pH in green ash, across all pollutant treatments. Significant pollutant x pH interactions occurred in leaf weight ratio (LWR) in green ash and LAR and RSR in white ash. Significant linear increases in LAR and decreases in RSR, with decreasing pH, were observed for O₃ and SO₂-treated white ash. These findings are discussed relative to implications of the effects of gaseous pollutants in combination with acid rain on green and white ash growth.