Three-year growth responses of Pinus taeda L. to simulated rain chemistry,soil magnesium status,and ozone

Height, diameter, and biomass were measured for loblolly pine (Pinus taeda L) seedlings grown in soil containing 15 or 35 Μg Mg g^?1 and exposed from May to October in 1987, 1988, and 1989 to three O₃ concentrations (sub-ambient, ambient, or twice-ambient) and to rain pH levels of 3.8 or 5.2. Reduction in biomass accumulation in seedlings exposed to twice-ambient O₃ vs sub-ambient O₃ was 14% (P = 0.03) in 1987, 11.4% (P = 0.002) by 1988, and 8% (P = 0.15) by 1989. The greatest height growth occurred in seedlings exposed to twice-ambient O₃, and the greatest stem diameter growth occurred in seedlings exposed to sub-ambient O₃. A comparison of stem volume (d²h) with stem biomass suggested that tissue density was reduced by elevated O₃. Biomass accumulation response to rainfall chemistry was small (5.5% reduction in the low pH treatment in 1989) and not statistically significant for most plant tissues. Growth response to soil Mg status was not significant. Hoewever, in 1989 treatment interactions between rainfall chemistry and soil Mg status were observed. Height was 5% greater (P = 0.02) and biomass was 6% greater (P = 0.10) in seedlings grown in higher-Mg soil and receiving higher-pH rainfall than seedlings grown in any of the other pH-Mg treatment combinations. The data suggest direct adverse effects of near ambient O₃ and indirect, slower acting and interacting adverse effects of rainfall chemistry and soil nutrient status on growth of loblolly pine.