Growth responses of 53 open-pollinated loblolly pine families to ozone and acid rain

Field exposures of 9950 containerized 12-wk-old loblolly pine (Pinus taeda L.) seedlings representing 53 commercially important, open-pollinated families were conducted to evaluate individual and interactive effects of acid rain and O₃ on growth response. A 36-plot field research facility comprised of 33 open-top chambers and three open plots was used to test effects of five O₃ levels that included ambient (A) and seasonally integrated levels that were 0.53, 1.10, 1.58, or 2.15 times ambient. Individual effects of three levels of simulated acid rain (pH 3.3, 4.5, and 5.2) as well as their interaction with O₃ at 0.53A, 1.58A, and 2.15A levels were also included. Exposure to ambient air reduced average growth in height (26%), diameter (5%), and volume (14%) compared with growth of seedlings exposed to a 47% lower dose in charcoal filtered (CF) air. Responses to increasing O₃ above ambient levels varied widely between families, became increasingly inhibitory at the highest O₃ levels, but did not significantly exceed growth reductions found in ambient air. Diameter growth was reduced in most families by all levels of O₃ addition. Acid rain caused a general stimulation of height growth at ambient levels (pH 4.5), while both height and diameter growth were reduced at a mean pH of 3.3. Significant antagonism between rainfall acidity and O₃ effects on height and biomass increment was detected with increasing pollutant concentrations. Ozone reduced root/shoot biomass in most families, while acid rain did not.