Effects of elevated carbon dioxide and acidic rain on the growth of holm oak

In order to study the interactive effects of elevated atmospheric CO2 and acid rain on an evergreen oak, holm oak (Quercus ilex L.) (which is the keystone tree species in the Mediterranean environment) potted seedlings were grown for 90 days at two CO2 concentrations, ambient and +400 μmolView the MathML source. Half of each group was sprayed once a week with deionized (pH 5.6) or acidified (pH 2.5) water. Elevated CO2 enhanced growth: shoot and leaf sprouting, shoot length, total leaf area, total and individual leaf mass, stem, and fine root mass were increased. Acid rain increased leaf and shoot turnover by stimulating both abscission and sprouting and, because abscission was more common than sprouting, the result was growth inhibition. The trend of these results was visible after 30–40 days. Total leaf area was increased by the increase in leaf number at elevated CO2, and decreased by the reduction in leaf size in the acid treatments. Elevated CO2 favored biomass partitioning to stems, whereas acid rain did not modify allocation. Root-to-shoot ratio was not significantly affected. Despite the elevated CO2-enhanced growth of acid-stressed holm oak seedlings, significant CO2 × pH interactions were recorded only in cases where the effect of acidity was null (biomass allocation to stem) or positive (shoot and leaf sprouting). Thus, we conclude that short-term CO2 enrichment did not alleviate the negative effect of acid rain on holm oak growth and that a strong acid load may inhibit the CO2-promoted biomass partitioning to stems.