Performance and Secondary Chemistry of Two Hybrid Aspen (Populus tremula L. x Populus tremuloides Michx.) Clones in Long-Term Elevated Ozone Exposure

The effects of moderately elevated ozone (ca. 35 ppb) on the growth and secondary chemistry of the leaves of two soil-grown Finnish hybrid aspen (Populus tremula L. x Populus tremuloides Michx.) clones with different ozone sensitivities were studied at an open-air exposure field in Kuopio, Finland. Stomatal conductance, photosynthetic rate, and chlorophyll fluorescence were measured during the third growing season. Foliar phenolic concentrations, ergosterol concentration of fine roots, and final dry mass of the trees were determined at the end of the third growing season. Elevated ozone increased the ectomycorrhizal status of the fine roots but had no effect on gas exchange or on the final biomass of either of the clones, indicating equal sensitivity to ozone and no effect of elevated ozone on the intraspecific competitive ability of the clones after three growing seasons. However, in agreement with the data from potted plants of the same clones after two growing seasons, significant differences between the clones were found in all parameters measured. A negative correlation between growth and high concentrations of foliar phenolics indicated that allocation to secondary chemistry also was costly in terms of growth under high resource availability.