Changes in growth, leaf abscission, and biomass associated with seasonal tropospheric ozone exposures of Populus tremuloides clones and seedlings

The effects of single-season tropospheric ozone (O-3) exposures on-growth, leaf abscission, and biomass of trembling aspen (Populus tremuloides Michx.) rooted cuttings and seedlings were studied. Plants were grown in the Upper Peninsula of Michigan in open-top chambers with O-3 exposures that ranged from 7 to 92 ppm-h. Depending on the genotype, total seasonal O-3 exposure in the range of 50-92 ppm-h had negative impacts on stem, retained leaf, and root biomass accumulation and on diameter growth. Leaf abscission generally increased with increasing O-3 exposure and was the principal cause of the decrease in leaf biomass of the O-3-treated plants. Considerable genetic variation in O-3 responses occurred, as shown by 3 differences in sensitivities among clones and among seedlings. However, the responses to O-3 of rooted cuttings and seedlings were similar when seedling means were compared with clonal means for leaf abscission, diameter growth, retained leaf biomass, and root biomass. Comparison of a single square-wave treatment (52 ppm-h) with 70 and 92 ppm-h episodic exposures suggested that the plant response to the square-wave exposure was similar to the response to the highest episodic exposure even though the 92 ppm-h episodic exposure was almost twice the square-wave exposure. Our results are consistent with previous studies that show that P. tremuloides is highly responsive to O-3 exposure and this response has a strong genetic component.