GROWTH AND BIOMASS ALLOCATION OF SYMPTOMATIC AND ASYMPTOMATIC POPULUS-TREMULOIDES CLONES IN RESPONSE TO SEASONAL OZONE EXPOSURES

The effect of single-season ozone exposures on growth and biomass production of numerous trembling aspen (Populus tremuloides Michx.) clones was studied, using open-top chambers, in the Upper Peninsula of Michigan. All clones studied had previously been ranked according to foliar sensitivity to short-term, high-dose ozone exposures. For this study, comparisons were made between plants grown in charcoal-filtered, nonfiltered, and ozone-added (consisting of 80 ppb ozone for 6 h per day, 3 days per week) chambers and in open-air plots. For the plants grown in ozone-added chambers, as compared with those grown in the charcoal-filtered air, stem biomass decreases were 0% for the 18 ozone-tolerant clones and 46.4% for the 18 ozone-sensitive clones, in 1988. In 1989, they were 5% for the tolerant clones and 74% for the sensitive clones. Thus, our results suggest that ozone can have either a negative effect on growth or no effect, depending on the clones tested. However, it is clear from this study that ozone consistently affected biomass production in sensitive clones in a negative manner. These results strengthen the case for ozone being a strong selective force, even at relatively low total doses.