Growth responses of yellow‐poplar(Liriodendron tulipifera L.) exposed to 5 years of O3 aloneor combined with elevated CO2

Field‐grown yellow‐poplar (Liriodendron tulipifera L.) werefumigated from May to October in 1992–96 within open‐topchambers to determine the impact of ozone (O3) aloneor combined with elevated carbon dioxide (CO2) on saplinggrowth. Treatments were replicated three times and included: charcoal‐filteredair (CF); 1 × ambient ozone (1 × O3);1·5 × ambient ozone (1·5 × O3);1·5 × ambient ozone plus 350 p.p.m.carbon dioxide (1·5 × O3 + CO2)(target of 700 p.p.m. CO2); and open‐air chamberlessplot (OA). After five seasons, the total cumulative O3 exposure (SUM00 = sumof hourly O3 concentrations during the study) rangedfrom 145 (CF) to 861 (1·5 × O3) p.p.m. × h (partsper million hour). Ozone had no statistically significant effecton yellow‐poplar growth or biomass, even though total root biomasswas reduced by 13% in the 1·5 × O3‐exposedsaplings relative to CF controls. Although exposure to 1·5 × O3 + CO2 hada stimulatory effect on yearly basal area growth increment aftertwo seasons, significant increases in shoot and root biomass (∼ 60% increaserelative to all others) were not detected until the fifth season.After five seasons, the yearly basal area growth increment of saplingsexposed to 1·5 × O3 + CO2‐air increasedby 41% relative to all others. Based on this multi‐yearstudy, it appears that chronic O3 effects on yellow‐poplargrowth are limited and slow to manifest, and are consistent withprevious studies that show yellow‐poplar growth is not highly responsiveto O3 exposure. In addition, these results show thatenriched CO2 may ameliorate the negative effects of elevatedO3 on yellow‐poplar shoot growth and root biomass underfield conditions.