Growth and crown architecture of two aspen genotypes exposed to interacting ozone and carbon dioxide

To study the impact of ozone (O-3) and O-3 plus CO2 on aspen growth, we pla nted two trembling aspen clones, differing in sensitivity to O-3, in the gr ound in open-top chambers and exposed them to different concentrations Of O -3 and O-3 plus CO2 for 98 days. Ozone exposure (58 to 97 mul l(-1)-h, tota l exposure) decreased growth and modified crown architecture of both aspen clones. Ozone exposure decreased leaf, stem, branch, and root dry weight pa rticularly in the O-3 sensitive clone (clone 259). The addition of CO, (150 mul l(-1) over ambient) to the O-3 exposure counteracted the negative impa ct of O-3 only in the O-3 tolerant clone (clone 216). Ozone had relatively little effect on allometric ratios Such as, shoot/ root ratio, leaf weight ratio, or root weight ratio. In both clones, however, O-3 decreased the sho ot dry weight/shoot length ratio and shoot diameter. This decrease in wood strength caused both current terminals and long shoots to droop and increas ed the branch angle of termination. These results show that aspen growth is highly sensitive to O-3 and that O-3 can also significantly affect crown a rchitecture. Aspen plants with drooping terminals and lateral branches woul d be at a competitive disadvantage in dense stands with limited light. Publ ished by Elsevier Science Ltd.