Growth and Leaf Gas Exchange in Three Birch Species Exposed to Elevated Ozone and CO_2 in Summer

摘要

We examined the effects of ozone and elevated CO_2 concentration in summer on the growth and photosynthetic traits of three representative birch species in Japan (mountain birch, Monarch birch, and white birch). Seedlings of the three birch species were grown in 16 open-top chambers and were exposed to two levels of ozone (6 and 60 nmol mo~(-1) for 7 h per day) in combination with two levels of CO_2 (370-380 and 600 μmol mol~(-1) for daytime) from July to October. No adverse effects of ozone were found in the Monarch birch or the white birch, but elevated ozone in summer reduced branch biomass and net photosynthesis, and accelerated leaf abscission, in the mountain birch. Elevated CO_2 promoted root development and thereby reduced the ratio of shoot dry mass (stem + branch) to root dry mass (S/R ratio) in the mountain birch and white birch. In contrast, there was no difference in dry mass between ambient and elevated CO_2 for the Monarch birch, due to down-regulation of photosynthesis. Studies of the combined effect of CO_2 and ozone revealed that elevated CO_2 did not ameliorate the effect of ozone on mountain birch in late summer. In considering the ameliorating effect of CO_2 on ozone damage, it is necessary to take account of the species and the season.