Effects of photochemical oxidants on Japanese forest tree species under elevated CO_2 concentration

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Two-year-old Betula platyphylla Sukatchev var.japonica, three-year-old Betula ermanii Cham., two-year-old Pinus densiflora Sieb.et Zucc.and Cryptomeria japonica D.Don seedlings were exposed to ambient or elevated ozone (O_3) and carbon dioxide (CO_2) during two growing seasons in Maebashi, Gunma Prefecture, Japan. Ten open-top chambers were used for five different gas treatments : filtered air (FA), ambient air (1.0x ambient O_3), elevated O_3(1.5x ambient), elevated CO_2 (1.5x ambient), and elevated O_3 and CO_2. At ambient CO_2 concentration, the dry weight growth of the four tree species was significantly reduced with increasing the concentration or dose of O_3. On the other hand, the dry weight growth of the four tree species grown at elevated CO_2 concentration was significantly increased as compared with that of the seedlings grown at ambient CO_2 concentration. Significant interactions between O_3 and CO_2 treatments could not be found on the growth and photosynthetic parameters of B.ermanii, P.densiflora and C.japonica, but on those of B.platyphylla. The effects of elevated CO_2 concentration on the O_3-induced reductions in the dry weight growth and net photosynthesis were counteractive in B.platyphylla, while not in the other three tree species. The results obtained in the present study indicate that the growth and photosynthetic responses to elevated O_3 and CO_2 concentrations are different among the tree species.