Abstract: One-year-old seedlings of two main tree species (Betula platyphylla, Populus alba × P. berolinensi) in Shenyang were exposed to high O₃ concentration (about 160 nmol·mol^-1) in opentop chambers (OTCs) and its effect on photosynthesis in leaves of the two tree species was studied. The results showed that the foliage of both species showed necroses induced by the elevated O₃. After 14 and 30 d exposures of elevated O₃, the percentages of visible foliar injury per leaf in B. platyphylla were 72.0% and 91.6%, significantly higher than that in leaf of P. alba × P. berolinens, which were 21.5% and 26.2% (P<0.01). Chl-a and Car contents decreased significantly in B. platyphylla, but Chl-b content increased in P. alba × P. berolinensi (P<0.05). Electrolyte leakage and malondialdehyde (MDA) contents were significantly increased in leaves of the two tree species (P<0.05), while superoxide dismutase (SOD) and ascorbic acid peroxidase (APX) activities decreased significantly (P<0.05). The net photosynthetic rate (P_n) was decreased significantly and intercellular CO₂ concentration (C_i) was significantly increased in the two tree species (P<0.05), but stomatal conductance (G_s) increased in P. alba × P. berolinensi and decreased slightly in B. platyphylla. We infer that the decrease in photosynthesis of the two tree species is related to nonstomatal factors. P. alba × P. berolinensi has higher tolerance to high O₃ concentration than B. platyphylla, and B. platyphylla can be considered as an O₃ indicator species.

Key words: ecological suitable region, Codonopsis pilosula, maximum entropy model, ecological characteristics