Individual and interactive effects of air warming and elevated O3 on carbon fixation and allocation in two urban tree species

With continuous urbanization and climate warming, increased air temperature and elevated ozone (O3) concentration often co-occur in many urban areas, but we still lack information about the interactive effects of warming and elevated O3 on urban trees. In the present experiment, the single and combined effects of increased air temperature (IT, ambient air temperature + 2 degrees C) and elevated O3 (EO, ambient air O3 concentrations + 80 ppb) on carbon (C) fixation and allocation in Quercus mongolica and Pinus tabuliformis, which are widely used as street tree species in urban areas of China, were investigated over two consecutive growing seasons by using 13C isotope techniques. The results showed that IT increased biomass, photosynthetic gas exchange parameters and total 13C content of both tree species. Compared to ambient temperature, IT significantly increased the total 13C content labelled by 56.6 % in Q. mongolica and by 31.2 % in P. tabuliformis in 2021. Elevated O3 induced a decrease in biomass and net photosynthetic rate (Pn) in both tree species. Compared to ambient O3, elevated O3 significantly decreased Pn by 52.6 % in Q. mongolica and by 37.4 % in P. tabuliformis in 2020. The treatment EO decreased 13C allocation to roots but increased 13C content and distribution in leaves in Q. mongolica. These findings demonstrated that EO inhibited the growth and photosynthesis of the two tree species. Our results showed that Q. mongolica was more sensitive to IT and EO than P. tabuliformis, but the former has a self-repair mechanism under increased O3 stress as it is able to invest more carbon to repair leaf damage to a certain extent. Our study also found that the total biomass, relative growth rate, Pn and total 13C content remained higher under the combination of IT and EO compared to EO alone, suggesting that moderate warming may mitigate the negative effects of elevated O3 stress to some extent.