Abstract: [Objective] This study investigated transpiration characteristics and carbon fixation and oxygen release, cooling and humidification of two typical afforestation tree species of Quercus acutissima and Cunninghamia lanceolata in Nanjing suburbs in order to provide basic scientific data for the reasonable management and eco environmental benefit evaluation. And this study could further enrich knowledge about eco-physiology of Q. acutissima and C. lanceolata production, which would be good for the reasonable operation and management of Q. acutissima and C. lanceolata forests, and benefit to improve the forestry production and provide theoretical basis in species selection for rapid growth, good water saving performance, high social benefit and economic benefit in the delta area of Yangtse River. [Method] A Li-6400 portable photosynthesis instrument was used to measure light photosynthetic and transpiration indexes, a LAI-2200 plant canopy analyzer was used to measure the leaf area index, and a mini weather station measured was used to collect weather data, of Q. acutissima and C. lanceolata forests of Nanjing suburb in spring, summer, autumn, and winter, respectively, to quantitatively study the seasonal variations in carbon fixation and oxygen release, cooling and humidification. With the collected data, the "optimal" multivariate regression model was constructed to determine the main impact factors in each season. [Results] Q. acutissima diurnal variation of net photosynthetic rate showed a bimodal curve in spring, showed a single peak curve in summer and autumn. C. lanceolata diurnal variation in net photosynthetic rate was a bimodal curve in spring, a unimodal curve in summer, autumn and winter. The diurnal variation in transpiration rate of Q. acutissima showed a bimodal curve in spring and summer, and a single peak curve in autumn. The diurnal variation in transpiration rate of C. lanceolata was bimodal curve in spring and winter, and a single peak curve in summer and autumn. The carbon fixation and oxygen release, and cooling and humidification of Q. acutissima and C. lanceolata had overall the similar seasonal variation patterns with the net photosynthetic rate and transpiration rate, that is, summer > spring > autumn > winter with relatively large seasonal fluctuations. Q. acutissima had higher daily light photosynthetic carbon fixation and oxygen release, transpiration cooling humidification amount per unit land area in spring, summer, and autumn and their corresponding annual total amount than C. lanceolata. [Conclusion] There existed significant differences in photosynthetic carbon release oxygen and transpiration cooling humidifying capacity, the same tree species in different season, and different tree species in the same season. The variation varied along with the geographical location of the trees, ecological environment and species characteristics. However, the overall performance was strongest for the summer photosynthetic carbon release oxygen and transpiration cooling humidifying capacity, and weakest in winter, with spring and autumn in the middle. Q. acutissima had stronger photosynthetic carbon fixation and oxygen release capacity and transpiration humidification and cooling capacity than C. lanceolata through the calculation and analysis, proving that Q. acutissima has stronger ability in using light to fix carbon dioxide and accumulation energy, cooling and humidification, microclimate regulation to improve and adapt the environment. It can be seen that Q. acutissima belongs to the water consumption and high yield, high input high output type. In contrast, C. lanceolata belongs to water saving middle, high input middle class. Thus, we can deploy the suitable tree species as the main afforestation tree species according to the local precipitation conditions to develop the suburban forestry. While better ensuring and improving the living environment and quality of urban residents, the output of forest industry is improved.

Key words: Quercus acutissima, Cunninghamia lanceolata, photosynthetic nitrogen oxygen release, transpiration cooling humidification