Abstract: Photosynthetic characteristics for different ages of 18-year-old Chinese fir shoots at different positions within crown were measured in 3 growing seasons. Results showed that the photonsynthetic rate of current shoots was the greatest in summer(mid-season) followed by late spring(fast-growing season) and autumn(late season), while that of 1-year-old and 2-year-old shoots decreased from late spring through summer to autumn. Light saturation point(LSP) for net photosynthetic rate and dark respiration rate(R_d) also decreased from late spring to autumn while apparent photon utilization efficiency(α) increased, especially those from summer to autumn. Except that light compensation point for photosynthesis (LCP) of current shoots in late spring was remarkably higher than in summer and autumn, there was no obvious difference of LCP among different seasons. In regard to differences of shoot ages, the photosynthetic rate of current shoots was highest followed by 1-year-old and 2-year-old shoots except for the late spring that the photosynthetic rate of current shoots was similar to 1-year-old shoots. The older the shoots, the lower the LSP,LCP and R_d,but the higher the α. For the different positions within tree crown, the photosynthetic rate, LSP,LCP and R_d of all shoots decreased from upper crown throught midcrown to lower crown while α increased, particularly differences between the upper-mid-crown and lower crown were quite remarkable. Differences among seasons were thought to be associated with photosynthetic biological activities of shoots, while differences among shoots at different positions within crown were related to acclimated light environment; however, differences among shoot ages were resulted from not only the biological activities but also the light environments. The daily courses of net photosynthetic rate(P_n) were also measured. The daily course of P_n in cloudy day was single peaked and P_n was mainly affected by photosynthetic photon flux density (PPFD) but also by changes of temperature, humidity, vapour pressure deficit(VPD) and ambient CO₂ to a small extent. However in clear day, the daily course of P_n was double peaked and PP_n was significantly infuenced by temperature, humidity and VPD as well as PPFD. The high temperature and VPD and low humidity resulted in the remarkable decrease of P_n at noon. The daily courses of P_n for current shoots and shoots at mid-upper crown were more remarkable than old shoots and shoots at lower crown.

Key words: Chinese fir (Cunninghamia lanceolata(Lamb.)Hook.), Photosynthetic characteristics, Position within tree crown, Shoot age, Seasonal change, Daily course