Objective To analyze the leaf traits of Chinese fir (Cunninghamia lanceolata) and nanmu (Phoebe chekiangensis) in pure and mixed plantations and to study the effect of nitrogen deposition on leaf economic characters.

Method In this experiment, two nitrogen application levels were set by artificial nitrogen addition: 0 kg·ha^-1yr^-1N (contrast) and 30 kg·ha^-1yr^-1N (nitrogen addition). Every level consisted of three treatments: pure Chinese fir seedlings, pure nanmu seedlings and mixed Chinese fir and nanmu seedlings (1∶1 row mixing of Chinese fir seedlings and nanmu seedlings). During the experiment, eight indicators, including height of plants, diameter, crown width, fluorescence parameters, specific leaf mass, net photosynthetic rate, maximum net photosynthetic rate per unit mass and respiration rate of both Chinese fir seedlings and nanmu seedlings were measured. SPSS was used to analyze the significance of their differences among different treatments.

Result The results showed that the respiration rate and photosynthetic capacity of pure Chinese fir seedlings were significantly higher than that of pure nanmu seedlings, and the specific leaf weight was significantly lower than that of nanmu seedlings. Mixed planting significantly increased the respiration rate of Chinese fir seedlings and the photosynthetic capacity of nanmu seedlings, but significantly reduced the specific leaf mass of Chinese fir seedlings and nanmu seedlings and the net photosynthetic rate of Chinese fir seedlings. This may be due to the mixing led to interspecific competition between the seedlings of the two species, both of which allocated more energy to competition, but the nanmu seedlings dominated in competition. Nitrogen addition significantly increased the specific leaf mass and photosynthetic capacity of pure Chinese fir seedlings, and also significantly increased the respiration rate and photosynthetic capacity of nanmu seedlings, but significantly decreased the specific leaf mass of two seedlings. In addition, in mixture, nitrogen application significantly reduced the photosynthetic capacity of Chinese fir seedlings and the respiration rate of nanmu seedlings, and the specific leaf quality of two seedlings also decreased significantly. This indicated that the addition of nitrogen changed the leaf economic characters of Chinese fir seedlings and nanmu seedlings under mixed state to some extent. In the condition of competition, the different responses of Chinese fir seedlings and nanmu seedlings to nitrogen adding may be associated with their own factors. Chinese fir is more sensitive to the environment than nanmu. The content of nitrogen addition in this experiment had relatively little effect on the growth of nanmu seedlings. It might also be related to the stronger acid resistance of nanmu seedlings and its higher photosynthetic-related enzymes content and enzymes activity.

Conclusion Chinese fir seedlings are closer to the fast end of leaf economic spectrum, while nanmu seedlings were closer to the slow end, and the simple mixing will not change the relative position of the two seedlings in leaf economic spectrum. However, nitrogen addition can make Chinese fir seedlings develop towards the slow end of leaf economic spectrum, while nanmu seedlings develop towards the fast end of leaf economic spectrum.