Abstract: Baccaurea ramifloraLour. is a native tropical tree species with different light adaptability at seedling stage and maturity stage. Clarifying the growth, physiological and biochemical response mechanisms of B. ramiflora seedlings to different shading environments can help determine the suitable light environment for seedling growth, which is of great significance for cultivation and ecological application. Five shading treatments were set up, including CK (natural light), S3 (shading rate of 45.3%), S4 (shading rate of 69.2%), S6 (shading rate of 80.0%) and S8 (shading rate of 90.2%). Growth and physiological changes of seedlings from Tingkou Mountain and Xiexie Mountain were measured after four months of treatment. With the increases in shading, leaf area and photosynthetic pigment content of B. ramiflora seedlings increased, while chlorophyll a/b value and catalase (CAT) activity decreased. Malondialdehyde content decreased firstly when shading rate was ≤S6, and then slightly increased in S8. Plant height and ground diameter firstly increased and then decreased, while electrical conductivity, proline (Pro) and soluble sugar (Ss) contents firstly decreased and then increased. Under strong light conditions, B. ramiflora seedlings eliminated membrane lipid peroxidation by increasing CAT activity and the relative content of carotenoids. Under low light conditions, seedlings increased light energy utilization by enhancing leaf area and photosynthetic pigment content. Pro and Ss, as comprehensive regulatory substances, could alleviate cell membrane stress caused by strong light and weak light by changing osmotic pressure. Pro and Ss contents of seedlings from Tingkou Mountain and Xiexie Mountain were the lowest under the treatments of S6 and S4, respectively, while the increase of plant height and ground diameter was the highest under the treatments of S6 and S4, respectively. Our results indicated that B. ramiflora at seedling stage exhibited the characteristics of shading plants. In other words, strong light or heavy shading will inhibit its growth. The seedlings from Tingkou Mountain were more shade-tolerant than that from Xiexie Mountain, highlighting the effects of long-term differences of light environment in their provenances.

Key words: Chinese fir, soil available N, litterfall., forest type, soil nitrogen transformation rate