The objective of this study was to explore the effects of small forest gap on the growth and survival of artificially regenerated Tilia amurensis seedlings by characterization of parameters such as leaf morphology, gas exchange parameters[net photosynthetic rate (P_n), transpiration rate (T_r), intercellular CO₂ concentration (C_i), stomatal conductance (G_s)], chlorophyll content(Chlt), carbon and nitrogen contents. These results provide a theoretical basis for the artificial regeneration of Tilia amurensis in small forest gaps and closed canopy.

The two-year-old artificially regenerated Tilia amurensis seedlings in Xiaojiu forest farm, Shangzhi City, Heilongjiang Province were selected as the research object. The plant height, ground diameter, survival rate and leaf morphology, gas exchange parameters, chlorophyll, carbon and nitrogen contents of Tilia amurensis seedlings in small forest gaps (G1, Forest gap area was greater than 30 m²; G2, The gap area was greater than 20 m² and less than 30 m²; G3, The gap area was less than 20 m²) and closed canopy (CK) were measured.

The plant height and survival rate of Tilia amurensis seedlings were higher in G1 and G2 than those in G3 and CK, and the height and survival rate of Tilia amurensis were positively correlated with the light intensity. The leaf area, specific leaf area and nitrogen content of Tilia amurensis were significantly increased with the decrease of gap area (P < 0.05). The specific leaf area and nitrogen content of Tilia amurensis were negatively correlated with light intensity. The P_n value was significantly higher in G1 and G2 than that in G3 and CK (P < 0.05), and P_n was positively correlated with light intensity. The contents of T_r, G_s and Chla/b were significantly decreased with the decrease of gap area (P < 0.05), while C_i and Chlt were significantly increased (P < 0.05). T_r and G_s were positively correlated with light intensity, while C_i and Chlt were negatively correlated with light intensity.

In small forest gaps, Tilia amurensis seedlings had strong photosynthesis in gaps greater than 20 m² and could maintain strong competitiveness in this habitat. However, Tilia amurensis seedlings could not capture enough light energy in gaps less than 20 m² in the closed canopy, which would inhibit its growth.