Abstract: Spatial structure of forest stands is closely related to understory light environment. However, the key structural variables influencing light conditions remain unclear. In this study, we quantitatively analyzed the relationship between stand spatial structure and light environment in mixed moso bamboo (Phyllostachys edulis) forest in central Fujian, China, aiming to identify the structural variables that significantly affect understory light conditions. The results showed that: (1) the mingling degree (M) ranged from 0 to 0.8519, opening degree (K) from 0.0689 to 0.3317, stand layer index (S) from 0 to 0.4306, neighborhood comparison (U) from 0.3696 to 0.5682, competition index based on intersection angle (UCI) from 0.2101 to 0.3734, and angle index (W) from 0.4384 to 0.6600. Canopy openness (CO) ranged from 7.47% to 33.73%, leaf area index (LAI) from 0.97 to 3.33, and mean leaf angle (MLA) from 10.62° to 85.50°. Diffuse radiation below the canopy (DifBe), direct radiation below the canopy (DirBe), and total radiation below the canopy (TotBe) ranged from 108.08 to 475.42 μmol·m^-2·s^-1, 1207.50 to 5027.92 μmol·m^-2·s^-1, and 1336.08 to 5503.25 μmol·m^-2·s^-1, respectively. (2) Stand spatial structure variables had significant correlations with understory light conditions. Specifically, M, K, and W were all highly negatively correlated (P<0.01) with CO, DifBe, DirBe, TotBe, and MLA. UCI was highly positively correlated with DirBe and MLA (P<0.01), and positively correlated with TotBe (P<0.05). (3) Among the structural variables, M, W, and K were identified as key factors influencing light environment, collectively explaining 24.8% of the total variation, with individual contributions of 14.14%, 5.92%, and 2.91%. (4) Light-related variables (CO, DifBe, DirBe, TotBe, MLA, and LAI) could be effectively fitted using the key spatial structural variables M, W, and K, and all models were statistically significant (P<0.05). In conclusion, mingling degree (M), opening degree (K), and angle index (W) are the primary spatial structure variables affecting light environment in moso bamboo forests. It is recommended that management practices aiming at improving understory light conditions can focus on regulating species mixture and optimizing horizontal spatial distribution.

Key words: mixed forest, light environment variable, mingling degree, angle index, opening degree