Objective The spatial heterogeneity of the forest understory light environment directly affects the distribution and growth of seedlings and saplings. Studying the ecological response mechanisms of seedlings and saplings under different light conditions aims to provide a scientific basis for forest regeneration and vegetation restoration.

Method Taking a Quercus variabilis plantation in the Western Hills of Beijing as the research object, detailed surveys were conducted on the diameter at breast height (DBH, ≥5 cm), tree height, and crown width of each tree, as well as the basal diameter, seedling height, and spatial coordinates of seedlings and saplings to explore the stand structure characteristics. Hemispherical photography technology was used to quantify understory light environment parameters. Combined with seedling and sapling growth indicators (basal diameter, tree height), two-way analysis of variance was employed to test growth differences under different light conditions, and regression analysis was used to explore the quantitative relationships between light environment factors and seedling/sapling growth indicators.

Result (1) The diameter structure of the Q. variabilis stand, as well as the basal diameter and tree height of seedlings and saplings, all followed a normal distribution (P < 0.05). (2) Light transmittance had a significant impact on the growth of Q. variabilis seedlings and saplings. Specifically, direct light transmittance of 0.2, diffuse light transmittance of 0.15, and total light transmittance of 0.15 could be identified as key light environment thresholds for distinguishing growth response differences among seedlings and saplings. When light transmittance was below these thresholds, there was no significant difference in seedling and sapling growth among different light transmittance gradients (P > 0.05). However, when exceeding these thresholds, both basal diameter and tree height showed a significant positive correlation with different light transmittance (P < 0.05). (3) .Within the same light transmittance interval, different types of understory light transmittance (direct light, diffuse light, and total light) had no significant differences in their effects on the growth of seedlings and saplings (P > 0.05). However, according to the effect sizes derived from linear regression analysis, the influence of different light types on the growth of seedlings and saplings showed a numerical trend: understory total light > diffuse light > direct light. (4) The spatial distribution of seedlings and saplings in the stand shows that they are mainly clustered in areas with low light intensity. This spatial pattern further verifies the important role of suitable light conditions in the growth of seedlings and saplings.

Conclusion The growth of Q. variabilis seedlings and saplings is mainly influenced by light transmittance, showing distinct threshold response characteristics. In practical production, supplementing planting in understory areas where light transmittance reaches the thresholds (direct light ≥ 0.2, and both diffuse and total light ≥ 0.15) would be beneficial for the growth of Q. variabilis seedlings and saplings, thereby facilitating the assisted natural regeneration of Q. variabilis plantations.