Abstract:

Objective: The purpose of this study was to improve the light conditions of the Korean pine saplings under the forest by simulating the cutting of the canopy layer of the secondary forest, so as to promote the growth and development of the Korean pine saplings and accelerate the recovery process of the secondary forest. Method: According to distance control and growth quality, Korean pine saplings were selected from the core area in a 1 hm² large plot of Mongolian oak secondary forest. Individual openness optimization schemes with different thresholds (K_T =1, 1.5, 2) were used to control the openness of target Korean pine saplings under the forest. The openness optimization scheme is implemented through a two-level iterative optimization method, encompassing both tree and stand levels. At the tree level, the objective is to ensure that the openness of each target Korean pine sapling meets the predefined threshold value. Simultaneously, at the stand level, the optimization involves controlling a specific target Korean pine sapling while taking into account other target Korean pine saplings within the stand. The changes of non-spatial structure (total diameter classes, total species and canopy density) and spatial structure (uniform angle index, neighbourhood comparison and mingling) of stands before and after simulated cutting under different openness thresholds were compared, as well as the differences in openness and openness change rate of Korean pine saplings under different treatments. Result: A total of 145 target Korean pine saplings were selected by distance control and growth quality. The target Korean pine saplings were 1.1?4.9 m in DBH, 1.5?4.7 m in height and 0.6?1.7 m in crown radius. When K_T was 1, 1.5 and 2, the target Korean pine saplings reach the openness condition after cutting 14, 130 and 262 canopy trees for 3, 6 and 8 rounds respectively. After the optimized cutting simulation, the openness of Korean pine saplings was improved as a whole: when K_T was 1, the openness increased by 5.7% on average compared with the control, but there was no significant difference; when K_T was 1.5, the openness was 44.7% on average higher than that of the control, with significant difference; when K_T was 2, the openness was 107.7% on average higher than that of the control, with significant difference. Pre- and post-simulated harvesting activities exhibited no alterations in the total diameter classes and total species within the forest stand. Moreover, spatial structural metrics exhibited minimal changes. Under three different thresholds of openness (K_T = 1, 1.5, 2), the stocking intensity of harvesting is 1.5%, 15.6%, and 37.2% respectively. Compared with the canopy density of the control (CK) at 0.646, the canopy density under the three openness thresholds (K_T = 1, 1.5, 2) decrease by 0.5%, 13.3%, and 33.1% respectively. Conclusion: The two-level iterative optimization method for the openness index of Korean pine saplings improved the light conditions of Korean pine saplings under the forest, while maintaining the stability of most stand structures, and the openness threshold K_T =1.5 was better than K_T =1 or 2. The optimization methodology introduced in this research enables the determination of key parameters, including the total count of cutting rounds, the number of cutting trees within each round, the openness regulation route of each target Korean pine sapling and the distribution of cutting trees under different openness thresholds. This comprehensive framework offers invaluable guidance for shaping the post-replanting thinning strategy of Korean pine saplings and informs best practices for managing secondary forest ecosystems.

Key words: canopy optimization, openness, Mongolian oak secondary forest, Korean pine saplings