Objective To investigate the species diversity, structural diversity, carbon stocks, and allocation patterns in the arboreal layer of Quercus aliena var. acuteserrata-dominated secondary forests across different types in Xiaolongshan Mountain forest region, Gansu Province of northwestern China, and to provide a scientific foundation for forest management practices and precision-based enhancement of carbon sequestration capacity in this region.

Method Based on stand basal area composition, the typical Quercus aliena var. acuteserrata secondary forests in the study area were classified into three types (oak-broadleaved mixed forest, oak forest, pine-oak mixed forest). The species diversity, structural diversity, carbon storage, and carbon allocation among different organs in the arborous layer of these secondary forest types were systematically calculated, with comparative analysis conducted using Kruskal-Wallis tests. Structural equation modeling (SEM) was subsequently employed to elucidate the relationship between species diversity, structural diversity, and carbon stocks.

Result (1) There were significant differences in the tree layer diversity of different types of Quercus aliena var. acuteserrata secondary forests. The tree species diversity of oak-broadleaved mixed forest was significantly higher than oak forest and the pine-oak mixed forest, and its average structural diversity was 4.133 ± 0.595, which was also significantly higher than oak forest (3.469 ± 0.537) and pine-oak mixed forest (3.201 ± 0.199). (2) There were significant differences in carbon storage accumulation and allocation among tree layers of three types of secondary forests of Q. aliena var. acuteserrata. The overall average carbon storage of oak forest was the highest ((94.528 ± 2.523) t/ha), followed by oak broadleaved mixed forest ((88.855 ± 16.297) t/ha), and the lowest ((82.094 ± 7.527) t/ha) in pine oak mixed forest. The allocation of carbon storage organs in the three types of secondary forests also showed a significant dominance of stem branch-root carbon storage, with a total proportion of stem and branch carbon storage ranging from 67.9% to 79.1%, and root storage ranging from 12.6% to 19.4%, totaling over 90%. (3) The structural equation modeling results showed that structural diversity had a significant positive impact on carbon storage (0.484, P < 0.001), and tree species diversity also had a positive impact on carbon storage, but it was not significant.

Conclusion Significant differences in species diversity, structural diversity, and carbon stocks are identified among the three Quercus aliena var. acuteserrata secondary forest types. The oak-broadleaved mixed forest consistently exhibites higher species diversity, structural diversity, and carbon stocks compared with the oak forest and pine-oak mixed forest. Structural diversity contributes more substantially to carbon stocks than species diversity, highlighting that enhancing forest ecosystem structural complexity may represent a more effective strategy for augmenting carbon sequestration potential.