Objective Larix gmelinii is the dominant species forming the cold-temperate forests of the Greater Khingan Mountains, and its biodiversity and carbon-density patterns are critical for regional carbon sequestration and ecological security. However, the combined effects of stand origin, stand age, and latitudinal gradients on forest structure and carbon dynamics remain insufficiently quantified. This study aimed to clarify the independent and interactive effects of these three factors on biodiversity and tree-layer carbon density.

Method Based on field-survey data from 929 L. gmelinii plots across the Greater Khingan Mountains, linear mixed-effects models and structural equation modeling were applied to evaluate the independent and interactive effects of stand origin, stand age, and latitude on biodiversity and carbon density.

Result Significant differences in tree-layer carbon density were observed between forest origins, with natural forests generally exhibiting higher carbon density than plantations. In plantations, carbon density was relatively low at the young stage, increased markedly with stand age, and declined at the mature and over-mature stages. Both biodiversity and tree-layer carbon density showed significant changes with increasing stand age, indicating that stand age is a key factor shaping forest structure and carbon storage. Along the latitudinal gradient, biodiversity indices and tree-layer carbon density generally decreased from lower to higher latitudes, reflecting strong climatic regulation. Multifactor analyses revealed significant interactions among stand origin, stand age, and latitude, suggesting that these factors jointly, rather than independently, determine the spatial patterns of biodiversity and tree-layer carbon density.

Conclusion Biodiversity and tree-layer carbon density in L. gmelinii forests are jointly driven by stand origin, stand age, and latitudinal gradients. Among these factors, stand age plays a dominant role, the effect of stand origin is more pronounced during early developmental stages, and latitudinal gradients indirectly influence carbon density patterns by regulating stand development. These findings provide a scientific basis for enhancing regional carbon-sink capacity, promoting climate adaptation of cold-temperate forests in Northeast China, and advancing ecological conservation and high-quality development.