Synergistic effects of site factors, spatial structure, and their interactions on carbon storage in conifer–broadleaf mixed forests in southwestern Zhejiang

Site factors and spatial structure factors are key determinants influencing forest carbon storage. However, existing studies have mainly focused on the effects of site or stand structural factors on forest productivity, while comparatively less attention has been devoted to the integrated effects and causal pathways of site factors and specific spatial structural attributes (e.g., mixing degree, aggregation index, and competition index) on forest carbon storage. Additionally, traditional competition indices have not fully accounted for the dominant role of tree crowns in resource competition. This study, conducted in mixed forests in Longquan, Lishui City, Zhejiang Province, utilized data from 40 fixed 20 m × 20 m sample plots. A structural equation model (SEM) was constructed to systematically quantify the direct and indirect effects, as well as the pathways of site factors (elevation, slope, aspect, slope position, humus layer thickness) and spatial structure factors (mixing degree, aggregation index, competition index) on forest carbon storage, at both the stand and individual species scales. Furthermore, a modified crown-width-based Hegyi competition index (C_Hegyi)—was introduced and comparatively evaluated against the traditional diameter at breast height (DBH) distance-based competition index (V_Hegyi) across different tree species. Results indicate that: (1) For site factors, humus layer thickness and elevation have significant negative direct effects; slope and aspect primarily influence carbon storage through indirect pathways; (2) Among spatial structure factors, mixing degree and aggregation index significantly promote carbon accumulation, while competition index has a notable suppressive effect on carbon storage; (3) Different tree species exhibited significantly different response patterns to site and spatial structural factors, with carbon storage levels decreasing in the following order: Other Conifers (e.g., Pinus massoniana , Taxus chinensis , etc.) > Cunninghamia lanceolata ≈ Other Broadleaves (e.g., Ormosia hosiei , Liquidambar formosana , etc.) > Schima superba , revealing pronounced interspecific heterogeneity in carbon accumulation within mixed forests. (4) The comparison of competition indices indicates that C_Hegyi exhibits a 95.1% improvement in explanatory power among coniferous species, whereas V_Hegyi performs better in broadleaf species. This study provides a systematic understanding of the response mechanisms of site factors, stand spatial structure, and their interactions to carbon storage in mixed conifer–broadleaf forests in southwestern Zhejiang. The findings offer new theoretical support for optimizing forest structure, enhancing carbon sink functions, and implementing precise carbon sink management strategies.