Connection between phyllosphere microbial composition and leaf stoichiometry of Phoebe bournei at upper and lower canopy positions

Closer-to-nature forest management models that transform plantations into uneven-aged multi-storied forests, can enhance long-term sustainability. The density of the upper canopy trees regulates light resource allocation for trees in the understory, thereby correlating with changes in their phyllosphere microenvironment. This study assessed the associations between the stand density of upper-layer Cunninghamia lanceolata (375–810 trees·ha −1 ) and the leaf stoichiometry as well as the phyllosphere microbial community of understory Phoebe bournei . Based on this study, within the stand density range of 570–630 C. lanceolata trees·ha −1 , the lower-canopy P. bournei leaves maintained higher carbon reserves under low-light conditions, characterized by an elevated soluble sugar/starch ratio and significantly greater non-structural carbohydrates (NSC) and soluble sugar contents compared to upper-canopy leaves ( p < 0.05). Although increased C. lanceolata density did not significantly alter the leaf C and P contents or the N/P ratio of P. bournei ( p > 0.05), the upper canopy leaves displayed increased N content and a lowered C/N ratio under shading. Concurrently, NSC, soluble sugar, and starch contents significantly decreased in the upper canopy leaves ( p < 0.05). In contrast, the lower canopy P. bournei leaves sustained relatively high carbon reserves as the C. lanceolata density increased. Vertical differentiation in both leaf chemistry and phyllosphere microbial communities was observed across the canopy profile, and these patterns varied systematically with overstory C. lanceolata density. Phyllosphere microbial community’s composition was correlated with leaf chemical patterns: some taxonomic clusters (e.g., Bryocella -associated) were positively correlated with a microenvironment characterized by a high C/N ratio and high starch content, while other clusters (e.g., including Amnibacterium ) were positively correlated with high leaf nitrogen and phosphorus contents. These findings reveal correlative patterns among overstory tree density, leaf stoichiometry, and the phyllosphere microbiota. This observational framework not only establishes a basis for future controlled inoculation experiments but also provides an ecology‑informed reference for closer-to-nature forest management. • Phoebe bournei leaves across canopy positions show functional differentiation in nutrient utilization or carbohydrate homeostasis. • Phyllosphere microbiota demonstrate synergistic adaptation with leaf chemical composition across canopy positions. • Phoebe bournei leaves increase soluble sugar-to-starch ratio to optimize energy storage and promote beneficial microbes.