Effects of forest gaps on root, stem, and leaf stoichiometric traits of Pinus koraiensis seedlings

Objective Forest gaps are considered microsites for tree species regeneration. Pinus koraiensis is a climax tree species in temperate forests, the nutrient strategies and regulatory mechanisms of Pinus koraiensis seedlings in forest gaps remain unclear. Methods In this study, four-year-old Pinus koraiensis seedlings of the same provenance were planted in different gap zones within forests, and after one growing season, the stoichiometric traits of their roots, stems, and leaves, as well as related physiological traits and environmental conditions, were measured. Results (1) Across different forest gap zones, there were no significant changes in the carbon, nitrogen, and phosphorus contents of the Pinus koraiensis seedlings’ root systems ( P > 0.05). The phosphorus content in the stems and leaves of the seedlings was significantly lower than that in a closed-canopy stand; leaf carbon content was significantly higher at the forest gap edges, while nitrogen content remained stable across all organs; (2) In all forest gap areas, the N:P and C:P ratios in stems and leaves were significantly higher than those in the closed-canopy stand. (3) Leaf carbon content was significantly positively correlated with the net photosynthetic rate (Pn), intercellular CO2 concentration (Ci), and transpiration rate (Tr); root phosphorus content was significantly positively correlated with chlorophyll a; and the root C:P ratio was significantly negatively correlated with chlorophyll a, suggesting coordinated variation among the plants' stoichiometric traits and their physiological traits. (4) Redundancy analysis (RDA) showed that canopy-structure variables were strongly associated with phosphorus-related stoichiometric variation of Pinus koraiensis seedlings. Conclusion Changes in the microenvironment following the formation of forest gaps are associated with shifts in the nutrient use strategies of Pinus koraiensis seedlings, with patterns showing organ specificity. Specifically, the forest gap environment is linked to altered photosynthetic physiology of Pinus koraiensis seedlings, which may in turn be associated with the differentiation of their carbon-phosphorus stoichiometric traits. This study recommends that, when selecting forest gaps as microsites for planting Pinus koraiensis seedlings, seedling adaptability should be considered, with priority given to small forest gaps, the edges of large forest gaps, or expanded gaps.