Soil pathogens shift root strategies from acquisition to defense under shaded understorey conditions in Pinus koraiensis seedlings.

Broadleaved Korean pine forest, a typical zonal vegetation type in Northeastern China, is important in maintaining regional ecological security. However, the regeneration of Korean pine (Pinus koraiensis) seedlings remains a major constraint in the primary forests. Although declining light availability and increasing soil pathogens are recognized as critical factors affecting seedling survival and growth, their interactive effects and underlying mechanisms remain unclear. In this study, we conducted a controlled experiment with three light levels (200, 400, and 600 μmol·m-2·s-1) and four soil pathogen concentrations (0, 1 × 104, 1 × 106, and 1 × 108 CFU/mL) to examine seedling survival, growth, physiology, and above- and belowground functional traits. Low light significantly reduced survival, relative growth rate, photosynthesis, and biomass of P. koraiensis seedlings, while the presence of high pathogen concentrations exacerbated these impacts, notably driving mortality rates up by 95%. Leaf traits were only regulated by light, with shaded conditions increasing specific leaf area and nitrogen (N) concentration, but decreasing total phenolics and carbon/nitrogen ratio. In contrast, root traits were jointly influenced by light and soil pathogens. Under low light, seedlings exhibited an acquisitive strategy characterized by higher specific root length and N concentration, along with reduced root diameter and defense investment. However, increasing pathogen concentration reversed this pattern, suppressing resource acquisition traits while enhancing root diameter and defensive compounds (total phenolics and tannins). These results indicate that light determines the baseline resource context, while soil pathogens impose additional constraints that shift root strategies from resource acquisition to defense. Such interactive regulation modulates seedling establishment, with critical implications for improving forest regeneration and ecosystem stability.