Effects of Light–Nitrogen Interactions on Leaf Functional Traits of (Picea neoveitchii Mast.)

<i>Picea neoveitchii</i> Mast., a critically endangered spruce species endemic to China, is classified as a national second-level key protected wild plant and listed as critically endangered (CR) on the International Union for Conservation of Nature (IUCN) Red List. Its habitat features complex forest light environments, and global climate change coupled with environmental pollution has increased regional nitrogen deposition, posing significant challenges to its survival. This study explores the effects of light-nitrogen interactions on the leaf functional traits of <i>Picea neoveitchii</i> Mast. seedlings by simulating combinations of light intensities (100%, 70%, and 40% full sunlight) and nitrogen application levels (0, 10, and 20 g N·m ^-2·a^-1, where g N·m^-2·a^-1 denotes grams of nitrogen applied per square meter per year). We examined changes in morphological traits, anatomical structures, photosynthetic physiology, and stress resistance traits. Results indicate that moderate shading (70% full sunlight) significantly enhances leaf morphological traits (e.g., leaf length, leaf area, and specific leaf area) and anatomical features (e.g., mesophyll tissue area and resin duct cavity area), improving light capture and stress resistance. Medium- to high-nitrogen treatments (10 or 20 g N·m^-2·a^-1) under moderate shading further increase photosynthetic efficiency, stomatal conductance, and antioxidant enzyme activity. According to the comprehensive membership function evaluation, the L2N0 (70% full sunlight, 0 g N·m^-2·a^-1) treatment exhibits the most balanced performance across both growth and stress-related traits. These findings underscore the critical role of light-nitrogen interactions in the growth and adaptability of <i>Picea neoveitchii</i> Mast. leaves, offering a scientific foundation for the conservation and ecological restoration of endangered plant populations.