Contrasting Leaf Adaptation Strategies of Pinus koraiensis and Fraxinus mandshurica Under Water and Nutrient Variation

To clarify interspecific differences in the adaptive mechanisms of Pinus koraiensis and Fraxinus mandshurica seedlings to water–nutrient conditions, we compared their leaf morphological, anatomical, and photosynthetic responses under contrasting water and fertilization regimes. A pot experiment was conducted using three water levels: drought (DR, 30% of field capacity), control (CK, 55% of field capacity), and water addition (W, 85% of field capacity), combined with two fertilization treatments: no fertilization (NF) and nitrogen–phosphorus fertilization (F). Leaf morphology, anatomy, and photosynthetic traits were systematically measured and analyzed. The results showed that water availability was the dominant factor regulating seedling responses in both species, including increasing water supply generally promoting leaf expansion, photosynthetic rate, and anatomical development, whereas fertilization mainly exerted a water-dependent modifying effect. Under improved water supply, P. koraiensis exhibited a more conservative strategy, characterized by greater investment in conductive tissues and a threshold response of water-use efficiency (WUE). In contrast, F. mandshurica showed a more acquisitive strategy, with coordinated increases in leaf expansion, photosynthetic performance, and assimilatory–vascular tissue development under improved water conditions. These results indicate that both species share a common water-driven response pattern. However, they differ markedly in the coordination of morphological, physiological, and anatomical adjustments, reflecting contrasting resource-use strategies that should be considered in species-specific nursery water and fertilizer management.