We investigated how different degrees of shading affected growth, morphology, and biomass allocation in seedlings from two coniferous and three broadleaved species. The experiment was conducted in a shade house over a 1-year period. Our results showed that under increasing shade, seedlings from most species exhibited lower total biomass, net assimilation rates, relative growth rates, root mass ratios, and root/shoot ratios. In contrast, the slenderness quotients, leaf area ratios, and specific leaf areas increased with increasing shade. For coniferous species, growth traits were relatively more plastic (responsive to shade) than morphology or biomass allocation traits, whereas for broadleaved species, growth and biomass allocation were the most shade-sensitive traits. When comparing coniferous versus broadleaved species, the former had a higher growth plasticity index and lower allocation plasticity than the latter. Root biomass and stem mass ratio were the most and least plastic traits in response to shading. Our results indicate that shade differentially affects coniferous and broadleaved species in terms of their growth, morphology, and biomass allocation. These findings have important implications for the establishment and maintenance of mixed-species stands.

Introduction: Cunninghamia lanceolata (Lamb.) Hook is a high-quality, fast-growing plantation-tree species widely distributed in southern China, and a commercial timber species unique to China that plays a vital role in meeting wood demand and maintaining ecological security. Although the morphological and physiological adaptations of C. lanceolata seedlings to light stress have been extensively documented separately, their interplay remains a critical gap in our knowledge and understanding of plant ecophysiology. Particularly, the synergistic mechanisms between phenotypic adaptations and metabolic regulation remain unclear.Methods: This study employed 1-year-old, clonal, C. lanceolata seedlings as test materials to investigate the coordinated effects of different light intensity gradients (100%, 68%, 27%, 12%, and 5% of full sunlight) on the morphological and physiological responses.Results: (1) Under decreasing light intensity, seedling height to diameter ratio and specific leaf area were 30.10% and 64.38% greater than of those recorded under 100% light intensity. The observed changes in growth maximized light capture capacity. Further, root growth, root to shoot ratio, and seedling quality index decreased with decreasing light intensity. (2) Leaf non-structural carbohydrate contents decreased significantly, along with key carbohydrate-metabolizing enzyme activities, and leaf carbon∶nitrogen and carbon∶phosphorus ratios. (3) High light intensities increased cytokinin and abscisic acid contents, whereas the lowest (5%) light intensity tested enhanced the accumulation of gibberellin, but had no significant effect on indoleacetic acid content.Discussion: These results indicate that C. lanceolata seedlings used a dual adaptation strategy that combined “photoprotection under high light intensity” with “efficient resource utilization under low light intensity” through coordinated morphological and physiological adjustments. Our study provides a scientific basis for managing nursery light conditions and plantation light environment during early development of C. lanceolata seedlings. Specifically, we recommend a 68% light intensity for optimal seedling production.

Phosphorus (P) is among the most intractable constraints on plant fertility, particularly in acidic soils with high P fixation capacities. The effects of nutrient limitation and the adaptive strategies of plants in infertile soils are central topics in plant ecology. The development of tree cultivars with greater P use efficiency (PUE), defined as the ability of a tree to grow and be productive in soils with reduced P availability, would substantially improve forest development. The ability of plants to redistribute and transfer P across fractions determines their adaptability to P limitations. However, the mechanisms of P utilization and transport remain unknown in Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) from the perspective of P fraction distribution. In this study, we investigated the distribution and translocation patterns of exogenous P and different P fractions in the M1 Chinese fir, which was identified as exhibiting high P-deficient resistance ability and maintaining higher yield under low P stress relative to the average clones, using ³²P tracking, which can accurately trace the migration pathways of exogenous P after plant absorption. We found that exogenous P in the roots was higher than in the stems or leaves under low-P conditions in which the amount of the exogenous P absorbed by M1 was significantly reduced. Under low-P conditions, the plants optimized P allocation, which led to higher PUE than under high-P conditions, with the highest PUE in the leaves, followed by the stems and roots. The M1 clone maintained a high ratio of soluble P (i.e., inorganic P and ester P) in its leaves and stems, which improved P mobility and recycling under the conditions of limited P. In the roots, the P fractions shifted from soluble inorganic P and ester P to insoluble P (i.e., nucleic P), but the total P concentration was relatively stable, which may ensure root growth and exogenous P absorption under the conditions of limited P. Our results confirm that the M1 Chinese fir reduces P demand, optimizes the allocation of P among P fractions, and increases PUE to maintain aboveground productivity in response to limited P conditions.

Low inner leaves in the thick canopy of dense Chinese fir plantations frequently show premature senescence and dieback regardless of age. To elucidate the underlying mechanisms, a 28-day growth chamber experiment was conducted under dark conditions to induce leaf senescence. Changes in leaf area, photosynthetic performance, and the responses of carbon metabolism and the antioxidant defense system were analyzed. Leaf area decreased significantly with time in darkness. The photosystem II reaction center was damaged, and fluorescence parameters and chlorophyll contents decreased, resulting in reduced light energy capture and conversion efficiencies. Photosynthetic rate, apparent quantum yield, stomatal conductance, transpiration rate, and light use efficiency all decreased, while the light compensation point and intercellular CO₂ concentration increased. Antioxidant enzyme activities initially increased but eventually collapsed as the stress continued and H₂O₂ and malondialdehyde accumulated, causing membrane conductivity, i.e., membrane permeability, to increase by 122%. Meanwhile, reduced non-structural carbohydrates, especially total non-structural carbohydrates content, decreased by 45.32%, triggering sugar starvation and accelerating aging. Our study provided new physiological evidence for light-stress response mechanisms in Chinese fir. Specifically, it revealed that dark-induced leaf senescence was mainly caused by irreversible damage to the photosynthetic apparatus and oxidative stress, which together led to carbon starvation and ultimately death.