Comparisons of Leaf Functional Traits between Juvenile and Mature Trees of 26 Subtropical Species

The leaf functional traits reflect the plasticity exhibited by the plants in morphology, function, and physiology during the environmental adaptation, embodying their fundamental physiological characteristics. The plants at different developmental stages may show varying patterns of change in leaf functional traits. Therefore, exploring the relationship between the traits of saplings and mature trees is helpful to reveal the dynamic adjustment of plant function during growth. By taking 26 subtropical tree species as the research objects, seven leaf functional traits of saplings and mature trees were systematically measured. Firstly, the overall differences in leaf functional traits among different tree species were compared. Secondly, the differences in leaf functional traits of different mycorrhizal tree species arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) at the sapling and mature tree stages were analyzed. Finally, the correlations among multiple leaf functional traits across the 26 tree species and their synergies or trade-offs were discussed. The results showed that the saplings exhibited higher specific leaf area and higher leaf nitrogen, phosphorus, potassium, and magnesium contents compared to the mature trees, whereas the mature trees generally had higher leaf carbon and calcium contents than the saplings. Among them, the arbuscular mycorrhizal fungal tree species showed a significant change trend of specific leaf area, leaf carbon, nitrogen, and phosphorus contents decreased, while leaf calcium content increased. In contrast, the leaf functional traits of ectomycorrhizal fungal tree species remained relatively stable, reflecting a conservative nutrient utilization pattern. In the saplings, SLA was significantly negatively correlated with leaf carbon content, and significantly positively correlated with leaf nitrogen, potassium, and magnesium contents, while in the mature trees, the specific leaf area was significantly positively correlated with leaf nitrogen, calcium, and magnesium contents. The pattern of leaf functional traits indicated that the saplings showed a strategy of rapid resource acquisition, while the mature tree species displayed different leaf phosphorus and potassium traits from the saplings, reflecting the changes in the combination of leaf functional traits at different developmental stages. By comparing the variation patterns of leaf functional traits between saplings and mature trees, the differences in leaf functional traits of tree species at different forest ages and the differentiation of resource utilization strategies indicated by the differences were clarified, which provided new insights into understanding the changes in traits of forest tree species during the growth and development and the dynamics of stand nutrients.