Does season regulate heterochronous leaf growth? Mechanisms of petiole-lamina trade-offs in broad-leaved woody plants of the Tianshan Mountains.

As the core of leaf functional traits, the trade-off relationship between the petiole and lamina expresses the plant's adaptability to the environment in terms of support structure and photosynthesis. We investigated the proportions of allometric growth in the relationship between the petiole and the lamina of broadleaf woody plants in temperate highland Tianshan Mountains montane forests through three dimensions (length, area, and mass), including the length of the lamina (LL) and the length of the petiole (PL), and the area of the lamina (LA) and petiole cross sectional area (PCA) versus the mass of the lamina (LM) and the mass of the petiole (PM), as well as exploring the characteristics of the variance in response to seasonal changes. We found that the functional traits in all three dimensions showed a clear convergent evolution as the seasons progressed, that is, a "seasonal effect" of increasing and then decreasing. The effect of the petiole-lamina relationship under spring was minimal in the area dimension; the effects of the three-dimensional relationships of the traits were all highest in summer, and the effect of the petiole-lamina relationship was lower in autumn. We also found that petiole traits are simultaneously and multiply affected by lamina traits, with LA and LM having additional effects on the length/mass and area dimensions, respectively. Compared to tree species, shrub species significantly require more light intensity and support capacity. Compound-leaved plants would invest more in photoluminescence, increasing leaf light capture efficiency and static load and dynamic resistance. Our results suggest that plants have rather complex trade-off mechanisms at the leaf level influencing their ability to adapt to the environment, emphasize the need for leaf-level studies on the relationships between functional traits in plants, and illustrate the importance of the season as a distinct time scale for plant trade-off mechanisms.