data and script for Small first leaves promote subsequent leaf development in Padus grayana

Why plants have evolved diverse leaf morphologies is central question in plant ecology and evolution. Plants often display morphological variation in leaves within a single individual, but the functional significance of this diversity remains understudied. In many tree species, the first leaves to emerge from winter buds are remarkably small. In this study, we explored the ecological and physiological roles of such small first leaves in Padus grayana. Using a shading experiment, we found that these early leaves exhibit higher SPAD values—indicating greater photosynthetic potential—and contribute to accelerating the growth of subsequent typical leaves. When the first leaves were shaded, the expansion of the third leaves was significantly delayed, although their final size remained unchanged. This finding suggests that the small first leaves, despite their size and short lifespan, play a critical role in initiating leaf development and maximizing photosynthetic gain in early spring. We also show that this phenomenon is widespread across phylogenetically distant species, suggesting possible convergent evolution driven by environmental pressures such as frost risk.