Irregularity at the edge: Developmental dynamics of leaf margins in six temperate tree species

Leaf teeth are marginal structures that play an important role in early-season gas exchange and have significance for both functional ecology and paleobotanical reconstructions. This study examined the dynamics of leaf margin development during early leaf expansion in six temperate tree species (Acer platanoides, Betula pendula, Carpinus betulus, Corylus avellana, Salix caprea, and Sorbus aucuparia). Leaf sampling was conducted in a temperate mixed forest near Zielona Góra, Poland. Leaves were collected from sun-exposed and shaded conditions over a 50-day spring period and analyzed using image-based software. Primary traits such as leaf area, perimeter, width, length, and the height and width of each tooth were recorded. From these, five derived variables were calculated: 1) leaf blade irregularity index (perimeter/area), 2) relative irregularity index (based on consecutive measurements), 3) compactness index (perimeter²/area), 4) shape index (width/length), and 5) simplified tooth area (tooth height × tooth width / 2). The results demonstrated a rapid decline in leaf margin irregularity during the first 20–30 days of development, after which it stabilized. This pattern was consistent across all species, regardless of their ecological preferences, indicating a universal developmental trend in analyzed woody plants. Among the tested variables, the leaf blade irregularity index (perimeter-to-area ratio) correlated most strongly with actual tooth area and was selected for further analysis. While shade leaves generally exhibited slightly higher irregularity values, no significant differences were detected after adjusting for leaf size. The findings support the hypothesis that margin serration enhances early photosynthetic efficiency, with its relative contribution diminishing as leaves expand. This study contributes to our understanding of species-independent developmental plasticity in leaf morphology and underscores the importance of standardized sampling in ecological and paleobotanical research.