Trade‑offs and timing of leaf defenses during development in two co‑occurring oaks with contrasting leaf habits

Key message Evergreen and deciduous oaks resolve the early ontogenetic “vulnerability window” with contrast‐ ing defense schedules: evergreens front‐load chemical deterrence (condensed tannins) and complete mechanical reinforcement later, whereas the deciduous prioritize rapid growth and earlier mechanical strengthening, delaying chemical investment. Context Young, expanding leaves are nutrient‐rich and mechanically weak, making them highly attractive to herbi‐ vores. How defense timing and allocation differ during development between co‐occurring evergreen and deciduous species remains unclear. Aims We tested whether evergreen oaks invest more in direct defenses during leaf development than deciduous oaks, and whether each habit prioritizes distinct defensive traits across stages. Methods In a Mediterranean common garden, we monitored co‐occurring evergreen Quercus rotundifolia and deciduous Q. faginea from budburst to leaf maturity, measuring growth and defense traits. Seasonal trajectories and toughness–tannin relationships were analyzed, and patterns validated with three additional evergreen and three deciduous congeners. Results Leaf area reached its maximum before LMA, creating a juvenile phase with low fiber content and tough‐ ness. The evergreen oak increased condensed tannins concentration early, then work of fracture; the deciduous oak grew faster initially, then reinforced mechanically and delayed tannins. Across stages, the evergreen kept higher work of fracture and condensed tannins, while the deciduous prioritized early growth; defense trade‐off patterns were habit‐specific and consistent across species. Conclusion Leaf‐habit‐specific defense schedules shape protection: evergreens front‐load chemical defenses before structural hardening, while deciduous oaks prioritize tolerance and earlier mechanical reinforcement, patterns with potential implications for herbivore pressure and associational dynamics in mixed stands.