Leaf Trait Divergence and Elevational Adaptation in Endangered

Fagus hayatae Palib. ex Hayata, an endangered East Asian paleoendemic species, dominates in Sichuan's Micang Mountain while persisting in fragmented populations across Taiwan and mainland China. Nevertheless, niche differentiation and adaptive strategies between this species and its close congener (F. engleriana) remain poorly understood. Leaf functional traits-key indicators of plant resource-use strategies-reveal ecological adaptations through their interrelationships and environmental responses. Here, we examined interspecific variation in leaf functional traits among F. hayatae, F. engleriana, and co-dominant tree species, and analyzed the trait variation in F. hayatae with respect to elevation to elucidate climate adaptation mechanisms (Micang Mountain Nature Reserve, China). F. hayatae and F. engleriana exhibited low niche overlap as community dominants. Compared to associated dominants, F. hayatae displayed higher leaf dry matter content (LDMC), specific leaf area (SLA), leaf carbon content (LCC), and leaf phosphorus content (LPC), alongside smaller stomata with lower density. Conversely, F. engleriana manifested stronger resource-acquisition traits: larger leaf width (LW), leaf area (LA), SLA, LCC, higher leaf nitrogen content (LNC), and greater stomatal density, yet lower LPC. Direct comparison revealed that F. hayatae possesses larger but sparser stomata and reduced resource-acquisition capacity relative to F. engleriana, evidenced by lower LL, LW, LA, SLA, LCC, and LNC. Elevation significantly modulated F. hayatae's leaf traits. At lower elevations, increased LA and SLA indicated acquisitive tendencies, whereas higher elevations favored conservative strategies: reduced LA, SLA, stomatal area (SA), and LPC. This underscores F. hayatae's superior phenotypic plasticity and reliance on defense-oriented adaptations compared to sympatric F. engleriana. Critically, mid-elevations supported optimal performance via coordinated stress-tolerance traits. These findings highlight the priority of protecting mid-elevation habitats where F. hayatae achieves peak fitness, and the need for proactive habitat preservation due to its conservative strategy, which entails slower post-disturbance recovery.