Leaf morphological trait integration and modularity provide insights into ecological adaptation in evergreen oaks

The integration and modularity of leaf morphological traits are fundamental to plant adaptations, yet their responses to diverse environmental pressures remain unclear. In this study, we investigate the roles of leaf trait integration and modularity and how they interact with environmental factors. We analyzed geometric, traditional, and functional leaf traits across 908 individuals from 72 populations of two alpine evergreen oaks, Quercus aquifolioides Rehder & E.H. Wilson and Quercus spinosa David ex Franch., distributed throughout the Himalayan-Hengduan Mountains (HHM), employing genetic assignment as a priori. Multivariate and redundancy analyses revealed that Q. aquifolioides, which inhabits harsher environments, exhibits lower trait integration and greater morphological flexibility, allowing for dynamic adaptation to fluctuating conditions. In contrast, Q. spinosa, thriving in milder environments, demonstrates stronger integration and stability in leaf morphology, facilitating resource optimization and providing a competitive advantage. Notable differences in modularity between the two species were observed, particularly in specific leaf traits, as revealed by structural equation modeling (SEM) analysis. These results underscore the adaptive significance of leaf trait integration and modularity in extreme environments and highlight the critical role of leaf morphology in enhancing species resilience.