Phylogenetic structure of understory communities in the semi-natural forest of Dalian island and its influencing factors

This study investigates the semi-natural forests on Dalian Island, Pingtan, Fujian Province, to explore the underlying mechanisms shaping the phylogenetic structure of the understory shrub and herbaceous layers. It examines the relationships between phylogenetic structure, environmental factors, and dominant species to uncover the mechanisms maintaining plant community diversity, thereby providing a scientific basis for biodiversity conservation on island ecosystems.

18 tree plots in the forest community of Dalian Island were established to investigate the understory species composition. Separate phylogenetic trees were constructed for the shrub and herb layers of the understory community and the N_TI (phylogenetic nearest taxa index) was calculated to quantify their phylogenetic structures. Climate, topography, soil properties, and importance values of dominant species were integrated into model selection and structural equation modeling (SEM) analyses to identify key factors influencing phylogenetic structure and their pathways of influence.

1) The phylogenetic structure of the understory shrub and herbaceous layers was predominantly clustered. Among the plots, 14 in the shrub layer exhibited phylogenetic clustering, while 15 in the herbaceous layer showed clustering; 2) The importance values of Casuarina equisetifolia and Acacia confusa significantly increased the N_TI of the shrub layer. While altitude, aridity index, and mainland distance indirectly regulated shrub-layer phylogenetic structure by influencing the distribution of dominant species; 3) Casuarina equisetifolia and Eurya emarginata significantly influenced the N_TI of the herbaceous layer by modulating soil alkaline hydrolyzable nitrogen levels. Soil nutrients (alkaline hydrolyzable nitrogen, available phosphorus, and available potassium) directly controlled the phylogenetic structure of the herbaceous layer, with alkaline hydrolyzable nitrogen and available phosphorus exerting significant negative effects on N_TI, while available potassium showed a positive effect.

Environmental filtering plays a crucial role in species coexistence and biodiversity maintenance in the understory shrub and herbaceous layers of the semi-natural forest on Dalian island. Dominant species (Casuarina equisetifolia and Acacia confusa) primarily shape the phylogenetic structure of the shrub layer, whereas soil nutrients and other environmental factors (e.g., altitude, aridity index, and coastal distance) jointly shaped the phylogenetic structure of the shrub and herb layers through both direct and indirect pathways. Future studies should further integrate functional traits to validate phylogenetic niche conservatism and investigate the impacts of human disturbances on community assembly.