Moisture moderate the effects of urbanization pressure on the growth of rare Cercidiphyllum Japonicum in a loess plateau reserve

Anthropogenic disturbance imposes significant environmental stress on plants, yet certain factors may mitigate these effects. However, robust causal evidence explaining how such factors moderate the impacts of disturbance on rare species remains scarce. This study applies Structural Causal Modeling (SCM) within a mixed-effects framework to provide novel insights into these mechanisms for the rare relict tree Cercidiphyllum japonicum in China’s Lishan National Nature Reserve. We surveyed 175 adult trees, linking cumulative growth (Diameter at Breast Height, DBH) to urbanization pressure (measured as the distance from human activity), topography, hydrothermal conditions, and soil properties. Results revealed that urbanization pressure was the primary growth inhibitor, with its direct effects and indirect pathways reducing moisture availability and crown width. While moisture had the strongest direct positive influence on DBH, it also functioned as a crucial mediator, alongside soil Total Potassium (TK) and total nitrogen. Crucially, moderation analysis revealed specific buffering mechanisms. Enhanced moisture directly ameliorated the negative effects of disturbance on tree growth. This moderating effect, however, manifested only at distances beyond 4 km from human activity centers. Our causal evidence highlights key environmental buffering mechanisms in protected ecosystems, informing conservation strategies for C. japonicum—such as prioritizing remote and hydrologically stable sites—and offering broader implications for safeguarding other rare species under anthropogenic pressure.