Background Cryptomeria japonica, an introduced species, is the most extensively planted exotic tree species on Mountain Lu. Due to the fact that its plantations are located within a national scenic area, management activities such as logging are subject to strict restrictions. However, these plantations are currently facing several ecological degradation issues, including irrational stand density, low understory biodiversity, difficulty in natural regeneration, and severe structural soil erosion caused by increased kinetic energy of raindrops after canopy interception.

Methods This study focused on C. japonica plantations in Mountain Lu and employed a combination of field monitoring and laboratory analysis, utilized stable isotope techniques to compare the isotopic composition of rainfall outside the forest and throughfall within the forest during rainfall events, and to examine their responses to leaf area index (LAI) and meteorological factors.

Results 1) The average throughfall rate at each collector ranged from 0.48 to 1.28 mm, exhibiting a significant negative correlation with LAI (best fitted by a quadratic polynomial). 2) Compared with rainfall outside the forest, the isotopic composition of throughfall was more enriched. 3) Δδ¹⁸O exhibited three distinct correlation patterns with LAI: Significant positive correlation, significant negative correlation, and insignificant correlation. In contrast, Δd-excess showed no significant correlation with LAI across all rainfall events. The correlation between LAI and Δδ¹⁸O varied distinctly under different meteorological conditions. 4) The isotopic composition of throughfall was influenced by the canopy interception process. This influence is further mediated by meteorological factors. Specifically, average rainfall intensity and wind speed emerge as key drivers that shape the behavior of rainwater and its isotopic variability.

Conclusions These findings reveal the spatial distribution of isotope composition of throughfall within the forest and clarify the behavioral characteristics of stable isotopes during canopy rainfall interception, providing a theoretical basis and technical support for the scientific management and renovation of Japanese cedar plantations in Mountain Lu in the future, and providing a certain data basis for the evaluation of the benefits of its water conservation service functions.