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Article History

Received: 23 August 2025

Accepted: 19 November 2025

First Online: 5 December 2025

Declarations

:

: The authors declare no competing interests.

: Figure 1 Map of the study area showing the distribution of the 175 surveyed Cercidiphyllum japonicum individuals. The map includes the boundary of the Lishan National Nature Reserve, Shanxi, China (blue line; from the China Nature Reserve Specimen Resource Sharing Platform, ( ), survey routes (orange lines), and key land cover types, including agricultural land (pink), built-up areas (red), and water bodies (light blue), derived from the ESA WorldCover dataset (Zanaga et al., ).

: Figure 2 Nighttime light intensity (mean of 2020–2024) in and around the study area, captured by the Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band. The two primary centers of high light intensity correspond to Yuanqu County (bottom left) and Qinshui County (top right). The lower-intensity lights near the protected area represent Xiyan Town. For our DHAC analysis, these two major counties were considered the primary sources of broad-scale urbanization pressure. DHAC, distance to human activity center; DNB, nighttime light intensity.

: Figure 3 Distributions (A) and pairwise correlation coefficients (B) of measured variables for Cercidiphyllum japonicum in the study area. DBH, diameter at breast height; DHAC, distance to human activity center; NDMI, normalized difference moisture index; SOC, soil organic carbon; TK, soil total potassium; TN, soil total nitrogen; TP, soil total phosphorus.

: Figure 4 Determining the number of predictors for the Structural Causal Model (SCM) (A) and selecting predictors (B) using a random forest model. NDMI, normalized difference moisture index; DHAC, distance to human activity center; TK, soil total potassium; TN, soil total nitrogen.

: Figure 5 The Structural Causal Model (SCM) for diameter at breast height (DBH) is shown in panel A, along with the direct (B), indirect (C), total (D), and mediated (E) causal effects of environmental factors on DBH. In panel A, solid black arrows/bars indicate positive effects, while red arrows indicate negative effects; the width of arrows reflects the magnitude of effects. Solid lines in panel A represent significant effects (P < 0.05), while dashed lines denote nonsignificant paths (P > 0.05). Bidirectional arrows represent residual correlations between factors. In panels B-E, bars without letter labels indicate non-significance; different letters denote significant differences (P < 0.05) based on multiple comparisons of absolute summed path coefficients. NDMI, normalized difference moisture index; DHAC, distance to human activity center; TK, soil total potassium; TN, soil total nitrogen.

: Figure 6 Moderation effects of NDMI (A) on the DHAC–DBH relationship and the visualization of NDMI’s modulation of the DHAC–DBH relationship (B). In panel A, solid black arrows/bars indicate positive effects, while red arrows indicate negative effects; the width of arrows reflects the magnitude of effects. Solid lines in panel A represent significant effects (P < 0.05), while dashed lines denote nonsignificant paths (P > 0.05). Bidirectional arrows indicate residual correlations between factors. In panel B, the blue line represents the top 30% of sites ordered by NDMI (high to low), and the red line represents the bottom 30%. The difference in the slopes of the regression lines in panel B is statistically significant (P < 0.05), as indicated by the significant interaction term in the structural causal model (SCM). DBH, diameter at breast height; NDMI, normalized difference moisture index; DHAC, distance to human activity center; TK, soil total potassium; TN, soil total nitrogen. R² denotes the coefficient of determination for a linear model.