Potential Stand Structural Drivers of Spatial Variability in Throughfall Kinetic Energy in Unmanaged Japanese Cypress Plantations

초록

Throughfall kinetic energy (TKE) is a physically based index of raindrop-driven splash erosion potential in forest ecosystems. In Japanese cypress plantations, TKE shows substantial spatial heterogeneity beneath the canopy, but the stand structural factors controlling this variability remain poorly understood. Under-canopy structures, such as the dead-branch layer in unmanaged stands, may contribute to localized variation in throughfall erosivity. This study quantified TKE in two unmanaged Japanese cypress plots and examined whether stand structural attributes, particularly the lowest dead-branch height (H-db), explained within-stand variability. Throughfall (TF) gauges and sand-filled splash cups were co-located at 20 measurement points in each plot, and gross rainfall (GR) and free kinetic energy (FKE) were measured in an adjacent open space. TKE beneath the canopy exceeded FKE, indicating canopy enhancement of rainfall erosivity. Stand-mean TKE was similar between the two plots, whereas TKE varied markedly among measurement points within stands. H(db )showed a significant but modest positive association with TKE (r = 0.447, R-2 = 0.20, p < 0.01) in both plots. These results suggest that the under-canopy structure, including H-db, is associated with within-stand spatial heterogeneity in throughfall erosivity. The normalized stand-mean TKE (unit TKE) values were consistent with the reported relationship between stem density (SD) and unit TKE . These findings highlight the importance of considering H(db )and SD in spatially explicit assessments of potential splash erosion risk.

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