Characteristics of throughfall kinetic energy in an unmanaged Japanese cypress plantation laden with dead branches

Among soil erosion processes, the initial stage of splash soil erosion caused by throughfall kinetic energy (TKE) plays a crucial role in soil conservation within forest ecosystems. Throughfall (TF) through the upper canopy structure considerably influences TKE in coniferous plantations. Recent studies have revealed that the under-canopy structure laden with dead branches in unmanaged coniferous plantations reduces the TF; however, its specific impacts on TKE and appropriate sampling strategies remain unexplored. This study used 40 splash cups (4 for free kinetic energy and 36 for TKE) for one event (total: 600 splash cups for 15 events) in an unmanaged 36-year-old Japanese cypress plantation laden with dead branches. Additionally, unstratified and stratified Monte Carlo simulations were used to determine optimal sample sizes for stand-scale TKE estimation. Results demonstrated a strong correlation between TKE (J m−2) and TF (mm) (R2 = 0.98) across all events. The stand-scale unit TKE of 12.5 J m−2 mm−1 was much lower than those in seven previous studies with fewer or no dead branches (range: 16.4 to 28.2, median: 18.8 J m−2 mm−1). The previously developed empirical stand-scale unit TKE model considering under-canopy structure solely exhibited a 1.7-fold overestimation. Among the stand structures, a positive relationship of TKE with the distance between a sampling point and its nearest trunk (Dmin) was observed, which was positively correlated with the lowest dead branch height. This indicated that under-canopy dead branches weakened the TKE, likely by reducing TF and raindrop fall velocity. Stratified Monte-Carlo simulation considering the Dmin related spatial patterns of TKE provided a more efficient approach for capturing variability. To achieve high precision with potential errors of ≤5–10 %, a total sample size of 6–15 was appropriate. Our findings implied that the presence of under-canopy dead branches mitigates the soil erosion risk in unmanaged Japanese cypress plantations.