Naturally grown understory vegetation after thinning reduces throughfall erosivity in Japanese coniferous plantations

Naturally grown understory vegetation after thinning can potentially mitigate throughfall erosivity.In this study, we observed throughfall kinetic energy (TKE) under the overstory canopies of Japanese cypress (Chamaecyparis obtusa Endl.) plantations in Fukuoka, Japan, using 12 sand-filled splash cups in each of three study plots, T 1 -T 3 , with different stem densities (SD) (T 1 : 1000 stems ha -1 , T 2 : 2400 stems ha -1 , and T 3 : 2900 stems ha -1 ).T 1 was thinned and covered with naturally grown understory vegetation, whereas T 2 and T 3 were unthinned, resulting in an absence of understory vegetation.We also measured TKE beneath the understory vegetation of T 1 (T 1U ) with 12 sand-filled splash cups, as well as free kinetic energy (FKE) in the nearby open space (OP) with four sand-filled splash cups as controls.The results showed that all stand-scale throughfall kinetic energy (TKE) values were much higher than FKE.TKE was highly correlated with stand-scale throughfall in all study plots (R 2 > 0.88, p < 0.001).The TKE beneath the understory vegetation in T 1U was approximately half that under the overstory canopies in T 1 (406.6 vs. 772.1 J m -2 ).This suggests that the understory vegetation layer decreased TKE by decreasing raindrop fall velocity.For the overstory canopies, the entire period unit rainfall stand-scale TKE in T 1 (uTKE total,1 ) of 17.1 J m -2 mm -1 was much higher than that in T 2 (uTKE total,2 ) and T 3 (uTKE total,3 ), whereas the uTKE total,2 and uTKE total,3 of 11.4 and 11.1 J m -2 mm -1 , respectively, were similar because of the negative relationship between SD and uTKE total (r = -0.658,p = 0.028).After thinning, naturally grown understory vegetation is important for soil conservation in Japanese cypress plantations.Our study further improves our knowledge of the mechanisms determining splash soil erosion processes and provides new insights into forest management strategies for soil conservation.