17:15 〜 19:15
[ACG37-P04] Changes in sediment transport following thinning combined with contour-felled logs in a Japanese cypress plantation: catchment and hillslope perspectives
キーワード：Catchment hydrology, Hillslope hydrology, Japanese cypress forest plantations, Sediment transport
Sediment transport is a major concern in monoculture plantations, particularly in steep terrains with intense rainfall, where limited understory vegetation exacerbates soil erosion. In Japanese cypress plantations, sediment transport is especially high due to insufficient ground cover. We examined whether thinning combined with contour-felled logs could effectively reduce sediment transport at both the hillslope and catchment scales. We investigated sediment transport dynamics using a paired-catchment and paired-plot approach. Annual sediment transport was measured from 2018 to 2024 in two adjacent catchments: one underwent 40% thinning with contour-felled logs from late 2019 to early 2020, while the other remained untreated. At the hillslope scale, monthly sediment transport was monitored from 2016 to 2024 using two open-type plots: one treatment and one control with same timeline of thinning operation in catchment. Thinning was performed using chainsaws, and the felled logs were placed parallel to contour lines. At the catchment scale, sediment transport increased in both catchments post-thinning. However, the increase was significantly greater in the control catchment, where sediment transport rose from 257.0 ± 7.5 m3 yr−1 km−2 pre-thinning to 421.4 ± 10.7 m3 yr−1 km−2 post-thinning (t-test, p < 0.054). In contrast, the treatment catchment increased from 129.2 ± 35.6 m3 yr−1 km−2 to 225.4 ± 8.7 m3 yr−1 km−2 (t-test, p < 0.116), suggesting that thinning combined with contour-felled logs mitigated sediment export. At the hillslope scale, sediment transport showed a significant seasonal pattern after thinning. The treatment plot had the highest sediment transport during the rainy season, likely due to increased surface erosion following canopy opening. The catchment-to-hillslope sediment transport ratio revealed contrasting responses (Fig 1). Total annual rainfall did not correlate with sediment transport trends; however, month with highest rainfall amount in each year showed a clear pattern. In the control catchment, sediment transport increased sharply during months of maximum rainfall, suggesting that intense storms drive higher sediment mobilization at the catchment scale. In contrast, in the treated catchment, the ratio remained near 1.0, indicating that thinning combined with contour-felled logs increased sediment storage within the catchment, reducing sediment connectivity and export downstream. These findings highlight the scale-dependent nature of sediment transport and the importance of extreme rainfall events in driving sediment export. While thinning increased hillslope erosion, its impact at the catchment scale was mitigated by contour-felled logs, which enhanced sediment storage capacity. This study suggests that thinning, when combined with contour-felled logs, can be an effective erosion control strategy in Japanese cypress plantations, particularly in areas prone to extreme rainfall events.