Soil movement on forest slope with impoverished understory vegetation induced by sika deer grazing in Shiiba Research Forest, Kyushu University

九州大学宮崎演習林では1980 年頃からシカの個体数増加に伴い落葉広葉樹林林床のスズタケ（Sasa borealis（Hack.）Makino et Shibata）の衰退がみられ, 三方岳団地の多くの林地では約20 年の間, 下層植生がほぼ存在しない状態が維持されている。本研究では, 長期間にわたるスズタケの消失が林地斜面における土砂移動に及ぼす影響を明らかにするために, スズタケが残存する林地（ササ有区）とスズタケが消失した林地（ササ無区）において, 土砂受け箱とスプラッシュカップを用いて土砂移動量と土砂飛散量を測定した。ササ無区のほうが樹冠通過雨量は多かったにもかかわらず, 土壌移動量と土壌飛散量は少なかった。ササ無区では表層土壌に含まれるレキの割合が大きかった。レキには雨滴衝撃からの保護, 透水性の増加などの効果があることから, ササ無区でのレキの多さが, 土砂移動量, 土砂飛散量を抑制した原因の一つと考えられた。移動土砂や飛散土砂に含まれるレキの割合は表層土壌よりも少なかったことから, 今後長期にわたり細土の流亡が継続すると, 残存する土壌に含まれるレキの割合が増加することが考えられる。Since around 1980, Sasa borealis (Hack.) Makino et Shibata on forest floor has been impoverished with increase in the density of sika deer (Cervus nippon) in the Shiiba Research Forest, Kyushu University. Understory vegetation in a considerable area of the Sampoudake-danchi had been eliminated for around 20 years. To evaluate the effects of the long term elimination of understory vegetation on the soil movement, we measured amounts of moved soil and of splashed soil with soil trap and splash cup, respectively, on slopes of the forest with and without S. borealis. The amounts of moved and splashed soils were smaller in the forest without S. borealis, though the amount of throughfall was larger. The ratio of gravel in the surface soil of the forest without S. borealis was larger than that with S. borealis. The larger amount of gravel is a reason of smaller amounts of the moved and splashed soils in the forest without S. borealis. The ratios of gravel in the moved and splashed soil were smaller than that of the surface soil. Long-term erosion of fine soil could create the higher ratio of gravel in the surface soil