択伐施業後における渓流水に含まれる微細土濃度の変化  [in Japanese] Changes in the Concentrations of Fine Solids in Stream Water after Selective-cut Logging in the Forested Catchment, Central Hokkaido, Northern Japan  [in Japanese]

2001年の冬に択伐が行われた森林小流域(流域面積9.2ha)において,2001年6月〜2002年11月までの渓流水中の微細土(TFS)とその有機成分(OFS)の濃度変化を測定し,択伐前(1997年6〜7月)および無施業流域の濃度と比較した。伐採流域におけるTFS濃度とOFS濃度の平均値は,それぞれ択伐前の値に比べて有意に高かった。伐採実施年における伐採流域のTFS濃度は,無施業流域に比べて有意に高かった。TFS濃度に占める有機成分濃度の割合(OFS割合)は,択伐前後ともに流量があるしきい値を超えたときに値が収束する傾向を示した。増水時のOFS割合は,択伐前に比べて択伐後に小さくなった。択伐後の流路状況の観察から,択伐施業にともなうTFS流出のうち,特に無機成分の流出促進は主に渓流流路直近での集材路敷設にともなう堆積土砂に起因すると推察された。

Concentrations of total fine solids (TFS) and organic fine solids (OFS) in the stream water were measured from June to November in 2001 and 2002, in a forested catchment of 9.2 ha where selective-cutting had been carried out in the winter of 2001. TFS and OFS concentrations from the selective-cutting catchment were significantly higher after cutting as compared with those before cutting (1997). The significant difference in TFS concentrations between selective-cutting and control catchments was observed during the first year following the logging; average TFS concentration was higher in the selective-cutting catchment than that in the control. Ratio of OFS to TFS (=OFS concentration/TFS concentration), which reached a constant value as the stream discharge increased over a critical value were significantly lower during the period after logging as compared with that prior to logging. According to the observation at the disturbed area in the selective-cut catchment, the increase of inorganic component of TFS after logging seemed to result from sediment deposition below the skid trails adjacent to the stream.