森林立地学会誌 森林立地40(2),1998,67〜74

Jpm.J.For.Environment40(2)67−74  1998

 

 

綾照葉樹林における表層土壌の動態および物理特性と微地形との関係

 

大貫靖浩・佐藤 保・藤本 潔**・稲垣昌宏

森林総合研究所九州支所

**森林総合研究所

 

 

 天然生常緑広葉樹林が分布する,宮崎県の綾リサーチサイトにおいて,実生の発生・生残を規定する地形・土壌要因を明らかにするために,100m×120m調査区域に規則配置した263地点において表層土壌の堆積・侵食量調査,および表層土壌の物理特性の測定を実施した。表層土壌の堆積・侵食量は,微地形単位と斜面の傾斜によって異なり,傾斜の緩い頂部斜面では堆積が卓越する地点が認められるなど侵食量が少なかったのに対し,急傾斜の谷頭急斜面・谷頭凹地では特に降水量が多かった時期に侵食量が多かった。堆積・侵食量の経時的変化について検討した結果,斜面下部に位置する谷頭凹地や麓部斜面では,降水量の多寡によって堆積・侵食量の変動が大きく,下部谷壁斜面では常に侵食傾向にあるが,頂部斜面・上部谷壁斜面・谷頭急斜面では春季〜秋季にかけては侵食作用が,秋季〜春季には堆積作用が卓越することが明らかになった。表層土壌の物理特性と微地形単位との関係を検討した結果,土壌の透水性に影響を与える低水分張力域孔隙率は,統計的な差異は認められなかったものの,尾根部の頂部斜面や上部谷壁斜面で比較的高い地点が多く,土壌の保水性に寄与する高水分張力域の孔隙は上部谷壁斜面と谷頭凹地で多かった。物理特性間の関係では,現場含水率と高水分張力域孔隙率は比例関係にあったが,低水分張力域孔隙率とは逆比例関係にあった。表層土壌の物理特性と侵食量との関係を検討した結果,低水分張力域孔隙率が高いほど侵食量が少なくなることが明らかになった。

 

 Yasuhiro OHNUKI, Tamotsu SATO, Kiyoshi FUJIMOTO and Masahiro INAGAKI : Dynamics and physical properties of surficial soil and microtopography at Aya evergreen broad-leaved forest, southwestern Japan. Jpn. J. For. Environment 40, 67-74, l998

  Soil movement and physical properties of surface soils were measured at 263 points in the evergreen broad-leaved Aya forest in southern Kyushu, in order to clarify the factors affecting the establishment and survival of seedlings. The mass of soil movement was found to be different in each microtopographical unit. Depositional poits were observed on crest slopes where the angle of slope was gentle, whereas the headmost wall and head hollow, where the angle of slope was steep, demonstrated heavy soil denudation. This denudation is thought to be caused by surface runoff during the heavy rains. Temporal changes in soil movement, the variations in accumulation and denudation, were large in the head hollow and the footslope and depended on the amount of precipitation. In the crest slope, the upper sideslope and the headmost wall, erosion was found to occur from spring to autumn, while deposition occurred from autumn to spring. Porosity in the low suction range, which increases permeability, showed high values on the crest slopes and the upper sideslopes. Porosity in the high suction range, which increases water retention, showed high values on the upper sideslopes and the head hollow. The soil water content in situ was found to be in proportion to the level of high suction range porosity. On the other hand, the soil water content in situ was in inverse proportion to the level of low suction range porosity. From the relationship between physical properties of surface soils and erosion rates, it is clear that the soils which have high level of low suction range porosity show less erosion rate.


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