森林立地学会誌 森林立地47(1), 2005, 47〜55
Jpn.J.For.Environment 47(1), 47-55 2005
ヒノキ林の間伐による枝条放置が土壌の水分動態に与える影響
荒木 誠＊・加藤正樹＊・小林繁男＊＊・有光一登＊＊＊
＊森林総合研究所・＊＊京都大学大学院・＊＊＊元高知大学
ヒノキ林の間伐時に林地に放置された枝条による地表状態の違いが土壌の水分状態に与える影響を明らかにするため，茨城県の筑波山の北東に位置する燕山山麓緩斜面において，表層土壌の孔隙組成および土壌水分状態の変動を観測した。間伐によって残された枝条が地表を被覆する箇所（枝条放置区）と被覆のない箇所（対照区）において調査を行い，以下の点が明らかとなった。
１）調査区内の土壌は，5cm深，10cm深，30cm深いずれも固相率が20%にも満たず，孔隙率が80%を超えており，火山灰を母材とする黒色土の一般的な特徴を示していた。
２）全孔隙率については，5cm深〜30cm深まで深さ別の大きな違いはなかった。また，枝条放置区と対照区の違いは認められなかった。
３）枝条放置区と対照区の土壌水分状態には違いが見られ，無降雨期間で比較すると5cm深は対照区が枝条放置区より乾燥する傾向が見られた。一方，30cm深では枝条放置区が対照区より乾燥する傾向が見られた。10cm深では枝条放置区と対照区の違いはあまり明瞭でなかった。
４）地表面下5cm深の土壌温度は枝条放置区では対照区に比べ，冬季に1〜2 ℃高く，夏季には逆に1〜2 ℃低く，放置された枝条が温度変化を緩和した。
５）放置された枝条が土壌水分状態に与える影響は，地表面直上での降雨遮断による土壌への雨水供給量の減少と地表面蒸発量の抑制によるものと考えられる。
６）枝条の降雨遮断効果は最表層より下層の乾燥，水分率の低下をもたらす。一方，枝条による蒸発抑制効果は，土壌温度の上昇の抑制などにより地表面からの蒸発を減らし，最表層の水分減少を抑制したと考察された。
Physical characteristics and moisture conditions of the soil in an experimental Hinoki (Chamaecyparis obutusa) stand located at Mt. Tsubakuroyama, Ibaraki Prefecture were observed and analyzed. This study examined slash that remained after thinning, and its effects of the surface soil moisture conditions. Observations and soil samples were collected in two plots in the stand : one was named the “Slash Plot”, in which the slash remained on the forest floor and covered the surface of the ground after thinning; the other was called the “Control Plot”, where no slash and litter covered the surface of the ground. Surface conditions there were similar to those observed before thinning.
The results of observations and analyses
are as follows.
1) Solid phase contents of respective
soils at 5-, 10-, and 30-cm depths were less than 20%. Pore contents of soils
at each depth were more than 80% in the experimental plot. These observations
emphasize the character of this black soil, which is volcanic in origin.
2) Total pore contents of
soils at 5-, 10-, and 30-cm depths did not differ. Significant differences of
pore contents were not observed among Control and Slash plot soils.
3) Matric potential of the
Control plot was slightly less than that of the Slash plot at 5-cm depth during
the period without rainfall. In contrast, the matric potential of the Slash
plot was markedly less than that
of the Control plot at 30-cm depth. The matric potential of each plot was not different
at 10-cm depth.
4) The Slash plot soil
temperature was 1-2℃ higher than
that of the Control plot in winter, and 1-2℃ lower than that of the Control plot in summer. Those differences
were engendered by temperature change
moderation that is attributable to slash.
5) Effects of slash on
surface soil moisture are derived from rainfall interception above the ground
and reduction of evaporation from the ground surface.
6) Effects of rainfall
interception by slash engender a decline in the water content of sub-surface
soil. On the other hand, effects of evaporation reduction by slash moderate the
increases in surface temperature and retain
surface-soil moisture.
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