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スギ人工林の光合成生産機構に関する研究(I) 林分における光減衰の解析


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  • Studies on the photosynthetic production processes in a Sugi (Cryptomeria japonica D.DON) plantation (I) Computation of light extinction in the forest stand


スギ人工林で起こる光減衰に対し, 樹冠や林冠の構造を考慮して解析する方法を示し, 68年生の高齢林に適用してみた。(1)林分上から林床に向かう光線は, 各樹冠を通過する距離と各樹冠における全器官面積密度に依存して減衰するとして, xyz-直交座標系に各林木の樹冠曲面を定め, 林床での光測定結果より光減衰係数を導く方法を明らかにした。(2)この光減衰係数は, 光線が樹冠を通過するさいに起こる光減衰の程度をよく表わすが, 光線入射角で変化した。光線入射角が50°以下では, ほぼ一定の値を示すようであったが, 50°を越えると著しく増大し, 両者の関係は, 双曲線で近似された。A model for light extinction in relationship to the geometrical structure of a Sugi, Japanese cedar (Cryptomeria japonica D.DON) plantation is proposed, and it is applied to a 68-year-old stand. The model assumes that the light extinction is determined by the pathlength of light traversing the tree crown and by the spatial density of the area of the crown components, and that the crown occupies a volume bounded by a conicoid. The calculation procedure for the extinction coefficient which uses light measurements on the forest floor as inputs is shown, where the coordinate system in space is introduced. The extinction coefficient, suggesting the degree of light interception by the crown, varies depending on the incident angle. The coefficient seems to be unchanged at incident angles below 50°, but above 50° it shows a marked increase. The relationship is well-defined as a hyperbola.



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