エンクローズドパス式渦相関法を用いた降雨中および降雨直後の蒸発散・光合成の実態把握 Evaluation of Canopy Evapotranspiration and Photosynthesis During and Immediately After Rainfall Using Enclosed-Path Eddy Covariance Method

<p> 日本は降雨の多い地域だが，これまで降雨中・直後の生態系CO₂/H₂Oフラックスを直接測定するのは困難であり，未解明の部分が多かった．本研究では，滋賀県南部の桐生水文試験地ヒノキ林において，エンクローズドパス型分析計を用いた渦相関法により生態系CO₂/H₂Oフラックス，及び各種気象条件，樹液流速を測定した．従来のオープンパス型，クローズドパス型CO₂/H₂O分析計による測定結果と比較したところ，エンクローズドパス型が降雨中・直後のCO₂/H₂Oフラックス測定に最も適していた．エンクローズドパス型分析計を用いた観測の結果，降雨中・直後のH₂Oフラックスに関しては，飽差の増大につれて放出が増大し，無降雨時と比べて大きな放出の値が観測された．この間は樹液流速の上昇が伴わなかったことから，遮断蒸発がおこっていたと考えられる．さらに強い日射が続くと，樹液流速が増大したことから，H₂Oフラックスの成分が遮断蒸発から蒸散へ移り変わったと考えられた． 一方でCO₂フラックスは，降雨中・直後も無降雨時と同じように日射の増大に応じて森林生態系への吸収の増大が観測された．</p>

<p> To clarify canopy evapotranspiration and photosynthesis during and immediately after rainfall, measurements of ecosystem CO₂/H₂O fluxes using enclosed-path eddy covariance method, sap flow velocity, and micrometeorological conditions were taken in a Japanese cypress forest in the Kiryu Experimental Watershed in southern Shiga Prefecture, Japan. An enclosed path gas analyzer is more suitable than open-path or closed-path gas analyzers to measure CO₂/H₂O fluxes during and immediately after rainfall. Results show that H₂O flux during and after rainfall in relation to the vapor pressure deficit was greater than those on other days. H₂O flux during and after rainfall under low sap flow velocity is regarded as interception evaporation. With radiation increasing after rainfall, the H₂O flux component is regarded as a transition from interception evaporation to transpiration because of increased sap flow velocity. However, CO₂ absorption occurred during and immediately after rainfall in response to solar radiation in the same manner as on rain-free days. Observation revealed that the abaxial side of leaf surface remained dry even during and immediately after rainfall, which explains why photosynthesis occurs immediately with increased solar radiation after a rain event.</p>