報告書番号

V04029

タイトル（和文）

シンチロメータを用いた落葉広葉樹林のCO2吸収量評価

タイトル（英文）

Evaluation of CO2 uptake amount by deciduous forest using scintillometer

概要 （図表や脚注は「報告書全文」に掲載しております）

長期フラックスデータから落葉広葉樹林のCO2吸収量を評価するために，シンチロメータと渦相関法による領域平均フラックス測定手法を適用し，2002から2004年にかけて領域平均フラックスを欠測少なく算出した。領域平均フラックスから導いた群落光合成量は，渦相関法の結果に比べ，個葉の光合成能力に葉面積を乗じて求めた理論的な群落光合成量により近い値を示し，領域平均フラックスは群落光合成をより妥当に評価できることが分かった。渦相関法によるCO2フラックスから算出した３年間の平均CO2吸収量は1.81tCha-1yr-1で，一方領域平均フラックスによる評価値は2.77tCha-1yr-1となった。渦相関法による算出値は既往のサイトにおける結果とほぼ同じ値であることから，既往のサイトにおいても本手法の適用によってCO2フラックス評価の信頼性向上が期待できる

概要 （英文）

Evaluation of CO2 uptake amount by forests is necessary to use carbon sinks for greenhouse gas removal according to the Kyoto Accord's mechanism. Precise amounts of CO2 sequestered by forests are obtainable using CO2 flux (mg m-2s-1) data observed over a forest canopy. Application of a small aperture scintillometer (SAS), which has large observation area, improves the spatial representation capability of flux measurement. This study estimates the precise amount of CO2 uptake by a forest based on flux measurements using a combination of SAS with the conventional eddy-covariance (EC) method. SAS is a turbulence measurement apparatus that measures flux parameters using an optical long path. From 2002-2004, we set up SAS at 10 m above an 18-m high deciduous forest canopy. Using advantages of the conventional eddy-covariance method, which can provide long-term steady measurements even under bad weather conditions, the spatial-averaged flux measurement method was developed using SAS and EC together. Validity of the evaluated flux was confirmed by checking the match of the energy balance of the scalar flux. The maximum photosynthetic rate of the forest ecosystem calculated from the spatial-averaged CO2 flux showed good agreement with the canopy photosynthetic rate derived from single-leaf photosynthetic characteristics; the eddy-covariance method was prone to underestimation. Results indicate that the spatial-averaged flux can properly evaluate the forest's CO2 exchange. The amount of three years averaged CO2 absorption calculated from the EC was 1.81 tCha-1yr-1. That value almost corresponds to the result obtained at another site with similar weather and tree conditions. The CO2 absorption from spatial-averaged flux was 2.77 tCha-1yr-1, which was about 1.5 times that obtained using the eddy-covariance method. In cases where EC underestimates flux at existing forest sites, SAS can improve the flux measurement reliability.

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