Predicting variations of evapotranspiration, photosynthesis and water use efficiency in a forest with the change in LAI by the forest management

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1.Observations in the fieldThe following observations were made at the micrometeorological observational tower, around the center of a coniferous forest catchment that was mainly planted with Japanese cedar and the area of which was 10.7 ha in Ozu.(1)Observation of fluxes by the eddy correlation method and Bowen ratio energy budget method(2)Transpiration and photosynthesis of a single leaf(3)Soil respiration(4)Water balance of the catchment2.ResultsVariations of daily evapotranspiration (ET) and CO_2 flux (CER) in 2005 reached maximum in May - September with minimum in winter. Correlation coefficients between these two fluxes and meteorological conditions, soil water content and four seasons revealed that solar radiation was the first and air temperature was the second factor influencing on the two fluxes. The significance test revealed that the difference between two groups of the fluxes under the condition of air temperature higher than 22 C and lower than 22 C was significant. Yearly ET was 1000 mm and absorption of CO_2 was 27.9t/ha in the forest catchment in 2005.The LAI measured was 4.93 at the observational site. By using a simple model of the canopy transpiration and photosynthesis that consists of leaf transpiration and photosynthesis model and multilayer radiation model within the canopy, variations of canopy ET and photosynthesis with the change in LAI were modeled. The predicted LAI that would make the water use efficiency highest was around 4.