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

1.野外观测以下观测是在小津市以日本雪松为主的针叶林集水区中心周围的微气象观测塔进行的，该集水区面积为10.7公顷。（1）涡动相关法和波文比能量收支法观测通量（2）单叶蒸腾和光合作用（3）土壤呼吸（4）流域水量平衡（2）结果2005年蒸散量（ET）和CO2通量（CER）日变化在5 ~ 9月最大，冬季最小。这两个通量与气象条件、土壤含水量和四季的相关系数表明，太阳辐射是影响这两个通量的第一因子，气温是第二因子。显著性检验表明，气温高于22 ℃和低于22 ℃时，两组间的通量差异显著。2005年森林集水区年蒸散量为1000 mm，吸收CO_2量为27.9t/ha，叶面积指数为4.93。利用一个由叶片蒸腾、光合作用模型和冠层内多层辐射模型组成的简单的冠层蒸腾、光合模型，模拟了冠层蒸散和光合作用随叶面积指数的变化。使水分利用效率最高的预测叶面积指数在4左右。

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.