温带混交林碳水通量模拟及其对冠层分层方式的响应--耦合的气孔导度-光合作用-能量平衡模型

Simulation of CO_2 and H_2O fluxes over temperate mixed forest and sensitivity analysis of layered methods:stomatal conductance-photosynthesis-energy balance coupled model

利用Leuning建立的耦合的光合作用、气孔导度和能量平衡方程,以将冠层分成多层的方式,包括Gaussian五点积分法、将冠层平均分为多层的方法,逐层计算温带混交林的碳水通量,最后累加至冠层尺度,以模拟CO2和H2O通量。该模型以常规气象观测数据作为驱动变量,计算出冠层与大气之间的碳水交换,与涡动相关系统的通量观测数据进行比较,分析了不同的冠层分层方式对多层模型模拟结果的影响。从3个温带混交林通量站涡动相关系统的能量平衡闭合度来看,中国长白山站CBS、韩国GDK和日本MMF站点的能量平衡比率(EBR)分别为0.76、0.66和1.07,居于国际同类观测范围(0.34—1.2)的中上水平,因此,涡动相关系统的观测数据较为可靠。从碳水通量的日变化来看,用Gaussian五点积分法将冠层分为五层的模型能较好的模拟碳水通量的"单峰形"日变化趋势。夜间Fc为负值且变化趋势较为平缓,表明生态系统进行呼吸作用释放CO2,从日出开始Fc逐渐变为正值,表明生态系统进行光合作用吸收CO2,Fc在中午时分达到最大值,下午Fc逐渐减小,日落之后又回复到夜间的负值。H2O通量的日变化曲线与CO2通量相似,且模拟值与涡动相关实测值具有较好的一致性。在多层模型中,对冠层采用不同的分层方法,对碳水通量模拟结果有一定的影响。以Gaussian五点积分法将冠层分为五层的方法作为对照,分别将冠层平均分为2、5、10、20层的方法得到的碳水通量与其进行比较。从平均值来看,分层越多,H2O通量模拟值越低,而CO2通量模拟值越高。不同的分层方法产生的差异,主要来自于不同层的辐射吸收、温湿度、风速等环境要素的垂直廓线差异,且叶片光合作用对光的响应是非线性的。

A stomatal conductance-photosynthesis-energy balance model developed by Leuning is used to estimate CO2 and HiO fluxes of temperate mixed forests. The canopy is divided into multiple layers, using both Gaussian integration and averagelayer method. The fluxes are calculated layer by layer and then accumulated for the whole canopy. The modelcalculates CO2 and H：O exchange between atmosphere and canopy based on the input of meteorological variables. We compare the modeled fluxes with those measured by eddy covariance system, and analyze the influence of different layermethods on the modeled fluxes. The energy balance ratios （EBR） of CBS, GDK and MMF sites are 0.76, 0.66 and 1.07, respectively, which is reasonable according to the international reported range （0.34-1.2）. The results of the Gaussian integration method agree well with the eddy covariance fluxes, showing the diurnal CO2 fluxes as bell shaped. The CO2 flux （Fc） is negative and stable during the night, which suggests that forest ecosystems release CO2 to the atmosphere. After sunrise, Fc increases gradually to positive values, peaking at noon, and decreases in the afternoon to the negative night values. The diurnal cycle of H20 fluxes （LE） is similar to that of Fc, and the modeled LE agree well with the measured LE. The estimated fluxes are influenced by the different layer-methods. We compared CO2 and H20 fluxes from dividing the canopy into 2, 5, 10 and 20 layers averagely with the fluxes from Gaussian integration method. Larger numbers of layers produce lower LE and higher Fc. The fluxes from different layer-methods are sensitive to the vertical profiles of absorbed radiation, temperature, humidity and wind speed. Furthermore, the leaf photosynthesis responds nonlinearly to the radiation.