东北山地森林涡动协方差碳通量观测关键技术研究

Eddy covariance technique has become an important tool of measurement of matter and energy exchange between terrestrial ecosystems and the atmosphere, but there are still some technical challenges on tall forest vegetations in cold climate region and complex terrain. This project focus on a few key technique issues, including the surface heating effect of open path eddy covariance system, anemometer tilt correction in complex terrain conditions, the causes and consequences of energy balance closure problem and the lack of cross-validation with ground carbon balance measurement. To solve these key technique issues, the methods of parallel comparison of various instrument systems and direct measurement of core processes are used at a temperate deciduous forest site in Northeast China. Five methods will be compared for CO2 flux and energy balance to determine suitable tilt correction method; For the heating effect and its influence on the fluxes of carbon dioxide and water vapor, the old-version open path system (LI-7500) is compared with the closed path system (LI-7000) and new open path system (EC150), meanwhile the heating effect is directly measured by fine-wire thermometer and thermocouple, and then the heating effect was related to the environmental factors; The energy balance closure problem is investigated by accounting all components of the energy balance equation, and the energy balance deficit is corrected by different methods to evaluate the consequences of different treatments of the energy balance on the CO2 flux; The decadal CO2 sequestration by eddy covariance and the net ecosystem carbon balance by inventory are cross-validated to improve the reliability of forest carbon flux estimates. Due to the application of eddy covariance become more and more widespread in complex terrain and large amounts of carbon flux data are obtained using the LI-7500, addressing these critical technologies has profound and practical significance for improve the reliability of global eddy covariance carbon flux measurement.

涡动协方差（EC）技术已成为陆地生态系统与大气间物质和能量交换观测的重要方法，但在地形复杂和气候寒冷地区森林植被的应用还存在一些问题。例如，开路系统的表面加热效应、复杂地形条件下的风速仪倾斜校正、能量平衡不闭合的原因与后果以及通量缺乏地面碳平衡测量的交互验证。本项目拟以东北天然次生林的核心分布区帽儿山为例，采用多种方法平行对比确定合适的倾斜校正法；平行对比旧式开路系统（LI-7500）与闭路系统（LI-7000）和新式开路系统（EC150），同时用极细热电偶直接测量表面加热效应，分析旧式开路系统的加热效应对碳水通量的影响，从而改进校正以往开路数据的经验方程；采用实测能量交换过程的方法研究能量平衡闭合问题，并评价不同处理方法的后果；比较年代尺度EC碳通量和地面碳平衡，通过不同途径的交互验证提高森林碳通量测定的可靠性。这些关键技术问题的解决方案对提高全球EC碳通量的观测的可靠性也具有深远的实践意义。

涡动协方差（EC）技术已成为陆地生态系统与大气间物质和能量交换观测的重要方法，但在地形复杂和气候寒冷地区森林植被的应用还存在一些问题。本项目主要研究了单一廓线测量CO2储存通量的误差、超声风速仪坐标旋转、OPEC系统加热效应校正、能量平衡闭合影响因素、EC方法的碳通量的地面对比与验证。重要发现：（1）CO2储存通量不确定性随计算CO2浓度平均值的时间窗口的增大而降低，但系统误差逐渐增大，30min平均可造成净生态系统碳交换（NEE）绝对值低估5%以上，因此单一廓线测定储存通量的系统偏差与随机误差是不可调和的矛盾，为开发下一代储存通量测定系统指明了方向。（2）平面拟合是最优坐标旋转方法，分段和分风向区旋转法表现并不理想。（3）LI-7500表面加热效应白天大于夜间，冬季大于夏季，底部镜头>支杆>顶部镜头。表面加热导致的CO2湍流通量的正误差夏季一般可达1.0 μmol m-2 s-1，冬季一般最大约2.0 μmol m-2 s-1。（4）水平安装的辐射表高估净辐射9%。简单经验模型将水平测量的辐射转换到坡面后，净辐射偏差减小了大约1/4。水平安装辐射表在北坡低估、在南坡则高估能量平衡闭合比率，误差的坡度敏感性约1.1%每度。本项目用实测数据指出辐射表应优先采用平行于坡面方式安装，如果条件不允许，也要建立以站点为基础的辐射倾斜校正模型。（5）全球来看，与测树学法和箱式法相比，EC法的净生态系统产力（NEP）估算值高估25％（0.52 Mg C ha-1 yr-1），生态系统呼吸（Re）低估10％（1.39 Mg C ha-1 yr-1），总初级生产力低估3％。EC法的偏差在在地形复杂、冠层稠密的森林站点更明显。开路EC系统可能因加热效应在NEP估计中引起较大的正偏差。帽儿山森林2008-2011年EC法的NEP（1.12± 1.28 Mg C ha-1 yr-1），明显低于4年测树学法的NEP为2.06 ± 1.03 Mg C ha-1 yr-1，但接近10年（2008-2017）平均生物量碳库增长量为1.05 ± 0.74 Mg C ha-1 yr-1。土壤呼吸对年Re的贡献率为84%。不同方法之间的差异表明通量塔可能低估了Re。这些发现对降低我国东北山地森林乃至全球森林的EC法通量观测的不确定性具有较大的启发意义。