基于EALCO模型对中亚热带人工针叶林CO_2通量季节变异的模拟

MODELING SEASONAL VARIATION OF CO_2 FLUX IN A SUBTROPICAL CONIFEROUS FOREST USING THE EALCO MODEL

EALCO模型是一个基于生理生态学过程,模拟生态系统下垫面与大气之间水、热和碳通量交换的综合模型。将该模型应用在亚热带常绿针叶林,对其生态系统过程进行了模拟,以深入探讨季节性干旱对生态系统过程的影响。对EALCO模型进行了参数化与初始化并对模型的光合作用时段和落叶机制进行了改进,以更好地模拟亚热带人工针叶林生态系统。千烟洲通量观测站自2002年底开始应用涡度相关技术对中亚热带人工针叶林生态系统进行通量观测,该站点2003年经历了一次较严重的季节性干旱(由高温与少雨综合作用造成),降水量仅为多年平均值的65%,而2004年的年降水量与多年平均值较为接近,利用该站点2003和2004年特殊的气候条件,使用其通量观测数据对模型的模拟效果进行检验。从模拟结果的总体趋势来看,模型能较好地从半小时、日及年尺度上反映两年内土壤-植被-大气之间的碳交换状况。总初级生产力(Gross primary production,GPP)在一年中呈现单峰型变化,遇高温及干旱胁迫GPP值下降。由于受到干旱胁迫的影响,2003年GPP值比2004年偏低12.9%。模拟结果显示,2003年GPP值比2004年偏低11.2%。观测数据与模拟结果均显示,水分胁迫期间净碳交换量(Net ecosys-temproduction,NEP)模拟值与实测值的日变化均呈现一种"偏态",即一天中生态系统碳交换量最大值出现在上午某一时刻,之后逐渐降低。模拟结果显示,水分匮缺对光合能力的影响比对生态系统呼吸作用的影响更为强烈,因而导致了净生态系统生产力的降低。进一步分析表明,水分匮缺期间,晴天正午之前,深层土壤(>20cm)水分的匮缺抑制了光合作用能力,正午之后,高温与深层土壤水分匮缺共同削弱光合作用能力,影响各占一半。

Aims Seasonal drought frequently occurs in the mid-subtropical region of China and commonly combines with high temperature.Our objectives were to test the sensitivity of carbon exchange to this seasonal drought and discuss the influence of seasonal drought on carbon assimilation.Methods We used flux measurements obtained from eddy covariance technology since October 2002 over a human-planted forest ecosystem at Qianyanzhou（QYZ）（26°44＇ N,115°03＇ E,110.8 m als.）.The EALCO（ecological assimilation of land and climate observations）model is parameterized to simulate the ecosystem carbon exchange process in the human-planted evergreen forest.Simulation results were validated using half-hourly carbon fluxes and daily and annual GPP（gross primary production）,NEP（net ecosystem production）and TER（total ecosystem respiration）estimated from eddy covariance measurements.Important findings In general,the model can effectively simulate the two years＇ carbon fluxes among soil-plant-atmosphere on hourly,daily and annual scales.Both simulations and observations showed strong impact of drought on GPP in 2003.Compared with 2004,the annual GPP in 2003 was 12.9% lower according to observations（1 610 vs.1 865 g C·m^-2）and 11.2% lower according to model results（1 637 vs.1 844 g C·m^-2）.The diurnal variations of NEP from both observations and simulations during the period of soil water deficit showed asymmetric format,i.e.,the peak value of carbon exchange accrued at a certain time in the morning and then decreased with time.Modeling results indicated that water stress has more influence on photosynthesis than TER,which led to the decrease of NEP.Further analysis suggested that deep soil water content controls canopy photosynthesis in sunny days before noon during soil water stress.Afternoon,both high temperature and deep soil water content eliminate the GPP,and their elimination percents are equal.On cloudy days,radiation and deep soil water content primarily determine the photosynthesis,and temperature becomes a generally minor controlling factor.