西双版纳热带季节雨林干热季林冠上小气候特征及CO_2通量的观测

Characteristics of microclimate and CO_2 flux above a tropical seasonal rain forest in dry-hot season of Xishuangbanna,southwest of China

热带森林在陆地生态系统中起着重要的作用,但是人们对它在碳循环中的作用却了解不多;近年来,为了对其进行深入研究,热带森林的CO2通量成为了研究的热点。应用微气象法中的开路系统涡度相关法,使用设置在西双版纳一片成熟的热带季节雨林中观测铁塔上的观测仪器所得的干热季7个晴好天气的CO2通量及小气候观测数据,对冠层的CO2通量及小气候特征进行了分析研究。研究结果表明:(1)热带季节雨林林冠上风速及摩擦风速在中午和上半夜较大,而后半夜和上午较小;风向有显著的昼、夜交替特征,昼间多为偏东风(45~135°),而夜间多为偏西风(250~280)°;(2)林冠上方气温和树冠面表温具有显著的日变化特征,树冠表温日变化幅度大于气温,热量由空气传向树冠中,在观测的7d中,气温有着较明显的升高趋势;(3)干热季林冠上湿度变化范围为26.5%~97.2%,饱和水汽压差数值大小介于0.3~30.5 hPa之间;(4)CO2浓度在364.5~408.5m l/m3之间变化,夜间浓度升高,而昼间CO2浓度降低;(5)地下5cm土壤温度与气温一样日变化规律明显,土壤含水量的变化幅度很小,在7d内其变化幅度维持在19.9%~23.3%之间,日变化幅度更小;(6)总体上讲林冠上方显热通量小于潜热通量,上午显热通量和潜热通量的数值基本相同,但是在中午和下午,潜热通量远大于显热通量,充分显示了西双版纳干热季热带雨林森林的热量支出主要是蒸腾耗热;(7)观测期间,生态系统净CO2交换(NEE)在-20.9~17.6μm o l/(m2.s)之间浮动,每天最大净CO2吸收速率在-20.9^-12.9μm o l/(m2.s)范围内。从生态系统净CO2交换的平均日变化看,昼间最大的净CO2吸收速率为-12.4μm o l/(m2.s),夜间最大的净释放速率为6.6μm o l/(m2.s)。净CO2交换的日累积量在-0.0665~0.0448m o l/m2范围内变化,7d的累积量为-0.0140 m o l/m2,表明在西双版纳干热季的7 d观测时间段里,热带季节雨林呈现弱的碳汇效应。

Tropical forests play a great role in the terrestrial ecosystem, however, little is known about its effects on carbon cycle. In recent years, many researches have focused on CO2 flux in order to further understand tropical forests how to influencing on carbon balance. Based on the eddy covariance method and by using of open-path CO2 analyzer placed on a tower in a tropical seasonal rain forest in Xishuangbanna, 7-clear day data about CO2 flux and microclimate factors in dry hot season were used to analyze canopy characteristics of CO2 flux and microclimate elements. The results showed that, （1） Wind speed and friction wind speed was larger at the noon and the first half of the night, while was less in the forenoon and the second half of the night; there had obvious alternation in wind direction above the canopy, with more east wind （45-135°） appearing in the daytime and more west wind coming forth in the nighttime （250-280°） ; （2） There had significant characteristics of daily variation in air temperature and canopy surface temperature, moreover, the daily range of canopy surface temperature was larger than that of air temperature which resulted that heat was transferred from air to canopy. During the 7 days, air temperature presented an obvious trend of rise. （3） In the dry hot season, relative humidity and water vapour deficit varied from 26.5 to 97.2% and from 0. 3 to 30.5 hPa, respectively; （4） CO2 concentration ranged between 364.5 and 408.5ppm, moreover, it increased in the nighttime and decreased in the daytime. （5） The characteristics of daily variation of the 5cm soil temperature were clear and similar with those of air temperature. The range of the soil water content was very small, with the values varying from 19.9 to 23.3% during the 7 days. The daily range of the soil water content was especially small. （6） Generally speaking, sensible heat flux was less than latent heat flux. In the forenoon, sensible heat flux was nearly equal to latent heat flux, but latent heat flux was much larger than sensible heat flux at the noon and in the afternoon, which showed that transpiration was mainly payout of the heat in Xishuangbanna tropical seasonal rain forest; （7） During the observation period, net ecosystem COz exchange fluctuated between -20. 9 μmol/（m^2·s） and 17.6 μmol/（m^2·s） with the maximum of net CO2 uptake was in a range from -20.9 μmol/（m^2·s） to -12.9 μmol/（m^2·s）. The average diurnal variation of NEE showed that the maximum of net CO2 uptake was -12.4 μmol/（m^2·s） and the minimum of net CO2 emission was 6.6 μmol/（m^2·s）. The daily accumulated net ecosystem CO2 exchange varied from -0. 0665 to 0. 0448 mol/m^2 and the total accumulated CO2 within 7 clear days was -0. 0140 mol/m^2, which indicated that the tropical seasonal rain forest was a small carbon sink within the 7 clear days of dry hot season in Xishuangbanna.