青藏高原高寒草甸生态系统CO2通量及其水分利用效率特征

以涡度相关技术为基础,研究了青藏高原当雄县高寒草甸生态系统2003-2005年共3个生长季的潜热通量L(E)、CO2通量F(c)和水分利用效率W(UE)的变化特征。结果表明:①该地区2004和2005年的太阳总辐射最高值可分别达到1563和1640Wm/2,瞬时净辐射最高值分别为896和925Wm/2,瞬时潜热通量最高值分别为592和597Wm/2。净辐射能量的转化形式季节变化特征明显,6-8月份,净辐射能量多用于潜热蒸发,5月和10月净辐射则多用于显热交换。就2004年5-10月份所选6个代表性晴天来说,LE占Rn的比例分别为0.355%、0.916%、0.738%、0.818%、0.609%、0.456%。②该地区的LE从早上8:30左右开始增加,在下午15:00左右达到最大值,而后逐渐下降;CO2通量从早上8:00左右通过零值开始上升,在10:30左右达到峰值后下降;水分利用效率的日变化特征是日出后迅速增加或直接达到全天的最高值,其后在一天内呈现下降趋势;2004年和2005年生长季的CO2吸收峰值都刚接近-0.3mgCO2.m-2.s-1F(c为负值时表示碳吸收),水分利用效率瞬时最大值接近8gCO2k/gH2O。③2004年当雄高寒生态系统白天CO2通量平均值从6月份初就开始表现为净碳吸收,而2005年在6月下旬才表现为碳吸收F(c为负值),但两者均在10月初就表现为碳排放F(c为正值);2004的水分利用效率日平均值从6月初通过零点开始上升,在7月中下旬左右达到最大值。相比之下,2005年的水分利用效率日平均值在6月底通过零点开始上升。另外,2004年的水分利用效率在总体水平上要高于2003年和2005年。就水分利用效率的日平均值而言,2003年和2005年的最大值分别为2.0gCO2k/gH2O和2.7gCO2k/gH2O,而2004年可以达到3.2gCO2k/gH2O。④当雄高寒草甸生态系统在2004年和2005年生长季(5月1日到10月31日)净CO2吸收量分别为0.257kgCO2.m-2和0.153kgCO2.m-2;2004年和2005年整个生长季的水分利用效率分别为0.496gCO2k/gH2O和0.365gCO2k/gH2O,与降雨量呈现正相关关系。

Carbon Dioxide Exchange and Water Use Efficiency of Alpine Meadow Ecosystems on the Tibetan Plateau

In order to study the diurnal and seasonal variations of CO2 fluxes and water use efficiency(WUE),the open-path eddy covariance systems were employed to continuously measure carbon and energy fluxes of the Alpine meadow during the three growing seasons(from May to October) of 2003-2005,respectively.The results showed that:(1)The maximum global solar radiations were 1 563W/m₂ and 1 640W/m₂ in 2004 and 2005(mean value of 30 minutes),respectively,greater than the solar constant;while the maximum net radiations were 896W/m₂ and 925W/m₂,and the maximum latent heat fluxes densities were 592 W/m₂ and 597W/m₂,respectively.As for the six selected days from May to October in 2004,the ratios of latent heat flux to net radiation were 0.355%,0.916%,0.738%,0.818%,0.609% and 0.456%,respectively.(2)The latent heat had an obvious diurnal variation.During the daytime of growing season,the latent heat fluxes density increased with radiation,and declined after reaching its maximum at 15:00p.m;CO2 flux reached its maximum at about 10:30a.m.and WUE presented a falling trend from morning to afternoon.The maximum CO2 fluxes and WUE(mean value of 30 minutes) were similar to 2004 and 2005,with values being 0.3 mg CO2·m-2·s-1 and 8g CO2/kg H2O,respectively.(3)The CO2 fluxes and WUE(mean value of the daytime) varied obviously at the seasonal and yearly scales.The carbon fluxes(mean value of the daytime) represented as a carbon sink from early June in 2004,but from late June in 2005;WUE(mean value of the daytime) became positive also from early June in 2004,but from late June in 2005;in addition,the overall values of carbon fluxes and WUE(mean value of the daytime) of 2004 were greater than 2005.The maximum carbon fluxes were 0.15mg CO2·m-2·s-1 and 0.13mg CO2·m-2·s-1 in 2004 and 2005,respectively.And the maximum values of WUE(mean value of the daytime) were 3.2g CO2/kg H2O and 2.7g CO2/kg H2O,respectively.(4)The total net CO2 absorption of the growing season(from May to October) in 2004 and 2005 were 0.257kg CO2·m-2 and 0.153kg CO2·m-2,respectively.The water use efficiency throughout the growing season in 2004 and 2005 were 0.496g CO2/kg H2O and 0.365g CO2/kg H2O,respectively,being positively correlated with seasonal precipitation.