盐生荒漠地表水热与二氧化碳通量的季节变化及驱动因素

以古尔班通古特沙漠南缘原始盐生荒漠为对象,利用涡度相关法,对原始盐生荒漠地表水热、二氧化碳通量进行了连续观测,对通量的季节变化、浅层土壤水分条件改变对盐生荒漠植物群落水汽、二氧化碳通量以及水分利用效率的影响进行了系统的分析.结果表明:净辐射通量、潜热通量和二氧化碳通量都具有明显的季节变化趋势,而显热通量的季节变化不明显.在有效能量的分配上,显热通量是有效能量的主要输出项.在降水影响期和非影响期,二氧化碳通量没有明显的变化;而在非降水影响期潜热通量明显降低,表明土壤水分处于亏缺状态,但二氧化碳通量并没有降低的趋势,而与前期保持高度的一致性.以此可以推断,该荒漠盐生植物群落并不以降水为主要水分来源,降水后水汽通量和二氧化碳通量变化的不一致性是该原始盐生荒漠独特植物特征决定的.降水影响期原始盐生荒漠植物群落的水分利用效率低于非影响期,是由于降水后土壤蒸发迅速增加,而植物蒸腾与光合并未随之增加造成的.

Seasonal variation in water, heat and CO2 fluxes and its driving forces over a saline desert

We studied the seasonal variation in carbon dioxide, energy and water vapor fluxes over a saline desert in western China using the eddy covariance technique. The plant community over the desert consisted of an over-storey dominated by T. ramosissima with sparse under-storey herb plants. Measurements presented here cover 6 month period from May to October. The following questions were addressed: (1) How did Rn, Fc, LE and H varied seasonally? (2) How did Fc and LE responded to environmental factors after raining events? (3) How did WUE varied after raining events? LE, Rn, and Fc exhibited a clear seasonal pattern. But H did not. Rn was highest (120-160W/m2) in mid-summer (June-August); the daily average of Rn was 72.87 28.3 W/m2. The daily average of H was 40.8 23 W/m2. In contrast, values of LE were very low (<30W/m2), and peak values appeared after rainfall, the daily average of LE was 29 13.3W/m2. Obviously all available energy was dissipated as sensible, rather than latent heat. In early May 2004, new leaf development was in progress, the respiratory processes were dominating, and the Saline Desert was a weak sink of carbon flux. By late May, plant came into the rapid growth; strong net sink activity started and lasted until the end of September. At the end of October the Saline Desert was net source of carbon. For the entire growing period, the Saline Desert was a weak sink, with a net sink estimated at only about -136mg/m2. During wet and dry periods after rain events, Fc displayed no obvious difference, the relationship between Fc and photosynthetically active radiation was described well by a hyperbolic fit, with the r2 values greater than 0.70, there appeared to be only PAR effect on Fc. However, in contrast to latent heat flux, variation in LE was rather different during the two periods. In wet period, the size of the rainfall pulse was positively related to the magnitude of the flux, the peak daytime LE was 238W/m2 whereas the maximum LE was approximately 100W/m2 in dry period, there appeared to be precipitation effect on LE. Low ecosystem WUE values measured during the wet period may be explained by soil evaporation, which resulted in the enhancement of LE.