长白山温带混交林林冠下层CO2通量对生态系统碳收支的贡献

作为ChinaFLUX的重要组成部分,从2002年年底开始利用涡度协方差技术在长白山温带混交林林冠上层和下层进行连续通量观测,这为量化林冠下层CO2通量对整个森林生态系统碳收支的贡献提供了一条有效途径.利用2003年林冠上层和林冠下层的观测数据,研究表明林冠下层夜间的CO2通量与5 cm深度的土壤温度存在明显的指数正相关关系.林冠下层的呼吸通量与箱式法观测的土壤呼吸通量之间具有很好的一致性(R2=0.77),二者在全年都与整个森林的光合产物量相耦合,且都在7～8月份达到最大值.林冠下层的呼吸量和土壤呼吸量分别为770 g Cm-2a-1和703 g Cm-2a-1,占整个森林生态系统呼吸年总量的比重高达59.88%和54.69%.林冠下层的光合作用呈双峰型季节变化,两个峰值分别出现在5月中旬和8月下旬.尽管全年林冠下层光合产物量为87 g Cm-2a-1,对整个森林光合产物量的贡献率仅为5.69%,但林冠郁闭度低的4、5月和10月份,林冠下层的光合产物贡献率也分别达到19.99%、21.06%和14.53%.林冠下层净初级生产力的季节动态受该层呼吸作用的季节变异控制,林冠下层在全年都表现为碳源,其净碳排放速率在8月份达到最大.

Contributions of below-canopy CO2 fluxes to ecosystem carbon budget of a temperate mixed forest in Changbai Mountain

As a part of ChinaFLUX projects, CO2 fluxes were measured below- and above-canopy of a temperate mixed forest at Changbai Mountain with the eddy covariance technique since late 2002. Using the CO2 fluxes data measured below- and above-canopy in 2003, this study quantified the contributions of below-canopy CO2 fluxes to ecosystem carbon budget of this mixed forest. The results showed that nighttime below-canopy CO2 flux increased exponentially with soil temperature at a depth of 5 cm, and below-canopy respiration was well consistent with soil respiration derived from chamber measurements (R2=0.77). The below-canopy respiration and soil respiration, coupling with gross ecosystem productivity (GEP) of the whole forest over the whole year, showed similar one-peak seasonal pattern over the whole year and reached their maximums in July through August. The annual below-canopy respiration and soil respiration were 770 g Cm-2a-1 and 703 g Cm-2a-1, respectively, contributing 59.88% and 54.69% to total ecosystem respiration. GEP of the below-canopy (GEPbc) showed clearly bimodal seasonal pattern, with higher rates in mid May and late August. Although GEPbc only accounted for 5.69% of the total forest photosynthetic productivity over the whole year, its contributions in seasons with low canopy density (i.e. April, May and October) reached 19.99%, 21.06% and 14.53%, respectively. The seasonal pattern of below-canopy net ecosystem productivity was controlled by the seasonal variability in respiration of this layer. The below-canopy acted as a net carbon source during the whole year, with peak carbon emission in August.