热带季节雨林林冠上方和林内近地层CO2浓度的时空动态及其成因分析

 为探讨西双版纳独特地方气候背景下,热带季节雨林CO₂浓度的时空变化特征和不同时间尺度上环境因素对森林CO₂浓度时间分布的作用,以及 为研究热带季节雨林的碳通量、净生态系统交换量(Net ecosystem exchange, NEE)等提供支持,我们利用热带季节雨林林冠上方和林内近地层 CO₂浓度连续监测资料,结合同步气象资料进行了统计分析。研究结果表明：在植被生理活动、土壤呼吸以及林内湍流的共同作用下,西双版纳 热带季节雨林CO₂浓度表现出明显的日变化、季节变化和林冠上下差异。在日尺度上,林冠上方的CO₂浓度时间变化曲线为“单峰型”,林内近 地层CO₂浓度时间变化曲线为“双峰型”,造成林内近地层傍晚第二个峰值的主要因子是地形因子作用下形成的局地环流。在季节尺度上,林冠 上方CO₂浓度主要受林冠代谢作用的影响,呈现雨季低、干季高的特点,而林内近地层的CO₂浓度则主要受地表呼吸过程所控制,季节变化趋势 与林冠上方相反。林冠上方CO₂浓度低于林内近地层CO₂浓度,且差异较大;在日尺度上,各月（除12月外）CO₂浓度的最大差值皆大于80 mg·m ^－3,且出现在傍晚;在季节尺度上,最大值为-62.9 mg·m^－3,出现在10月,最小值为-8.4 mg·m^－3,出现在12月。

SPATIAL AND TEMPORAL DYNAMICS OF CO2 CONCENTRATION AND ITS CAUSES IN XISHUANGBANNA TROPICAL SEASONAL RAIN FOREST, CHINA

Aims Because of its highly diverse flora and unique climate, Xishuangbanna tropical seasonal rain forest (XTSRF) is one of few areas in China characterized by well-protected primary forest types considered to be tropical rainforest. Compared with other rainforests around the world, XTSRF occurs at higher latitude and altitude. Our aim was to investigate variation of CO₂ concentration at different time scales under this unique climate and special geographical situation and determine how environmental variables formed the temporal pattern of CO₂ concentration. Methods CO₂ concentration in XTSRF was measured continuously by infrared gas analyzers (Li-7500, Li-Cor, Lincoln, NE, USA) and recorded every 30 min with a data-logger (CR5000); meteorological variables were also measured and recorded. The static chamber method was used to measure soil respiration weekly. Leaf area index was measured in the middle of every month using a forest canopy analyzer (LAI-2000).  Important findings The diurnal pattern of CO₂ concentration above canopy (AC) was consistent through the year, with higher values at daytime and lower values at night due to photosynthesis uptake and respiration release, respectively. In contrast, near the forest floor (NF) diurnal patterns were bimodal in most months, with a second peak in late afternoon. Average variation of CO₂ concentration in a year was larger at AC (622.8-686.5 mg·m^－3) than at NF (659-700 mg·m^－3). Difference of CO₂ concentration between AC and NF was higher in the rainy season than in the dry season, with the largest value (-62.9 mg·m ^－3) in October and the smallest (-8.4 mg·m^－3) in December. The “flushing" phenomenon in the early morning was primarily affected by stability of the atmospheric boundary layer. In long foggy days and under low wind velocities, forests in the valley had little air circulation, but local circulation resulted in sharp increases of CO₂ concentration in the afternoon. CO₂ concentration of NF and AC were dominated by soil respiration and canopy metabolism, respectively.