中国亚热带-暖温带过渡区锐齿栎林净生态系统碳交换特征

Abstract

在2017年1月1日-2017年12月31日期间，采用涡度相关法对位于亚热带-暖温带气候过渡区的河南宝天曼国家级自然保护区的65年生锐齿栎（Quercus aliena）天然次生林的碳通量进行了连续观测。结果表明：在观测期间，该森林生态系统在生长季5-10月份为碳汇，非生长季各月为碳源，净碳吸收量与释放量分别在7月和4月达到最大。净生态系统生产力为569.4 g C m^-2a^-1，生态系统呼吸为529.9 g C m^-2a^-1，总生态系统生产力为1099.3 g C m^-2a^-1。30min尺度上夜间净生态系统碳交换量与5cm深度土壤温度的关系可用指数方程表示（R²=0.21，P < 0.001），其温度敏感性系数（Temperature sensitivity coefficient，Q₁₀）为2.2。如果排除夜间通量观测的误差，处在海拔较高地区的夜间低温和非生长季的低温抑制了生态系统呼吸排放，可能导致全年生态系统呼吸量较低。在生长季5-10月份，各月的白天净生态系统碳交换量对光合有效辐射的响应符合直角双曲线模型，初始光能利用效率、平均最大光合速率和白天平均生态系统呼吸强度呈明显的季节变化，范围分别是0.06-0.12 μmol CO₂ μmol^-1 photon、0.44-1.47 mg CO₂ m^-2s^-1和0.07-0.19 mg CO₂ m^-2s^-1。夏季7、8月份，较高的饱和水汽压差对白天锐齿栎林的碳吸收有明显的抑制作用；生长季末期9月份较高的土壤含水量对白天锐齿栎林的碳吸收也产生了抑制作用，表明生长末期降水过多影响森林的碳吸收。;Oak forests comprise the largest forest area in central China and are the potential carbon sink, while we know little about the carbon dioxide flux of oak forests in the transitional zone from subtropics to warm temperate, China. Using an open-path eddy covariance system and micro-climate instruments, the CO₂ flux, photosynthetic active radiation (PAR), air temperature, soil temperature and precipitation were simultaneously observed in a natural oak (Quercus aliena) forest at Baotianman National Nature Reserve. Based on the data sets during January to December 2017, dynamic change of CO₂ flux at different temporal scales and its underlying mechanism were analyzed. The results indicated that the diurnal and seasonal variations of CO₂ fluxes showed an obvious single peak pattern. The oak forest ecosystem was a carbon sink during the growing season (May-October), while a carbon source occurred during the non-growing season. Net carbon sequestration and emissions peaked in July and April, respectively. Mean annual net ecosystem productivity (NEP), ecosystem respiration (Re) and gross ecosystem productivity (GEP) were 569.4, 529.9 and 1099.3 g C m^-2 year^-1, respectively. The relationship between net ecosystem carbon exchange (NEE) measured at half-hour interval during night and soil temperature at depth of 5 cm can be expressed by an exponential equation (R²=0.21, P < 0.001), with its temperature sensitivity coefficient (Q₁₀) of 2.2. A low temperature at night and in the non-growing season at high elevation resulted in lower Re throughout the year. The relationship between NEE and PAR at daytime could be well expressed by a rectangular hyperbolic equation during growing seasons. Monthly initial light use efficiency, maximum photosynthetic capacity and daytime Re were 0.06-0.12 μmol CO₂ μmol^-1 photon, 0.44-1.47 mg CO₂ m^-2s^-1 and 0.07-0.19 mg CO₂ m^-2s^-1, respectively. Both higher vapour pressure deficit in July and August and higher soil moisture at the end of the growing season inhibited carbon uptake of the oak forest, indicating a negative effect of increased precipitation on carbon sequestration at the end of the growing season.