干旱对青藏高原腹地高寒草地生态系统净CO2交换的影响

干旱对草地生态系统NEE有深刻影响。基于涡度相关技术提供的碳通量及小气候数据，研究了2009年当雄高寒草地生态系统的碳交换特征及其主控因子，同时分析了干旱的可能影响。5—7月初及9月发生的干旱导致草地GLAI、ALB和GPP较低，6月中旬到7月初碳吸收一度下降。干旱使6、7月份NEE日变化进程发生改变。同时，NEE和GPP的季节变化也受到干旱影响。由于干旱导致生态系统吸收能力降低，75]3日出现NEE日净碳排放最高值（0．9gCm-2d-1）。5-7月的NEE月总量均大于0，且逐月增加。该草地2009年的GPP和NEE分别为-158．1和52．4gCm。日均0〈01时，0成为影响白天NEE变化的主控因子。GLAI、r和目是3个对NEE季节变异影响最大的指标，且其影响程度依次降低。GPP季节变化的主控因子是GLAI、θ、PPT、VPD和瓦，生态系统水分状况（0、PPT或VPD）对GPP的影响大于T20。Rcco主要受控于t、GLAI、PAR和PPT，且其影响力依次降低。GLAI的季节变化可解释NEE和GPP变异的60．7％和76．1％。当雄高寒草地生态系统水分条件的年际变化可能是影响NEE年际变异的主要因子。

Impact of Drought Stress on Net CO2 Exchange above an Alpine Grassland Ecosystem in the Central Tibetan Plateau

Drought may impact the net ecosystem exchange of CO2 （NEE） between grassland ecosystems and the atmosphere during growth seasons. Here, carbon dioxide exchange and controlling factors in alpine grassland under drought stress in the hinterland of Tibetan Plateau （Damxung, Tibet, China） were investigated. Data were obtained using the covariance eddy technique in 2009. Severe drought stress appeared in the early growing season （May to early July） and September. Drought conditions during the early growing season limited grass production and the green leaf area index （GLAD increased slowly, with an obvious decline in June. When encountering severe water stress, diurnal patterns of NEE in the growth season altered with a peak carbon release around 16：00 h or a second carbon uptake period before sunset. NEE variations in daytime related most closely with O other than PAR when daily averaged @〈0.1 m3 m 3. Seasonal patterns of gross primary production （GPP） and NEE were also influenced by drought： the maximum and minimum of daily-integrated NEE were 0.9 g C m-2 d-1 on 3 July 2009, and -1.3 g C m-2 d-1 on 12 August 2009 with a GPP peak （-2.3 g C m-2 d-1） on the same day, respectively. Monthly NEE from May to July remained as carbon release and increased gradually; peak values of monthly NEE and GPP both appeared in August, but that of ecosystem respiration （R~co） was reached in July. Annual NEE, GPP and Reco of the alpine grassland ecosystem were 52.4, -158.1 and 210.5 g C m-2, respectively. Therefore, the grassland was a moderate source of COs to the atmosphere in this dry year. Interannual variation in NEE was likely related to different water conditions in the growing season. The three greatest contributors to seasonal variation in NEE, GPP and R~co respectively were GLAI〉Ta〉O, GLAI〉O〉PPT, and Ta〉GLAI〉PAR. Seasonality of GLAI explained 60.7% and 76.1% of seasonal variation in NEE and GPP, respectively. GPP or NEE was more sensitive than Reco to variation in GLAI, and ecosystem water conditions.