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The role of sky conditions on gross primary production in a mixed deciduous forest

机译：混合落叶林中天空条件对初级总产值的作用

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摘要

The role of different sky conditions on diffuse PAR fraction (phi), air temperature (T-a), vapor pressure deficit (vpd) and GPP in a deciduous forest is investigated using eddy covariance observations of CO2 fluxes and radiometer and ceilometer observations of sky and PAR conditions on hourly and growing season timescales. Maximum GPP response occurred under moderate to high PAR and 0 and low vpd. Light response models using a rectangular hyperbola showed a positive linear relation between phi and effective quantum efficiency (alpha = 0.023 phi+0.012, r(2) = 0.994). Since PAR and phi are negatively correlated, there is a tradeoff between the greater use efficiency of diffuse light and lower vpd and the associated decrease in total PAR available for photosynthesis. To a lesser extent, light response was also modified by vpd and T-a. The net effect of these and their relation with sky conditions helped enhance light response under sky conditions that produced higher phi. Six sky conditions were classified from cloud frequency and phi data: optically thick clouds, optically thin clouds, mixed sky (partial clouds within hour), high, medium and low optical aerosol. The frequency and light responses of each sky condition for the growing season were used to predict the role of changing sky conditions on annual GPP. The net effect of increasing frequency of thick clouds is to decrease GPP, changing low aerosol conditions has negligible effect. Increases in the other sky conditions all lead to gains in GPP. Sky conditions that enhance intermediate levels of phi, such as thin or scattered clouds or higher aerosol concentrations from volcanic eruptions or anthropogenic emissions, will have a positive outcome on annual GPP, while an increase in cloud cover will have a negative impact. Due to the phi/PAR tradeoff and since GPP response to changes in individual sky conditions differ in sign and magnitude, the net response of ecosystem GPP to future sky conditions is non-linear and tends toward moderation of change
机译：使用CO2通量的涡度协方差观测，辐射计和天空和PAR的云高仪观测，研究了不同天空条件对落叶林中的PAR分布分数（phi），气温（Ta），蒸气压亏缺（vpd）和GPP的作用。每小时和生长季节时标上的条件。最大GPP响应发生在中等到高PAR和0和低vpd下。使用矩形双曲线的光响应模型显示phi与有效量子效率之间呈正线性关系（alpha = 0.023 phi + 0.012，r（2）= 0.994）。由于PAR和phi呈负相关，因此在较高的散射光使用效率和较低的vpd与可用于光合作用的总PAR的相关降低之间需要权衡。在较小程度上，光响应也被vpd和T-a修饰。这些的净效应及其与天空条件的关系有助于在产生较高phi的天空条件下增强光响应。根据云的频率和phi数据分类了六个天空条件：光学上厚的云，光学上薄的云，混合天空（小时内的部分云），高，中和低光学气溶胶。生长季节中每种天空条件的频率和光响应被用来预测不断变化的天空条件对年度GPP的作用。增加厚云的频率的净效应是降低GPP，改变低气溶胶条件的影响可忽略不计。其他天空条件的增加都会导致GPP的增长。增强phi中间水平的天空条件，例如稀薄或分散的云朵或来自火山喷发或人为排放的更高的气溶胶浓度，将对年度GPP产生积极影响，而云层覆盖的增加将产生负面影响。由于phi / PAR的权衡，并且由于GPP对单个天空条件变化的响应在符号和大小上有所不同，因此生态系统GPP对未来天空条件的净响应是非线性的，并且趋于缓和变化

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