Light fluctuations have been shown to affect photosynthesis at the scale of individual leaves, but effects at a canopy scale are not documented. This study was conducted to determine the relationships of light fluctuations in an alfalfa canopy with wind and turbulence, and their effects on canopy photosynthesis. The photosynthetically active radiation (PAR) was measured at three levels within the canopy, while fluxes of CO2 (Fc), sensible heat (H), and latent heat (LE) were measured above the canopy with eddy covariance. Soil CO2 efflux was measured with a chamber system.; Numbers of sunflecks and their contribution to total received PAR increased with wind speed. Short (2 seconds) sunflecks represented the largest number, cumulative time, and contribution to total received PAR.; The proportion of U-shaped PAR frequency histograms in the upper canopy increased with wind, reflecting light oscillations between low and high values. The dominant peaks of PAR power spectra were 1 to 2 Hz. Relative contributions of high frequencies to total variance increased with wind speed.; The turbulence fluxes above the canopy were underestimated, with daytime-averaged energy balance closure of 0.66–0.79. The lowest closure values were associated with advection of sensible heat from the surroundings and large LE under strong turbulence conditions. Forcing closure increased LE and H significantly. When Fc was increased by forcing closure in the same proportion as LE, the resulting photosynthesis-light response curves were more realistic than for the uncorrected values, suggesting that forcing closure resulted in more reasonable flux values.; Statistical tests suggested that under windy conditions, the Cupid model predictions of canopy photosynthesis for steady-sate light were larger than measured values, suggesting that light fluctuations decrease canopy photosynthesis. However, the mean differences between the measurements and model values were within the uncertainty of the measurements. Therefore, the effects of light fluctuations on alfalfa photosynthesis in a canopy scale appear to be insignificant under these conditions.
展开▼
机译：研究表明，光的波动会影响单个叶片的光合作用，但没有记录到冠层的影响。进行这项研究来确定苜蓿冠层的光波动与风和湍流之间的关系，以及它们对冠层光合作用的影响。在冠层内的三个水平上测量了光合有效辐射（PAR），而CO 2 （F c ），显热（H）和潜热（ LE）在冠层上方测量，并具有涡动协方差。用箱式系统测量土壤CO 2 的流出。随着风速的增加，日晒的数量及其对总PAR的贡献增加。短（<2秒）的日晒代表最多的次数，累积的时间以及对总已接收PAR的贡献。 U形PAR频率直方图在上冠层中的比例随风的增加而增加，反映出低值和高值之间的光振荡。 PAR功率谱的主峰为1至2 Hz。高频对总方差的相对贡献随风速增加。冠层上方的湍流通量被低估，日平均能量平衡关闭为0.66-0.79。最低的闭合值与周围环境中显热的平流以及强湍流条件下的大LE的平流有关。强迫闭合显着增加LE和H。当通过以与LE相同的比例强迫封闭增加F c 时，所得到的光合作用-光响应曲线比未校正值更真实，这表明强迫封闭导致更合理的通量值。统计测试表明，在大风条件下，稳定状态光的冠层光合作用的丘比特模型预测值大于实测值，表明光的波动会降低冠层的光合作用。但是，测量值与模型值之间的平均差异在测量值的不确定性之内。因此，在这些条件下，光波动对冠层苜蓿光合作用的影响似乎微不足道。
展开▼
