There is an emerging need to understand the variation in the relationship between solar induced fluorescence (SIF) and canopy photosynthesis as the satellite-based SIF data starts to be widely used to map the regional and global gross primary production (GPP). Without mechanistic understanding of this relationship and its variation diurnally and seasonally at the leaf to canopy scales, the satellite SIF cannot be fully interpreted and validated. We measured chlorophyll fluorescence and SIF at the leaf scale using a traditional pulse amplitude modulation fluorometer and a novel portable SIF system, respectively, and SIF at the canopy scale using a newly developed SIF measurement system at the Harvard Forest. We also measured photosynthesis at the leaf scale and GPP from the eddy covariance data. We retrieved the full-wavelength SIF spectrum and calculated the red and infrared SIF peaks. We upscaled the leaf SIF to canopy SIF using a canopy radiative transfer model and then compared leaf-upscaled canopy SIF, directly measured canopy SIF, and canopy GPP.
Results/Conclusions
Directly measured canopy-SIF correlated significantly with canopy GPP at the seasonal pattern, but the diurnal pattern of canopy-SIF did not match well with the GPP. During the midday, the heat dissipation and stomatal closure may induce the above mismatch. Incorporating the temporal and spatial variation on the relationship between SIF and GPP improved our estimation of GPP via SIF measurement at the ecosystem to regional and global scales. Our work shed light to directly estimate GPP and examine its variation and drivers.