Solar induced fluorescence (SIF) has been widely used as a powerful tool and proxy to estimate photosynthesis and gross primary production (GPP) from canopy to global scales. However, without mechanistic understanding of the relationship between GPP and SIF and its variation diurnally and seasonally from 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 using a newly developed SIF measurement system at the canopy scale at the Harvard Forest and a coastal salt marsh. 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
We found that canopy structure influenced the relationship between canopy-surface SIF and canopy GPP because SIF within canopy varied with canopy structure. 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.