Comparison of total emitted solar-induced chlorophyll fluorescence (SIF) and top-of-canopy (TOC) SIF in estimating photosynthesis

Many studies have shown that solar-induced fluorescence (SIF) has a good potential to predict gross primary production (GPP) of vegetation. What we measured by remote sensing or near-surface platforms is top-of-canopy (TOC) SIF (SIFtoc), which is not necessarily equal or proportional to total emitted SIF (SIFtot) from the entire canopy due to the (re)absorption and scattering effects. However, photosynthesis, the process that plants use to fix carbon from the atmosphere, occurs at the entire vertical canopy. Here, by using the recollision theory, we calculated SIFtot at 760 nm from the measured SIFtoc, hyperspectral reflectance (R), canopy interception (i) and leaf albedo (omega(l)). Among them, both SIFtoc and R can be obtained from concurrent TOC spectral measurements; i and omega(l) in the near-infrared region can be estimated from the open access datasets with a good accuracy. Our result confirms that the measured SIFtoc only accounts for a small fraction of SIFtot: the above-the-canopy sensor can only see on average 22.9% of SIFtot at Harvard Forest. SIFtot has the following advantages over SIFtoc in estimating GPP: (1) SIFtot improves the diurnal estimate of canopy GPP, especially capable to capture the midday depression of photosynthesis which may cause the large discrepancies between SIFtoc and GPP on a diurnal basis, (2) SIFtot produces a stronger correlation with GPP from plants with complex canopy structure or under sky conditions with more diffuse irradiance, and (3) the SIFtot-GPP relationship shows a stronger resilience to environmental stresses. The fluorescence escape ratio (f(esc)), the ratio between SIFtoc multiplied by pi and SIFtot, is mostly determined by the sun-canopy-sensor geometry and leaf inclination distribution. The effect of LAI and the leaf chlorophyll concentration on f(esc) is marginal at the 760 nm wavelength. Our results suggest that converting SIFtoc into SIFtot provides a better tool to understand and estimate GPP.