Numerous tower- and satellite-based platforms have recently been established to measure solar-induced fluorescence (SIF), which, as a proxy for photosynthesis, shows great promise for constraining global estimates of gross primary productivity. Nonetheless, published SIF retrievals span two orders of magnitude, illustrating an opportunity for improved characterization of the SIF signal in the context of instrument noise, detector calibrations and limitations, and viewing geometry. Companion field measurements of SIF, carbon fixation, and photosynthetic efficiency will advance our understanding of the signal sensitivity to light conditions, leaf age, and plant stress. In 2017, the Forest Optical Reference for Evaluating Sensor Technology (FOREST) site was established at the National Institute of Standards and Technology (NIST) as a test-bed for solar-induced fluorescence (SIF) instrument intercomparison. Radiometric and spectral calibrations have been performed on all instruments, and rigorous characterization of hardware has been investigated to identify potential sources of measurement error. Using an optimized instrument setup, we empirically characterized the physiological and ecological meaning of SIF by directly linking to carbon exchange with an extensive suite of ground measurements. We further measured trends in the SIF signal following branch-level manipulations to induce stomatal closure and decrease photosynthetic rate.
Results/Conclusions
Following optimizations to our spectrometer deployment, we detect changes in SIF which closely track simultaneous pulse amplitude modulated (PAM) fluorescence measurements of the Kautsky effect in dark-acclimated leaves, indicating the successful measurement of a physiologically meaningful signal. Results of branch-level manipulations show clear inhibition of photosynthesis and stomatal conductance, alongside changes in Photosystem II efficiency and quantum yield of photosynthesis. Leaf punches taken pre- and post-treatment show a change in xanthophyll epoxidation state, indicating increased non-photosynthetic quenching at the leaf level. However, neither SIF measured from a tower nor steady state fluorescence measured with a PAM fluorometer tracks these changes, potentially indicating limitations of the chlorophyll fluorescence signal to indicate physiological state on fine spatial and temporal scales. Additionally, deviations from ideal measurement conditions, including low light or intermittent cloud cover, introduce significant noise. It is critical that common standards be developed for SIF measurement systems to ensure validation of data quality and allow for clear linkages to physiological and biophysical parameters. SIF is a promising technique to improve measurement and understanding of local to global trends in primary productivity, but data quality control is a key challenge to tackle with the rapid deployment of new sensors across the globe.