Instrumentation Sensitivities For Tower-Based Solar-Induced Fluorescence Measurements

Solar-induced chlorophyll fluorescence (SIF) has been widely cited as a proxy for photosynthesis and is being incorporated as a common input in terrestrial primary productivity models. Though satellite-based SIF products show close relationships with terrestrial gross primary productivity (GPP), there is wide variability in the magnitude of published SIF retrievals made at intermediate scales. In a meta-analysis of the tower-based and airborne SIF literature, we found that mean SIF retrievals from unstressed vegetation spanned a wide range, from 0.041 mW m- 2 nm- 1 sr-1 to 14.8 mW m- 2 nm- 1 sr-1, with a majority of values falling below 4 mW m- 2 nm- 1 sr-1. We compiled information on reported spectrometer calibration procedures, hardware characterizations, and associated corrections from these same papers, and found inconsistent reporting on if and how key calibration methodology was performed. In order to quantify the importance of such methodological differences on final SIF retrievals made at a proximal scale, we performed radiometric calibrations and corrections for electronic dark current, detector noise, atmospheric O2 absorbance, and cosine corrector effects on three field-deployed spectrometers. We found dramatic changes in SIF retrieval magnitude before and after applying calibrations and corrections, as well as significant differences between instrument performance in the field and expected performance based on laboratory characterizations. Based on these tests, and on a Monte Carlo simulation of uncertainty estimates associated with each of these corrections, it is likely that calibration methodologies and hardware characterizations explain some of the observed variability in published SIF retrievals. This wide range in baseline SIF retrieval methodologies and resultant magnitudes severely limit researchers? ability to synthesize and advance the utility of SIF in modeling GPP across scales. Further, variability in calibration and correction methodology may explain the weak SIF-GPP relationship across studies at tower scales.