Advancing Terrestrial Ecosystem Science With a Novel Automated Measurement System for Sun‐Induced Chlorophyll Fluorescence for Integration With Eddy Covariance Flux Networks

Abstract

Sun‐induced chlorophyll fluorescence (SIF) provides critical information on the dynamics of gross primary productivity, a unique role not readily achievable using other methods. Long‐term continuous SIF observations have the potential to advance terrestrial ecosystem science. Realizing this potential, however, requires synergistic implementation of SIF measurements within eddy covariance (EC) flux networks. There is a need for SIF systems that can integrate seamlessly with EC instrumentation to maximize synergistic use of obtained data. Here we introduce the Fluorescence Auto‐Measurement Equipment (FAME) and protocol that fulfill such a purpose. FAME is designed specifically for plug‐and‐play integration with existing EC data acquisition systems. Its innovative hardware and software designs provide versatility, extensibility, autonomous operation, and ease of maintenance for acquiring SIF data of high quality and quantity. A major novel feature of FAME is its synchronized sampling of spectral irradiance and environmental variables, allowing for more precise interpretation of the SIF signal. FAME has been deployed since September 2016 at the Missouri Ozark AmeriFlux site, providing high‐quality measurements even when air temperatures approached 40 °C. Results reveal that canopy SIF saturated or even slightly decreased at high light, similar to leaf‐level photosynthesis. Clear diurnal hysteresis was observed: For the same light, morning SIF was higher than afternoon. Dynamic energy dissipation processes and stress‐induced movements of chloroplasts and leaves may explain the observed pattern. The technology and measurement protocol introduced here advances the coordinated observation of SIF and EC fluxes and represents a step change in observational ecosystem and carbon cycle science research.

Reviews and syntheses: Turning the challenges of partitioning ecosystem evaporation and transpiration into opportunities

“…GPP and T are coupled through stomatal function, and studies of GPP have recently been revolutionized by spaceand ground-based observations of solar-induced fluorescence (SIF) (Frankenberg et al, 2011;Gu et al, 2018;Köhler et al, 2018;Meroni et al, 2009), the process by which some of the incoming radiation that is absorbed by the leaf is reemitted by chlorophyll. SIF emission is related to the light reactions of photosynthesis, but GPP estimation also requires information on the dark reactions and stomatal conductance such that the remote sensing community is currently challenged by how to use SIF to estimate GPP.…”

“…These ratios vary with assumptions regarding the potential evaporation calculation as well (Fisher et al, 2010). SIF can be measured at multiple spatial and temporal scales (Köhler et al, 2018), including the scale of the eddy covariance flux footprint (Gu et al, 2018), and this information can in turn be incorporated into remote-sensing-based approaches for estimating ET using remote sensing platforms (see Appendix A) following additional mechanistic studies of its relationship with T .…”

Comparison of Bi-Hemispherical and Hemispherical-Conical Configurations for In Situ Measurements of Solar-Induced Chlorophyll Fluorescence

“…A shorter IT could help to improve the SNR ( Figure A2) and enable the almost simultaneous measurements of downwelling and upwelling radiation to minimize the temporal mismatch between them as much as possible [35]. This will also be helpful for high-quality data acquisition under fast changing weather conditions and in the precision of SIF retrieval [34].…”

“…Two general protocols can be considered, namely the single field of view (SFOV), whereby one fiber optic is directly connected to the spectrometer and the fore optic, where light enters the system, rotates upward and downward to collect solar and vegetation irradiance (cosine corrector is mandatory) [17,34] and the duel field of view (DFOV) whereby two fore optics are fixed to measure downwelling and upwelling radiation, respectively, and an extra device (e.g., shutter or prism) is needed to switch between these two paths before light reaches the spectrometer [1,34,35]. Both protocols have their own advantages and disadvantages, e.g., the SFOV one can avoid spectral shift but the mobile joint where the fore optic rotates repeatedly may cause a bending or loss of light and noise to the spectrum, and the DFOV one keeps both two optical paths steady and may be easier to automate without a moving part but hard to avoid light loss and spectral shift in the switching part [1,34,35]. A couple of systems with different constructions are derived from the second protocol [1,13,17,18,20,22,36,37], which is also used in this study.…”

“…The upwelling sensor of the hemi-con configuration can be installed at the nadir or off-nadir point at any places of interests but is strongly influenced by the sun-target-viewer geometry, which needs to be considered at either a diurnal or seasonal scale [16,[38][39][40]. The bi-hemi configuration collects downwelling and upwelling irradiance in the nadir view direction, having a great advantage of covering a wide area of the target from its almost hemispheric view [17,34] whereas, in contrast, it may also capture non-vegetation signals due to its large view. Another drawback of the bi-hemi configuration is the attenuation of light by the cosine corrector (may up to 70%) that may increase the integration time (IT) of the spectrometer and reduce the SNR.…”