Potential of hotspot solar‐induced chlorophyll fluorescence for better tracking terrestrial photosynthesis

Abstract

Remote sensing of solar‐induced fluorescence (SIF) opens a new window for quantifying a key ecological variable, the terrestrial ecosystem gross primary production (GPP), because of the revealed strong SIF–GPP correlation. However, similar to many other remotely sensed metrics, SIF observations suffer from the sun‐sensor geometry effects, which may have important impacts on the SIF–GPP relationship but remain poorly understood. Here we used remotely sensed SIF, globally distributed tower GPP data, and a mechanistic model to provide a systematic analysis. Our results reveal that leaf physiology, canopy structure, and sun‐sensor geometries all affect the SIF–GPP relationship. In particular, we found that SIF observations in the sun‐tracking hotspot direction can be a better proxy of GPP due to the similar responses of light use efficiency and SIF escaping probability in the hotspot direction to the increasing incoming solar radiation. Such conclusions are supported by a variety of modeling simulations and satellite observations over various plant function types, at different time scales and with satellite observational modes. This study demonstrates the potential and advantage of normalizing SIF observations to the hotspot direction for better global GPP estimations. This study also demonstrates the great potentials of current and future spaceborne sun‐tracking satellite missions for a significant improvement in measuring and monitoring, at a wide range of spatial and temporal scales, the changes in terrestrial ecosystem GPP in response to anticipated changes in the Earth's environmental conditions.

Unveiling spatial and temporal heterogeneity of a tropical forest canopy using high-resolution NIRv, FCVI, and NIRvrad from UAS observations

“…In previous studies, the directional effects on NIRv have been examined on coarse spatial scales (i.e. satellites) and have been proposed as an area of need for improving NIRv agreement to GPP or as a potential tool to normalize SIF for these effects to improve SIF agreement to GPP (Hao et al, 2021;Dechant et al, 2020;Baldocchi et al, 2020;Zhang et al, 2020). Our results demonstrate that UAS-based data are suitable for normalizing SIF at high spatial resolution in addition to recording structural heterogeneity of a tropical forest.…”

“…However, recent studies have highlighted the importance and difficulties of measuring fPAR and APAR, the strong dependence of measurements such as SIF on illumination and viewing geometry, SIF escape potential, as well as the need for increased understanding of structure-related radiation regime information more generally e.g. (Hao et al, 2021;Dechant et al, 2020;Baldocchi et al, 2020;Rocha et al, 2021;Zhang et al, 2020). For NIRv, FCVI, and NIRvrad, inclusion of the NIR spectral region makes the emerging indices more sensitive to incoming, absorbed, and scattered radiation, and in the sense of remote sensing vegetation measurements, this includes changing illumination and viewing geometry, changes in canopy leaf angles or associated structure changes.…”

“…the number of fluorescence photons generated as a byproduct of photosynthesis) and structural component (e,g., arrangement of leaves, clumping, and canopy structure, which determine the light capture and SIF photon scattering regime of plants). The aforementioned complexity of measuring SIF and subsequent interpretation limit the number of studies using SIF in comparison to RIs (Hao et al, 2021;Zhang et al, 2020;Liangyun Liu, 2016;Van Wittenberghe et al, 2015;Baldocchi et al, 2020;Wang et al, 2020;Wu et al, 2020;Badgley et al, 2019). An additional barrier in tropical regions is that SIF data in tropical forests is even more scarce than in other biomes and, likely due to data scarcity and combined complexities of the SIF measurements and heterogeneity of tropical forest canopies, relationships to plant function in tropical forests are even more poorly understood than other biomes (Merrick et al, 2019;Wang et al, 2019;Yang et al, 2018a;Liu et al, 2017;Castro et al, 2020;Köhler et al, 2017).…”

Seasonal changes in GPP/SIF ratios and their climatic determinants across the Northern Hemisphere

“…In the boreal/arctic region, however, increased summer radiation benefits carbon assimilation. Finally, we note that the viewing geometry may vary among different satellite products and across different months (Hao et al, 2021;, which may also lead to seasonal variations in GPP/SIF. However, as we discussed before (Chen, Mao, et al, 2021), such view geometry difference is much smaller than seasonal climatic variations (Biriukova et al, 2020).…”

Attributing differences of solar-induced chlorophyll fluorescence (SIF)-gross primary production (GPP) relationships between two C4 crops: corn and miscanthus

“…SIF-GPP relationships depend on a number of factors, including canopy structure (Migliavacca et al, 2017;Dechant et al, 2020), leaf physiology (Celesti et al, 2018;Magney et al, 2019),environmental conditions (Verma et al, 2017;Paul-Limoges et al, 2018;Chen et al, 2021b, a), and sun-view geometry (Hao et al, 2021;Zhang et al, 2021). The role of these factors in affecting SIF-GPP relationships can be well explained when both SIF and GPP are conceptually described by the light use efficiency framework (Monteith, 1972).…”