Abstract
Remote sensing of far-red sun-induced chlorophyll fluorescence (SIF) has emerged as an important tool for studying gross primary productivity (GPP) at the global scale. However, the relationship between SIF and GPP at the canopy scale lacks a clear mechanistic explanation. This is largely due to the poorly characterized role of the relative contributions from canopy structure and leaf physiology to the variability of the top-of-canopy, observed SIF signal. In particular, the effect of the canopy structure beyond light absorption is that only a fraction (fesc) of the SIF emitted from all leaves in the canopy can escape from the canopy due to the strong scattering of near-infrared radiation. We combined rice, wheat and corn canopy-level in-situ datasets to study how the physiological and structural components of SIF individually relate to measures of photosynthesis. At seasonal time scales, we found a considerably strong positive correlation (R2 = 0.4–0.6) of fesc to the seasonal dynamics of the photosynthetic light use efficiency (LUEP), while the estimated physiological SIF yield was almost entirely uncorrelated to LUEP both at seasonal and diurnal time scales, with the partial exception of wheat. Consistent with these findings, the canopy structure and radiation component of SIF, defined as the product of APAR and fesc, explained the relationship of observed SIF to GPP and even outperformed GPP estimation based on observed SIF at two of the three sites investigated. These results held for both half-hourly and daily mean values. In contrast, the total emitted SIF, obtained by normalizing observed SIF for fesc, improved only the relationship to APAR but considerably decreased the correlation to GPP for all three crops. Our findings demonstrate the dominant role of canopy structure in the SIF-GPP relationship and establish a strong, mechanistic link between the near-infrared reflectance of vegetation (NIRV) and the relevant canopy structure information contained in the SIF signal. These insights are expected to be useful in improving remote sensing based GPP estimates.
Highlights
Sun-induced chlorophyll fluorescence (SIF) is increasingly used as a proxy for gross primary productivity (GPP) at large scales (Mohammed et al, 2019; Ryu et al, 2019)
We found clear differences in both the seasonal patterns and the degree of diurnal variability of fesc, ΦF, LUEF and light use efficiency of the canopy (LUEP) (Figs. 2, S2). fPAR, fesc and LUEF all appeared to be mainly characterized by considerable seasonal variation (Figs. 2 and S2)
4.1 Hypothesis evaluation In this study, we comprehensively investigated the relationships between SIF and GPP by decomposing observed SIF (SIFobs) into its canopy structure and radiation component, APAR × fesc, and its total emission component, APAR × ΦF, which contains the physiological emission yield term, ΦF (Fig. 1, Table 1)
Summary
Sun-induced chlorophyll fluorescence (SIF) is increasingly used as a proxy for gross primary productivity (GPP) at large scales (Mohammed et al, 2019; Ryu et al, 2019). The precise reason for the SIF-GPP relationship at the canopy scale, lacks a clear mechanistic explanation. This is mostly due to an insufficient understanding of the relative contributions of leaf physiological and canopy structure effects to SIF and how the physiological and structural components of SIF relate to photosynthetic light use efficiency. It is helpful to revisit the basic processes and equations relevant for SIF and GPP as the basis for more closely examining the possible mechanisms that might underlie their strong empirical correspondence
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