Multi-scale insights into the diurnal dynamics of far-red solar-induced chlorophyll fluorescence emission efficiency in an evergreen needle forest

Solar-induced chlorophyll fluorescence (SIF), emitted by photosystems, has become a key remote sensing signal for monitoring photosynthetic activity. SIF is sensitive to both structural and physiological influences, and its physiological component provides direct insight into the photosynthetic status of vegetation. While previous studies have extensively characterized the seasonal, diurnal, and environmental dynamics of SIF, the physiological component embedded in the signal remains less explored. Recent developments, such as the fluorescence correction vegetation index (FCVI), have enabled the extraction of fluorescence emission efficiency (Φ F ), offering a pathway to isolate physiological information from SIF. In this study, we examined the characteristics of Φ F in an evergreen needleleaf forest and its responses to environmental drivers. We first validated the reliability of the FCVI-based approach for retrieving Φ F under the canopy structure of this forest using Soil Canopy Observation of Photosynthesis and Energy fluxes (SCOPE) simulations. We then combined the estimated canopy Φ F with relative Φ F from leaf-scale MONI-PAM measurements to investigate the diurnal dynamics of Φ F and their environmental responses. The results revealed that canopy Φ F exhibited a bimodal diurnal pattern, with peaks in the early morning and late afternoon and a pronounced midday depression. During the growing season, Φ F exhibited clear diurnal hysteresis, with distinct morning and afternoon light-response relationships. Leaf-level measurements demonstrated that accelerated relaxation of non-photochemical quenching (NPQ) after solar noon enhanced both fluorescence emission and photosystem II (PSII) efficiency. We hypothesize that hydraulic limitations are responsible for the relatively high morning values of NPQ. Together, these findings underscore the value of integrating canopy- and leaf-scale observations to disentangle structural and physiological controls on Φ F and improving SIF-based representations of photosynthesis.