Tidal influence on the relationship between solar-induced chlorophyll fluorescence and canopy photosynthesis in a coastal salt marsh

Accurately estimating the dynamics of ecosystem photosynthesis in coastal wetlands is of paramount importance to the quantification of blue carbon and climate change. In this study, we investigated the relationship between the solar-induced chlorophyll fluorescence at 760 nm (SIF₇₆₀) and the gross primary productivity (GPP), as well as the underlying mechanisms in coastal salt marshes that are regularly flooded by tides, based on continuous observations of the SIF₇₆₀ and GPP throughout the growing season of Phragmites australis during 2019. The results show that the SIF₇₆₀ was significantly correlated with the GPP on the half-hourly, daily, and weekly scales in different phenological stages, and the linearity of the SIF₇₆₀–GPP relationship generally improved as the time scale increased. Moreover, we found that the canopy structure (i.e., leaf area index and spatial distribution of leaf angles) plays a major role in explaining the relationship between the SIF₇₆₀ and photosynthesis on half-hourly and daily scales at the study site. Tidal flooding significantly suppressed the strength of the correlation between SIF₇₆₀ and GPP in the early and rapid growth vegetation stage. Furthermore, the tides generally lowered the degree of the correlation between the light use efficiency for photosynthesis (LUE_p) and the SIF yield, and between the LUE_p and the canopy SIF₇₆₀ escape probability. This indicates that tidal inundation diminished the roles of the plant physiology and canopy structure in explaining the relationship between the SIF₇₆₀ and photosynthesis, and thus, it had a negative effect on the SIF₇₆₀-based GPP estimation. The results of this study demonstrate that tidal inundation exerts a significant influence on the relationships between the SIF₇₆₀ and GPP and their responses to the absorbed photosynthetically active radiation, which leads to a better understanding of the mechanism linking the SIF₇₆₀ and photosynthesis in tidal wetlands and provides new insights into reliable blue carbon quantification in a large scale domain.