High Temperature and VPD Amplify Tidal Suppression of the SIF–GPP Relationship in a Mangrove Restoration Area in Southeast China

Monitoring gross primary productivity (GPP) of mangroves is crucial for quantifying the carbon sequestration potential of these blue carbon ecosystems (BCEs). Solar-induced fluorescence (SIF) plays an important role in detecting plant photosynthesis. However, research on the relationship between SIF and GPP in mangroves remains limited. We collected high-frequency SIF observations in a restored mangrove ecosystem in Southeast China during 2021–2023 using tower-based remote sensing. We then investigated the SIF–GPP relationship at different temporal scales and the influence of environmental factors on the SIF–GPP relationship using high-frequency SIF observations and high-frequency GPP data. Our results demonstrate that the robust relationship between SIF and GPP is predominantly driven by absorbed photosynthetically active radiation, with the SIF–GPP relationship transitioning from a nonlinear to a linear pattern from the half-hourly to the daily scale. Sensitivity thresholds for air temperature (Ta $=~17.01~^{\circ }$ C) and vapor pressure deficit (VPD = 9.59 hPa) were identified, beyond which their effects on the SIF–GPP relationship shifted from inhibition to promotion. The suppressive effect of tidal inundation on the SIF–GPP relationship in mangroves was significantly amplified under high Ta, high VPD, or their combined conditions. Tides weakened the role of canopy structure in explaining the SIF–GPP relationship by suppressing the relationship between light use efficiency (LUE) and escape factor of SIF photons ( $f_{\mathrm {esc}}$ ). Although the high accuracy of SIF-based GPP estimation models underscores the potential of SIF for tracking GPP, the negative impact of tidal influences on SIF-based GPP estimation cannot be overlooked. These findings highlight the complex interplay between tidal effects and environmental conditions in modulating the SIF–GPP relationship in mangrove ecosystems.