How Do Sky Conditions Affect the Relationships Between Ground-Based Solar-Induced Chlorophyll Fluorescence and Gross Primary Productivity Across Different Plant Types?

Solar-induced chlorophyll fluorescence (SIF) has been used as a proxy for gross primary productivity (GPP) estimations. However, knowledge on how links between SIF and GPP across different plant types vary in response to sky conditions remain unclear. Here, we investigated the effects of sky conditions on the GPP-SIF relationship based on continuous measurements of SIF and flux across four different plant types. Our analysis shows that the GPP-SIF links are affected by sky conditions and these linking patterns respond differently across plant types. We propose that the inconsistent responses of SIF and GPP to sky conditions are primarily driven by variations in light use efficiency (LUE = GPP/absorbed photosynthetic active radiation (APAR)). Furthermore, we explore a quantitative variation in LUE and SIFyield (SIF/APAR) separately via a decoupling of clearness index (CI) and photosynthetic active radiation under different sky conditions. LUE is more sensitive to sky conditions for the C-3 plants (Forest, Wheat and Rice) than the C-4 plant (Maize), and SIFyield shows more sensitivity to sky conditions for the forest than croplands. Due to the tight link between CI and other environmental factors, the incorporation of CI into the SIF-based GPP model improves GPP estimates for all C-3 plants at both instantaneous and daily scales. Our study implies that a consideration of sky conditions into the SIF-based GPP model can significantly advance the GPP modeling under all sky conditions. Plain Language Summary An emission of radiation by chlorophyll under the solar light, known as solar-induced chlorophyll fluorescence (SIF), has been widely used to track ecosystem carbon uptake by plants (gross primary productivity, GPP). However, it remains unclear that how responses of links between SIF and GPP to sky conditions vary across plant types. This study explains the responses of GPP-SIF links to various sky conditions across four plant types based on continuous measurements of in situ SIF and flux (the amount of gases exchanged between land surface and atmosphere). We conclude that light use efficiency (LUE, the efficiency of vegetation converting absorbed light into biochemical energy through photosynthesis) dominates the responses of GPP-SIF links to sky conditions for the C-3 plants (such as forest, rice, and wheat). However, LUE for the C-4 plant (Maize) is less sensitive to sky conditions than the C-3 plants, indicating the negligible effects of sky conditions on GPP-SIF links for the C-4 plant. Furthermore, our study also suggests that the integration of sky conditions can improve the accuracy of SIF-based GPP estimation models.