## 1 Introduction  
GPP is the largest flux in the global carbon cycle [1], yet accurate estimation of GPP at regional and global scales is still a major challenge [2]. Solar-induced chlorophyll fluorescence (SIF) has recently emerged as a process that can be detected using Earth observation technologies, thus having the potential to radically improve terrestrial GPP estimation [3; 4]. SIF is the energy emitted directly from the core of photosynthetic machinery during the return photosystem II from excited to non-excited states, nanoseconds after light absorption, with a wavelength range from 600 to 800 nm [5; 6]. Light energy absorbed by the leaf chlorophyll molecules has three different pathways: photochemistry, non-photochemical quenching (NPQ, i.e., heat dissipation), and a small fraction re-emitted as SIF [6]. SIF is highly correlated with photosynthesis when NPQ dominates at high light levels [6], and it shows stronger capability in general in characterizing the temporal and spatial dynamics of photosynthesis or gross primary productivity in terrestrial ecosystems than traditional vegetation indices (e.g., NDVI and EVI) [7], as it is directly related to the actual photosynthetic rate [8]. Constructing a direct relationship between satellite-derived SIF and eddy covariance (EC) flux tower based GPP is crucial for using SIF to estimate GPP at large scales [2], but this has been hindered by the spatial and temporal coverage of SIF datasets [9]. Current SIF products are derived from Greenhouse Gases Observing Satellite (GOSAT) [10], SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) [11], Global Ozone Monitoring Instrument (GOME) [12] and Global Ozone Monitoring Mission Experiment-2 (GOME-2) [13], Orbiting Carbon Observatory-2 (OCO-2) [14], TanSat [15], and TROPOspheric Monitoring Instrument (TROPOMI) [16]. Among these products, SIF retrieved from OCO-2 showed the smallest footprints (1.30 \(\times\) 2.25 km) and slightly higher signal-to-noise ratios than others, and provided new opportunities to directly link satellite-derived SIF to flux tower GPP at the ecosystem scale [17]. Many studies have reported the relationship between SIF derived from various satellite missions with GPP derived from EC flux tower [18] and gridded Moderate-resolution Imaging Spectroradiometer (MODIS) products [19] at different spatiotemporal scales. The relationship between SIF measurements obtained with remote passive techniques (i.e., remote sensing SIF signal (OCO-2 SIF)) and photosynthesis (i.e., GPP) is not well understood [20] due to large uncertainties when establishing the relationship between SIF and EC flux tower GPP across different ecosystems [21]. [PERSON] and [PERSON] [22] found a linear SIF-GPP relationship that is sensitive to crop type (corn vs. soybean) and invariant across spatiotemporal scales in the Corn Belt. This study only investigated two types of crops in a small part of the United States; therefore, it is not a systematic study of the SIF-GPP relationship, and more studies should be conducted regarding of the differences of C3 and C4 crops [21]. It was found that the strength of this linear relationship in temperate forests was scale-dependent, and its linearity was stronger at the midday time scale [23]. Similar results have been found across several vegetated biomes, especially for OCO-2 SIF at 757 nm [21; 24]. [PERSON] and [PERSON] [21] reported a nearly universal linear SIF-GPP relationship between OCO-2 SIF and EC-GPP from a total of 64 sites across eight major biomes. Recently, [PERSON] and [PERSON] [25] improved the SIF-GPP relationship using the photochemical reflectance index. However, some studies based on GOSAT and GOME-2 analysis indicated that the SIF-GPP relationship varied across biomes [19]. Indeed, [PERSON] and [PERSON] [26] found that the linear SIF-GPP relationship diverges somewhat across 10 biomes at the global scale.