## 1 Introduction  
Accurate estimation of the CO\({}_{2}\) flux uptake by terrestrial vegetation photosynthesis, that is, gross primary production (GPP), is crucial in quantifying the gas exchanges between the biosphere and atmosphere [1]. Great developments have been achieved in the field of solar-induced chlorophyll fluorescence (SIF) research in recent years. The development of high-resolution sensors deployed on aircraft [2, 3] and satellites [4, 5, 6, 7, 8, 9, 10] especially has opened the door to detecting SIF signals at regional and global scales. SIF has been demonstrated to be an excellent proxy for plant photosynthetic function, and a high correlation between SIF and GPP has been found using remote sensing SIF products, including GOSAT SIF [4; 11; 12], GOME-2 SIF [13; 14; 15; 16], OCO-2 SIF [17; 18], and airborne SIF data [2; 3]. However, the relationship between SIF and GPP is still not clear, and the effects of the temporal and spatial scale, vegetation canopy structure, and climate variability on the physiological link between SIF and GPP remain to be investigated. Therefore, in situ observations need to be used to explore this link. Techniques based on flux measurement systems have been widely used to provide CO\({}_{2}\) and GPP flux data at the ecosystem level [19]. In addition, several ground-based flux observation networks have been established to combine the ground-based instruments and to define the calibration protocols and data processing procedure, which is crucial to the comparison of the flux measurements across different sites. FLUXNET is a network including hundreds of flux sites distributed across the world and is used to measure the exchange of CO\({}_{2}\) and water between the biosphere and atmosphere [19]. The Spectral Network (SpecNet) was initiated with the aim to establish a global network for collecting canopy spectra coordinated with flux measurements and standardizing optical measurements across flux stations [20; 21; 22]. In this context, ESSEM Cooperation in Science and Technology (COST) Action EUROSPEC (ES0903) was initiated in 2009 to coordinate optical measurements on heterogeneous terrestrial ecosystems and with the aim of promoting collaboration between scientists across Europe [23]. Subsequently, ESSEM COST action OPTIMISE (ESI309) was launched based on previous European and international initiatives. Recent advances in optical sensors provide unprecedented opportunities for SIF ground-based spectral observation networks. In particular, SIF remote sensing has developed rapidly in recent decades owing to its important role in monitoring vegetation photosynthesis. However, long-term SIF in situ measurements are still at an early stage. Several ground-based SIF automated measurement systems have been developed along with the growing interest in in situ SIF observations in recent years. The hyperspectral irradiometer (HSI) is the first instrument dedicated to SIF detection; this instrument is equipped with two spectrometers with spectral resolutions of 1 nm and 0.1 nm. FluoSpec, which uses HR2000+ spectrometer with a spectral resolution of 0.13 nm, is one of the first instruments to obtain continuous measurements of SIF over deciduous forests [24]; however, it does not include a temperature control and the signal-to-noise ratio (SNR) is relatively low (about 250). Two similar systems, MRI and SFLUOR box, are based on the use of an optical switch, a fiber optic multiplexer (MPM-2000-2 \(\times\) 8-VIS) [25]. Two spectrometers, HR4000 with a spectral range of 400-1000 nm and a spectral resolution of 1 nm, and SPEC\({}_{\text{Fluo}}\) with a spectral range from 700 to 800 nm and a spectral resolution of 0.1 nm, are embedded in these systems [25]. FluoSpec 2 replaces the HR2000+ spectrometer with a QEpro spectrometer, which has a higher SNR and a better temperature control and data collection flow compared with the FluoSpec instrument [26]. However, the spectral domain of FluoSpec 2 ranging from 730 nm to 780 nm covers only the near-infrared region.