# FluoC Spec 2--An Automated Field Spectroscopy System to Monitor Canopy Solar-Induced Fluorescence
## 1 Introduction  
Terrestrial vegetation photosynthesis contributes to the largest CO\({}_{2}\) flux on Earth [1]. Remote monitoring of vegetation photosynthesis can provide information on the spatial and temporalvariability of photosynthesis and its biotic and abiotic controls [2; 3]. The widely-used eddy covariance (EC) technique provides a measure of the net ecosystem exchange (NEE) which can be used to estimate the ecosystem level photosynthesis: the Gross Primary Productivity (GPP) [4]. Yet, calculation of GPP based on the EC heavily relies on the assumption of the respiration-temperature relationship, which may overestimate the daytime respiration, and could potentially lead to an overestimation of GPP [5]. Independent and long-term observations of canopy photosynthesis using vegetation spectroscopy are thus needed. An additional benefit of this approach is its ability to link with satellite observations, and thus can be up-scaled to regional and global scales [6; 7]. Recent developments in solar-induced chlorophyll fluorescence (SIF) suggest that it is an indicator of canopy photosynthesis [2; 8; 9; 10; 11; 12; 13; 14]. Long-term and high frequency measurements of SIF that cover a wide variety of terrestrial ecosystems are needed for multiple reasons: (1) high temporal frequency measurements of canopy photosynthesis (every a few minutes) provide important insights into canopy responses to environmental variables and have the potential to reveal photo protection mechanisms [15; 16]; (2) ground observations provide a network of sensors that can be used to validate satellite data [2; 17; 18] and also scale up photosynthesis to regional and global scales [19; 20; 21]; and (3) SIF observations can be used as additional constraints in addition to the EC fluxes to ecosystem models [22; 23]. In recent decades, various types of proximal remote sensing instruments have been developed for monitoring different ecosystem functioning. In one of the first continuous measurements of SIF, [PERSON] et al. [2] developed FluoSpec and made measurements over a temperate mixed forest. [PERSON] et al. [24] developed a similar type of instrument that measures both SIF and canopy reflectance over an agricultural field and a grass field. [PERSON] et al. [25] measured SIF over a grain sorghum field and found that SIF and PRI (Photochemical Reflectance Index) are more sensitive to drought compared with NDVI. [PERSON] et al. [26], one of the earliest groups to measure canopy SIF, showed that plants under water stress have lower SIF. Adopting the Differential Optical Absorption Spectroscopy (DOAS) concepts, PhotoSpec has been developed recently, and it measures both the red and far-red fluorescence [27]. In addition to SIF systems, automated field instruments have been developed to measure multi-angle canopy reflectance [28; 29]. AMSPEC (Automated, Multi-angle SPECTro-radiometer) has been deployed in several ecosystems and has provided insights in ecosystem light use efficiency [18; 30; 31]. Upward-looking cameras have been used to monitor the leaf area index (LAI) at an ecosystem scale in an oak-savanna ecosystem [32]. Light-emitting diode (LED) sensors have been developed as an economic and effective way to monitor canopy reflectance in an annual grassland [33]. These methods utilize the electromagnetic radiation in the visible and near-infrared region (0.4-1.1 \(\upmu\)m). Continuous measurements of full canopy spectra (0.4-2.5 \(\upmu\)m) are still rare, even though they provide additional information on leaf traits, like nitrogen and lignin concentration [34; 35]. Thermal cameras or point infrared thermometers have been used to measure canopy temperature for a few decades, and continuous measurement of canopy temperature with a camera has been demonstrated recently [36]. These studies demonstrated both the importance and the technical difficulties of long-term observations. With growing interest from the scientific community [37], it is important to provide a detailed description of the SIF instrument that we are using at multiple sites.