## 1 Introduction  
Photosynthesis is one of the most important physiological activities in the existence of plant life; it is the source of all biological energy and is an important promoter of terrestrial ecosystem circulation [1, 2]. Therefore, the research of photosynthesis is essential for understanding the energy flow of terrestrial ecosystems [3]. With the development of techniques for measuring photosynthesis, great progress has been made in photosynthetic measurements at the leaf and even the crop canopy scale [4]. However, measuring the photosynthetic capacity of tall trees and forests at the canopy scale presents technological challenges due to the limitations of canopy uniformity and inhomogeneous landscapes. With the deepening of plant physiology research, it has been discovered that chlorophyll fluorescence (ChlF), a co-product of plant photosynthesis, has potential for reflecting the photosynthetic capacity and photosynthetic status of plants. Light energy absorbed by chlorophyll pigments in photosystem II (PSII) in leaves dissipates by three main pathways: photochemistry (photochemical quenching, PQ), non-radioactively as heat (non-photochemical quenching, NPQ), and re-emitted as fluorescence [5]. It is the theoretical foundation of a unique relationship between ChlF and photochemistry efficiency as long as the NPQ is considered constant during the growing season while the pigment pool only varies slightly. The amount of ChlF changes with the ratio of absorbed light energy available for photosynthesis; hence, it can be used as a proxy of GPP on the canopy scale and beyond. As a form of light, ChlF can be captured by a hyperspectral resolution spectrometer [6; 7; 8]. SIF captured on the plant canopy scale in the natural state through instruments mixed with a reflectance signal can be retrieved based on the Fraunhofer lines discriminator (FLD) method [9; 10] and is used for estimating GPP of terrestrial ecosystems, which provides favorable conditions for studying the spatial-temporal variation characteristics of GPP. An increasing number of studies have begun to explore and use chlorophyll fluorescence as a probe to reveal the physiological state of plants [11]. The emission spectrum of chlorophyll fluorescence is continuous within the wavelength range of 650-800 nm, with two emission peaks of red light and near-infrared light at 688 nm and 740 nm, respectively, of which the peak of red light is higher. However, the emission spectra of ChlF overlaps with the absorption spectra of chlorophyll, part of the red fluorescence (FR) can be reabsorbed by leaves or canopy, varying greatly depending on the canopy structure [5; 12; 13]. The ratio of FR being reabsorbed can reach up to 90% [14; 15; 16]. Near-surface SIF obtained at the top of the canopy depends on the re-absorption and scattering process of the canopy, this action varying by leaf optical properties, direction, leaf area index, and canopy structure [12; 17]. When using canopy SIF to estimate GPP, the influence of the leaf and canopy structure on SIF signal also needs to be considered [5]. The SIF signal at 760 nm is less affected by canopy re-absorption and can be separated from the reflectance signal based on the FLD principle [12; 13], so it is widely used [18]. For certain plant functions, SIF and GPP have an indirect relationship through the absorbed PAR (APAR) [19]; the relationship between SIF and GPP is affected by factors such as canopy structure [17; 20], vegetation biochemical parameters [21], and sampling time [22]. SIF is driven by APAR and can be used to estimate GPP well in ecosystems where vegetation productivity is mainly determined by chlorophyll content [23]. Previous studies have shown that SIF is sensitive to changes in the photochemical processes in the leaves [24], canopy [8; 25; 26], and ecosystem scale [12; 23; 27]. On seasonal or interannual scales, model simulation and field measurement results testify that a clear linear relationship exists between SIF and GPP [12; 23; 28; 29].