Tower-based automatic observation methods and systems of solar-induced chlorophyll fluorescence in vegetation canopy

Sun-Induced chlorophyll Fluorescence (SIF) is a by-product of plant photosynthesis and is closely related to plant photosynthesis. The study on SIF and its relationship with Gross Primary Productivity (GPP) is of great significance in understanding the mechanism of photosynthesis. Recent instrumental and methodological developments of the tower-based SIF observation system provide a complementary capacity for measuring and interpreting chlorophyll fluorescence in the context of physiological processes. In addition, a tower-based system can also support satellite-based measurements through validation, interpretation, and data inputs provision for models. Recently, the tower-based SIF observation system has developed rapidly with varied observation methods and system characteristics. In this paper, we discuss and summarize the recent developments of tower-based SIF observation methods and propose technical specifications by comparing different tower-based SIF observation systems. Tower-based SIF observation systema can be built with either two spectrometers or one spectrometer combined with an optical path switching trigger. A two-spectrometer SIF system measures the solar incident radiance and the radiance reflected by the canopy independently to realize synchronous measurement. This system can obtain high frequency spectral data, and nearly no time gap exists between the solar incident spectrum and the spectrum reflected by the canopy, reducing the uncertainty of the retrieved SIF caused by the mismatch between the two optical channels under varied weather conditions. However, the spectral response characteristics of the two spectrometers are not completely consistent. The spectral drift between the two optical channels is difficult to correct, which may lead to the increase of the Sif retrieval uncertainty. A single-spectrometer SIF system realizes the sequential switching between the two optical channels by using an optical path switch, which allows the measurement of the solar incident radiance and the canopy reflected radiance with reliable data quality. Although a certain time gap exists between the solar incident spectrum and the reflected spectrum, it can be used for SIF retrieval because of the second disparity. In cloudy and other rapidly changing light conditions, the acquisition time gap between the spectra from the two optical channels may increase the SIF retrieval uncertainty. Compared with the dual spectrometer system, the single spectrometer system is simpler, has lower cost, and avoids the risk of spectral drift, which is the mainstream tower-based SIF system. The tower-based SIF system can be employed with bi-hemispherical and hemispherical-conical observation configurations for field installation. The bi-hemispherical observation mode refers to the configuration in which both down welling and upwelling bare fibers are equipped with cosine correctors, while the hemispherical-conical observation mode refers to the configuration in which only the upwelling bare fiber is equipped with a cosine corrector. The bi-hemispherical observation mode has a larger field of view, which is suitable for canopy measurements with high canopy heterogeneity or height with a limited installation height. The hemispherical-conical observation mode is suitable for low canopy, homogeneous canopy, and multi angle observation. In addition, if the canopy area is limited or the experimental observations have control factors, hemispherical-conical observation is more appropriate. The SIFprism system is a novel optical-prism-based SIF automatic observation system. This article introduces the software and hardware components and the flow of spectral data collection of the SIFprism system. Taking the SIFprism system as an example, the spectral data processing process is expounded, and the potential uncertainty of SIF retrieval is analyzed. The tower-based SIF observation system has experienced rapid development in recent years. Despite the essential and incremental research on near-surface SIF, further development of hardware and mechanistic theory is still urgently required. Several prospective areas for future work include improving the signal-to-noise ratio and radiation stability of the spectrometer and appraising the capabilities and efficacy of different retrieval algorithms in varied light conditions. Finally, research should strengthen the cooperation with industry to jointly develop a more efficient and stable field tower-based SIF system and formulate corresponding field observation technical specifications. © 2021, Science Press. All right reserved.