interpretation of sun-induced chlorophyll fluorescence for remote sensing of photosynthesis
Peiqi Yang is a PhD student in the research group Water Resources. Her supervisor is prof.dr.ing. W. Verhoef from the faculty of Geo-Information Science and Earth Observation.
Plants are a dynamic part of our planet. They have made the world habitable and formed the climate, through a fundamental process, photosynthesis. Photosynthesis absorbs energy from the sun, removes CO 2 from the atmosphere and records it, and creates O 2 for life on Earth. Tracking the speed of photosynthesis via Earth observation is needed to better understand the interactions of different processes in the environment, and it is essential from the point of view of food safety. It is not feasible to obtain a global and continuous time series of photosynthesis by measuring the exchange of gases alone. Earth observation with satellites offers a possible outcome, but measuring reflection with satellites offers no more than a rough estimate of photosynthesis. The sunlight-induced fluorescence of chlorophyll in plants (SIF) is a promising candidate to fill this gap. Measurements of SIF contain information about both the collection of sunlight, the energy source of photosynthesis, and the efficiency with which the captured light is used for capturing carbon. During the past decades SIF has been used as a direct measure for primary production (GPP), but also as an imposed restriction on models for GPP. The main challenges in the use of SIF for photosynthesis are:
1) obtaining quantitative information on parameters related to photosynthesis from measurements of SIF above the crop via model inversion, and 2) quantifying the mechanisms by which SIF and photosynthesis are linked to each other at the sub-cellular level of photo systems. This dissertation examines the first problem, and aims to deepen the understanding of SIF as measured above the vegetation. The three processes that are responsible for SIF are quantified in this study, namely the absorption of sunlight, the emission of fluorescence, and the (re) absorption of fluorescence. By separately quantifying these three processes, SIF can be corrected for the (re) absorption in the vegetation. The resulting signal is a better measure for photosynthesis than the uncorrected SIF. Further normalization through the light absorption provides information about the efficiency of the emission: a measure for the distribution of the energy in the photo systems over various processes, including photochemistry. The study uses models for radiation transport (RTMs) for the interpretation of simultaneous measurements of SIF and reflection above the vegetation. The approach proved to be well suited for evaluating the effects of the absorption of sunlight and fluorescence. It turned out to be possible to trace the functional response of various agricultural crops to a heat wave from measurements with the HyPlant sensor on an airplane. After analysis of the radiation transport, it was possible to estimate the scattering and reabsorption of the fluorescence from the reflection by means of a simple comparison. This simple comparison makes it possible to calculate the total produced fluorescence in the vegetation from the measured SIF and reflection. A new model for fPAR has emerged from a further analysis of the absorption of sunlight (fPAR). This model for fPAR, combined with the equation for total fluorescence, resulted in a very simple reflection index, FCVI, for both the absorption of sunlight and the scattering of fluorescence. FCVI is the difference between the near infrared and the panchromatic visible reflection. With this index, measurements of SIF with the satellite GOME-2 were subsequently corrected, resulting in global time series of the efficiency of the emission of fluorescence. The final part of this thesis describes a general radiation model for vegetation with a complex composition, modeled in the mSCOPE model. This model simulates the interaction of light and the energy balance in vegetation consisting of several layers. This model will also contribute to the interpretation of SIF measurements. This thesis contains a number of suitable approaches to interpret SIF measurements and to use them well. The measured SIF can be converted into an estimate of the total production of fluorescence in the vegetation and the efficiency of that production. This makes it possible to go beyond the empirical correlation between SIF and photosynthesis that until recently was common. Future studies should show how fluorescence depends on photosynthesis at the level of photo systems.