Measuring and scaling environmentally induced dynamics of solar-induced chlorophyll fluorescence (SIF) from the ground to the aircraft

Abstract

Chlorophyll fluorescence is the absorbed photosynthetically active radiation (PAR) re-emitted as a faint red glow when the photosynthetic apparatus is not able to run photochemistry. This relation that Chlorophyll fluorescence has with photosynthesis makes it an indicator of plant physiological status. After half a century research the emission of solar-induced Chlorophyll fluorescence (SIF) from the canopy can be measured from ground up to satellite scales. Though the first models of ground sensors were created since the 90’s, operational platforms for the remote sensing of SIF were only possible in the last decade. The increasing accessibility of SIF information has aroused the interest of ecology, forestry and agriculture scientists to use it as a reference of plants functioning response to varying environmental factors. Such a challenge demands integrated knowledge from diverse disciplines like biology, plant physiology, agronomy, geography and data analytics and specific sub-disciplines depending on the spatiotemporal scale of the plant process being studied. The knowledge network on each specific area and scale is woven by the interconnection of every single study published on the matter. In this regard, the present thesis aims to contribute with new knowledge about the use of SIF data on ground, aerial and satellite scales where a study addressing the spatial relation of airborne-SIF with the plant available water in the root zone (PAW) is considered as the main contribution of this doctoral work. <br /> Further, applications of Chlorophyll fluorescence data for field phenotyping were addressed in a ground-level study, where lower photosynthetic efficiency at noon was found in bean genotypes cultivated in a free-air CO₂ enriched (FACE) experiment. In the same study, a significant relation between SIF and the Chlorophyll fluorescence measured from active sensors is reported. Moreover, the potential use of unmanned aerial vehicles (UAVs) for the retrieval of SIF was discussed in a ‘state-of-the-arte’ article. Despite its great potential, the UAV-based SIF retrieval is challenged mainly by the complex characterization of the sensor pose and therefore its projected footprint on the ground. A higher progress on the retrieval of SIF has been achieved through airborne-based methods; e.g., with the recent development of the high-Performance airborne Imaging spectrometer (HyPlant), as well as with the improvement of the spectral fitting method (SFM) to retrieve SIF from hyperspectral information. With these novel advances it is possible to monitor SIF in high spatial resolution, enabling e.g. a deeper analysis of the water stress impact on the crop physiology at large scale. In this regard, the spatial relation between airborne-SIF and PAW was observed to be significant in non-irrigated sugar beet fields, whilst it was not detected in cases where there was enough natural or artificial water supply. In winter wheat, a stronger response of SIF to a low PAW zone was observed when compared with temperature- and reflectance-based information. Besides, in order to provide initial understandings about how the relation of SIF with the soil moisture may behave at satellite level, a SIF-soil moisture comparison was done in the context of the gross primary productivity (GPP). It was found that the positive SIF-GPP relation was decoupled in the lower soil moisture areas during a heat wave at European scale. Additionally, a new potential direction for SIF downscaling approaches (based on fractal theory) is introduced. <br /> The contributions of this doctoral work serve to advance our knowledge concerning the use of SIF data for vegetation functioning assessment at multiple scales. This becomes possible when the studies herewith presented are analyzed as a complement with related investigations published nearly in the same time window. Details about the context and methodologies of each study published within the frame of this thesis will be presented in the following pages