Mapping sun-induced fluorescence (SIF) for mechanistic stress responses of vegetation using the high-performance imaging spectrometer HyPlant

Abstract

Variations in photosynthesis that are not related to greenness of vegetation cannot be measured by traditional optical remote sensing techniques and still cause substantial uncertainties in predicting photosynthetic CO2 uptake rates and monitoring plant stress. Several activities were underway to evaluate the sun-induced fluorescence signal on the ground and on a coarse spatial scale using space-borne imaging spectrometers. Intermediate-scale observations using airborne-based imaging spectroscopy, which are critical to bridge the existing gap between small-scale field studies and global observations, are still insufficient. Here we present validated maps of sun-induced fluorescence in that critical, intermediate spatial resolution, employing the novel airborne imaging spectrometer HyPlant. HyPlant has an unprecedented spectral resolution, which allows for the first time quantifying sun-induced fluorescence fluxes in physical units according to the Fraunhofer Line Depth Principle that exploits solar and atmospheric absorption bands. Fluorescence maps show a large spatial variability between different vegetation types, which are not detected with classical remote sensing approaches.It could be shown that different crop types largely differ in emitting fluorescence that is related to the activity of the photosynthetic machinery and allows separating annual and perennial C3 and C4 crops and grasses. Additionally, it could be shown in different case studies that the two peak feature of sun-induced fluorescence emission is related to (i) the total absorbed radiation by photosynthetically active chlorophyll (far-RED peak) and (ii) the functional status of photosynthesis and vegetation stress (RED peak). Thus, the dynamic changes of the two peaks of fluorescence code for structural and functional variability within canopies. Sun-induced fluorescence thus can be used to better understand and to monitor the dynamic adaptations of the photosynthetic machinery of plants to the ever changing environmental conditions. Sun-induced fluorescence thus constitutes a novel and highly relevant remote sensing signal to understand and manage our natural and managed ecosystems in times of global change and to facilitate a sustainable use of plants and plant resources