Sun‐induced fluorescence – a new probe of photosynthesis: First maps from the imaging spectrometer HyPlant

Abstract

Variations in photosynthesis still cause substantial uncertainties in predicting photosynthetic CO2 uptake rates and monitoring plant stress. Changes in actual photosynthesis that are not related to greenness of vegetation are difficult to measure by reflectance based optical remote sensing techniques. Several activities are 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 the first 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. Maps of sun-induced fluorescence show a large spatial variability between different vegetation types, which complement classical remote sensing approaches. Different crop types largely differ in emitting fluorescence that additionally changes within the seasonal cycle and thus may be related to the seasonal activation and deactivation of the photosynthetic machinery. We argue that sun-induced fluorescence emission is related to two processes: (i) the total absorbed radiation by photosynthetically active chlorophyll and (ii) the functional status of actual photosynthesis and vegetation stress. Abstract Variations in photosynthesis still cause substantial uncertainties in predicting photosynthetic CO 2 uptake rates and monitoring plant stress. Changes in actual photosynthesis that are not related to greenness of vegetation are difficult to measure by reflectance based optical remote sensing techniques. Several activities are 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 the first 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. Maps of sun-induced fluorescence show a large spatial variability between different vegetation types, which complement classical remote sensing approaches. Different crop types largely differ in emitting fluorescence that additionally changes within the seaso...

Understanding Forest Health with Remote Sensing-Part II—A Review of Approaches and Data Models

“…The regulation of photosynthetic electron transport, as well as its feedback processes and the assessment of photosynthetic efficiency and stress in crops, grassland and forest canopies, have been investigated with sun-induced chlorophyll fluorescence methods at leaf [41], field, and regional levels [42][43][44][45]. Spectroscopic techniques for the detection of chlorophyll fluorescence forms the basis for the European Space Agency (ESA) Fluorescence Explorer Sensors (FLEX, [45,46,70]) to be launched in 2018.…”

“…Spectroscopic techniques for the detection of chlorophyll fluorescence forms the basis for the European Space Agency (ESA) Fluorescence Explorer Sensors (FLEX, [45,46,70]) to be launched in 2018. With its very high spectral resolution across the 0.3-3.0 µm range, FLEX will be the first satellite that is able to directly measure solar-induced chlorophyll fluorescence and thus forest stress indicators and other vegetation parameters.…”

“…This research on chlorophyll fluorescence and its acquisition using spectroscopic techniques forms the basis for developing the European Space Agency (ESA) Fluorescence Explorer Sensors (FLEX, [45,46,70]). …”

Understanding Forest Health with Remote Sensing -Part I—A Review of Spectral Traits, Processes and Remote-Sensing Characteristics

“…For example, plant photosynthesis conditions and their changes are crucial for modelling carbon fluxes and water exchanges in soil-landscape-atmosphere modelling. However, the spatio-temporal variation of photosynthesis activity of the canopy vegetation shows great variations in seasonal, diurnal and spatial patterns but cannot be predicted sufficiently with conventional RS techniques [153,495]. At the same time, forecasts on stress and shifts in the photosynthesis system are currently only indirectly possible through plant traits of greenness and the amount of chlorophyll.…”

“…At the same time, forecasts on stress and shifts in the photosynthesis system are currently only indirectly possible through plant traits of greenness and the amount of chlorophyll. The development of sensors with a high spectral resolution (0.3-3.0 µm, Fluorescence Explorer Satellite, FLEX, ESA, 2018, [152] means that it is now possible for the first time to directly measure functional plant traits such as the solar-induced chlorophyll fluorescence signal as an indicator of photosynthesis conditions and efficiency in order to make better estimates of global photosynthetic activity, CO 2 fluxes and budgets [152][153][154][155]496].…”