Abstract
Imaging and non-imaging spectroscopy employed in the field and from aircraft is frequently used to assess biochemical, structural, and functional plant traits, as well as their dynamics in an environmental matrix. With the increasing availability of high-resolution spectroradiometers, it has become feasible to measure fine spectral features, such as those needed to estimate sun-induced chlorophyll fluorescence (F), which is a signal related to the photosynthetic process of plants. The measurement of F requires highly accurate and precise radiance measurements in combination with very sophisticated measurement protocols. Additionally, because F has a highly dynamic nature (compared with other vegetation information derived from spectral data) and low signal intensity, several environmental, physiological, and experimental aspects have to be considered during signal acquisition and are key for its reliable interpretation. The European Cooperation in Science and Technology (COST) Action ES1309 OPTIMISE has produced three articles addressing the main challenges in the field of F measurements. In this paper, which is the second of three, we review approaches that are available to measure F from the leaf to the canopy scale using ground-based and airborne platforms. We put specific emphasis on instrumental aspects, measurement setups, protocols, quality checks, and data processing strategies. Furthermore, we review existing techniques that account for atmospheric influences on F retrieval, address spatial scaling effects, and assess quality checks and the metadata and ancillary data required to reliably interpret retrieved F signals.
Highlights
Imaging and non-imaging spectroscopy enables the detailed assessment of biochemical, structural, and even functional attributes of vegetation in natural environments at multiple scales [1,2]
Because F has a highly dynamic nature and low signal intensity, several environmental, physiological, and experimental aspects have to be considered during signal acquisition and are key for its reliable interpretation
Besides the remarkable advancements in technology, more sophisticated measurement setups and protocols have been defined and implemented, while standardization has mainly evolved as a result of standardized systems, such as the FluoWat, FloX, and Piccolo
Summary
Imaging and non-imaging spectroscopy enables the detailed assessment of biochemical, structural, and even functional attributes of vegetation in natural environments at multiple scales [1,2]. Many high spectral resolution spectroradiometers (often used synonymously with spectrometer, the latter is radiometrically calibrated) are available, and they are operated from different measuring platforms: (1) ground-based at the leaf level [5,6,7]; (2) ground-based at the canopy level, either carried by people or mounted on towers and other platforms [8,9,10,11]; (3) airborne-based at the canopy level, flown at low-altitudes on, for example, UAVs [12,13,14,15,16,17,18]; (4) at high altitudes by aircraft [19,20,21]; and (5) spaceborne-based [22,23,24,25] This diversity provides the opportunity to monitor the dynamics of vegetation on different spatial and temporal scales. F can be quenched by photochemistry or NPQ dissipation, the latter being partially under the control of photoprotective mechanisms that help to safely dissipate excess energy [42]
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