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Remote Sens. 2017, 9(10), 1005; doi:10.3390/rs9101005

Spectral Similarity and PRI Variations for a Boreal Forest Stand Using Multi-angular Airborne Imagery

1
Land Remote Sensing, VTT Technical Research Centre of Finland Ltd., P.O. Box 1000, Espoo FI-02044, Finland
2
Department of Geosciences and Geography, University of Helsinki, Helsinki FI-00014, Finland
3
Global Environmental Modelling and Earth Observation (GEMEO), Department of Geography, Swansea University, Swansea SA2 8PP, United Kingdom
*
Author to whom correspondence should be addressed.
Received: 1 August 2017 / Revised: 14 September 2017 / Accepted: 22 September 2017 / Published: 29 September 2017
(This article belongs to the Special Issue Recent Progress and Developments in Imaging Spectroscopy)
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Abstract

The photochemical reflectance index (PRI) is a proxy for light use efficiency (LUE), and is used in remote sensing to measure plant stress and photosynthetic downregulation in plant canopies. It is known to depend on local light conditions within a canopy indicating non-photosynthetic quenching of incident radiation. Additionally, when measured from a distance, canopy PRI depends on shadow fraction—the fraction of shaded foliage in the instantaneous field of view of the sensor—due to observation geometry. Our aim is to quantify the extent to which sunlit fraction alone can describe variations in PRI so that it would be possible to correct for its variation and identify other possible factors affecting the PRI–sunlit fraction relationship. We used a high spatial and spectral resolution Aisa Eagle airborne imaging spectrometer above a boreal Scots pine site in Finland (Hyytiälä forest research station, 61°50′N, 24°17′E), with the sensor looking in nadir and tilted (off-nadir) directions. The spectral resolution of the data was 4.6 nm, and the spatial resolution was 0.6 m. We compared the PRI for three different scatter angles ( β = 19 ° , 55 ° and 76 °, defined as the angle between sensor and solar directions) at the forest stand level, and observed a small (0.006) but statistically significant (p < 0.01) difference in stand PRI. We found that stand mean PRI was not a direct function of sunlit fraction. However, for each scatter angle separately, we found a clear non-linear relationship between PRI and sunlit fraction. The relationship was systematic and had a similar shape for all of the scatter angles. As the PRI–sunlit fraction curves for the different scatter angles were shifted with respect to each other, no universal curve could be found causing the observed independence of canopy PRI from the average sunlit fraction of each view direction. We found the shifts of the curves to be related to a leaf structural effect on canopy scattering: the ratio of needle spectral reflectance to transmittance. We demonstrate that modeling PRI–sunlit fraction relationships using high spatial resolution imaging spectroscopy data is suitable and needed in order to quantify PRI variations over forest canopies. View Full-Text
Keywords: multi-angular airborne imaging spectroscopy; photochemical reflectance index; shadow fraction; sunlit fraction; Scots pine; hyperspectral imaging; spectral information divergence multi-angular airborne imaging spectroscopy; photochemical reflectance index; shadow fraction; sunlit fraction; Scots pine; hyperspectral imaging; spectral information divergence
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Markiet, V.; Hernández-Clemente, R.; Mõttus, M. Spectral Similarity and PRI Variations for a Boreal Forest Stand Using Multi-angular Airborne Imagery. Remote Sens. 2017, 9, 1005.

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