{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,12]],"date-time":"2025-10-12T04:15:45Z","timestamp":1760242545258,"version":"build-2065373602"},"reference-count":43,"publisher":"MDPI AG","issue":"10","license":[{"start":{"date-parts":[[2017,9,29]],"date-time":"2017-09-29T00:00:00Z","timestamp":1506643200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100002341","name":"Academy of Finland","doi-asserted-by":"publisher","award":["266152","272989","303633"],"award-info":[{"award-number":["266152","272989","303633"]}],"id":[{"id":"10.13039\/501100002341","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>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\u2014the fraction of shaded foliage in the instantaneous field of view of the sensor\u2014due 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\u2013sunlit fraction relationship. We used a high spatial and spectral resolution Aisa Eagle airborne imaging spectrometer above a boreal Scots pine site in Finland (Hyyti\u00e4l\u00e4 forest research station, 61\u00b050\u2032N, 24\u00b017\u2032E), 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 (    \u03b2 = 19 \u00b0    ,     55 \u00b0     and     76    \u00b0, defined as the angle between sensor and solar directions) at the forest stand level, and observed a small (0.006) but statistically significant (p &lt; 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\u2013sunlit 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\u2013sunlit fraction relationships using high spatial resolution imaging spectroscopy data is suitable and needed in order to quantify PRI variations over forest canopies.<\/jats:p>","DOI":"10.3390\/rs9101005","type":"journal-article","created":{"date-parts":[[2017,9,29]],"date-time":"2017-09-29T12:24:04Z","timestamp":1506687844000},"page":"1005","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":18,"title":["Spectral Similarity and PRI Variations for a Boreal Forest Stand Using Multi-angular Airborne Imagery"],"prefix":"10.3390","volume":"9","author":[{"given":"Vincent","family":"Markiet","sequence":"first","affiliation":[{"name":"Land Remote Sensing, VTT Technical Research Centre of Finland Ltd., P.O. Box 1000, Espoo FI-02044, Finland"},{"name":"Department of Geosciences and Geography, University of Helsinki, Helsinki FI-00014, Finland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4434-8346","authenticated-orcid":false,"given":"Roc\u00edo","family":"Hern\u00e1ndez-Clemente","sequence":"additional","affiliation":[{"name":"Global Environmental Modelling and Earth Observation (GEMEO), Department of Geography, Swansea University, Swansea SA2 8PP, United Kingdom"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2745-1966","authenticated-orcid":false,"given":"Matti","family":"M\u00f5ttus","sequence":"additional","affiliation":[{"name":"Land Remote Sensing, VTT Technical Research Centre of Finland Ltd., P.O. Box 1000, Espoo FI-02044, Finland"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2017,9,29]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"9","DOI":"10.1016\/j.foreco.2015.06.014","article-title":"Dynamics of global forest area: Results from the FAO Global Forest Resources Assessment 2015","volume":"352","author":"Keenan","year":"2015","journal-title":"For. Ecol. Manag."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"988","DOI":"10.1126\/science.1201609","article-title":"A large and persistent carbon sink in the world\u2019s forests","volume":"333","author":"Pan","year":"2011","journal-title":"Science"},{"key":"ref_3","doi-asserted-by":"crossref","unstructured":"Gibbs, H.K., Brown, S., Niles, J., and Foley, J. (2007). Monitoring and estimating tropical forest carbon stocks: Making REDD a reality. Environ. Res. 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