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Remote Sens. 2018, 10(7), 1071; https://doi.org/10.3390/rs10071071

In Situ Observations Reveal How Spectral Reflectance Responds to Growing Season Phenology of an Open Evergreen Forest in Alaska

1
Institute of Arctic Climate and Environment Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3173-25, Showa-machi, Kanazawa-ku, Yokohama 236-0001, Kanagawa, Japan
2
Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3173-25, Showa-machi, Yokohama 236-0001, Kanagawa, Japan
3
International Arctic Research Center, University of Alaska, Fairbanks, P.O. Box 757320, Fairbanks, AK 99775-7320, USA
4
Center of Environmental Remote Sensing, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba-shi 263-8522, Chiba, Japan
5
Institute for Agro-Environmental Sciences, National Agriculture and Food Research Organization, 3-1-3, Kannondai, Tsukuba 305-8604, Ibaraki, Japan
6
Nuclear Science and Engineering Center, Japan Atomic Energy Agency, 2-4 Shirakata, Tokai-mura, Naka-gun 319-1195, Ibaraki, Japan
Deceased.
*
Author to whom correspondence should be addressed.
Received: 30 May 2018 / Revised: 29 June 2018 / Accepted: 4 July 2018 / Published: 5 July 2018
(This article belongs to the Special Issue Optical Remote Sensing of Boreal Forests)
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Abstract

Plant phenology timings, such as spring green-up and autumn senescence, are essential state information characterizing biological responses and terrestrial carbon cycles. Current efforts for the in situ reflectance measurements are not enough to obtain the exact interpretation of how seasonal spectral signature responds to phenological stages in boreal evergreen needleleaf forests. This study shows the first in situ continuous measurements of canopy scale (overstory + understory) and understory spectral reflectance and vegetation index in an open boreal forest in interior Alaska. Two visible and near infrared spectroradiometer systems were installed at the top of the observation tower and the forest understory, and spectral reflectance measurements were performed in 10 min intervals from early spring to late autumn. We found that canopy scale normalized difference vegetation index (NDVI) varied with the solar zenith angle. On the other hand, NDVI of understory plants was less sensitive to the solar zenith angle. Due to the influence of the solar geometry, the annual maximum canopy NDVI observed in the morning satellite overpass time (10–11 am) shifted to the spring direction compared with the standardized NDVI by the fixed solar zenith angle range (60−70°). We also found that the in situ NDVI time-series had a month-long high NDVI plateau in autumn, which was completely out of photosynthetically active periods when compared with eddy covariance net ecosystem exchange measurements. The result suggests that the onset of an autumn high NDVI plateau is likely to be the end of the growing season. In this way, our spectral measurements can serve as baseline information for the development and validation of satellite-based phenology algorithms in the northern high latitudes. View Full-Text
Keywords: spectral reflectance; NDVI; in situ; interior Alaska; black spruce; phenology; autumn; net ecosystem exchange spectral reflectance; NDVI; in situ; interior Alaska; black spruce; phenology; autumn; net ecosystem exchange
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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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Kobayashi, H.; Nagai, S.; Kim, Y.; Yang, W.; Ikeda, K.; Ikawa, H.; Nagano, H.; Suzuki, R. In Situ Observations Reveal How Spectral Reflectance Responds to Growing Season Phenology of an Open Evergreen Forest in Alaska. Remote Sens. 2018, 10, 1071.

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