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- 저자 : Verfaillie, Joseph (검색결과 1 건)
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Filippa, Gianluca,Cremonese, Edoardo,Migliavacca, Mirco,Galvagno, Marta,Sonnentag, Oliver,Humphreys, Elyn,Hufkens, Koen,Ryu, Youngryel,Verfaillie, Joseph,Morra di Cella, Umberto,Richardson, Andrew D. Elsevier 2018 Agricultural and forest meteorology Vol.249 No.-
<P><B>Abstract</B></P> <P>Time series of vegetation indices (e.g. normalized difference vegetation index [NDVI]) and color indices (e.g. green chromatic coordinate [<I>G</I> <SUB> <I>CC</I> </SUB>]) based on radiometric measurements are now available at different spatial and temporal scales ranging from weekly satellite observations to sub-hourly <I>in situ</I> measurements by means of near-surface remote sensing (e.g. spectral sensors or digital cameras). <I>In situ</I> measurements are essential for providing validation data for satellite-derived vegetation indices. In this study we used a recently developed method to calculate NDVI from near-infrared (NIR) enabled digital cameras (NDVI<SUB> <I>C</I> </SUB>) at 17 sites (for a total of 74 year-sites) encompassing six plant functional types (PFT) from the PhenoCam network.</P> <P>The seasonality of NDVI<SUB> <I>C</I> </SUB> was comparable to both NDVI measured by ground spectral sensors and by the moderate resolution imaging spectroradiometer (MODIS). We calculated site- and PFT-specific scaling factors to correct NDVI<SUB> <I>C</I> </SUB> values and recommend the use of site-specific NDVI from MODIS in order to scale NDVI<SUB> <I>C</I> </SUB>. We also compared <I>G</I> <SUB> <I>CC</I> </SUB> extracted from red-green-blue images to NDVI<SUB> <I>C</I> </SUB> and found PFT-dependent systematic differences in their seasonalities. During senescence, NDVI<SUB> <I>C</I> </SUB> lags behind <I>G</I> <SUB> <I>CC</I> </SUB> in deciduous broad-leaf forests and grasslands, suggesting that <I>G</I> <SUB> <I>CC</I> </SUB> is more sensitive to changes in leaf color and NDVI<SUB> <I>C</I> </SUB> is more sensitive to changes in leaf area. In evergreen forests, NDVI<SUB> <I>C</I> </SUB> peaks later than <I>G</I> <SUB> <I>CC</I> </SUB> in spring, probably tracking the processes of shoot elongation and new needle formation. Both <I>G</I> <SUB> <I>CC</I> </SUB> and NDVI<SUB> <I>C</I> </SUB> can be used as validation tools for the MODIS Land Cover Dynamics Product (MCD12Q2) for deciduous broad-leaf spring phenology, whereas NDVI<SUB> <I>C</I> </SUB> is more comparable than <I>G</I> <SUB> <I>CC</I> </SUB> with autumn phenology derived from MODIS. For evergreen forests, we found a poor relationship between MCD12Q2 and camera-derived phenology, highlighting the need for more work to better characterize the seasonality of both canopy structure and leaf biochemistry in those ecosystems.</P> <P>Our results demonstrate that NDVI<SUB> <I>C</I> </SUB> is in excellent agreement with NDVI obtained from spectral measurements, and that NDVI<SUB> <I>C</I> </SUB> and <I>G</I> <SUB> <I>CC</I> </SUB> can complement each other in describing ecosystem phenology. Additionally, NDVI<SUB> <I>C</I> </SUB> allows the detection of structural changes in the canopy that cannot be detected by visible-wavelength imagery.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We analyze 74 site-years of green chromatic coordinate (GCC) and camera NDVI data. </LI> <LI> Camera NDVI is comparable to traditional NDVI measurements. </LI> <LI> Camera NDVI and GCC can complement each other in describing ecosystem phenology. </LI> <LI> Both can be used as validation tools for satellite phenology products. </LI> </UL> </P>
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