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@ARTICLE{Hornero:889930,
      author       = {Hornero, A. and North, P. R. J. and Zarco-Tejada, P. J. and
                      Rascher, Uwe and Martín, M. P. and Migliavacca, M. and
                      Hernandez-Clemente, R.},
      title        = {{A}ssessing the contribution of understory sun-induced
                      chlorophyll fluorescence through 3-{D} radiative transfer
                      modelling and field data},
      journal      = {Remote sensing of environment},
      volume       = {253},
      issn         = {0034-4257},
      address      = {Amsterdam [u.a.]},
      publisher    = {Elsevier Science},
      reportid     = {FZJ-2021-00536},
      pages        = {112195 -},
      year         = {2021},
      abstract     = {A major international effort has been made to monitor
                      sun-induced chlorophyll fluorescence (SIF) from space as a
                      proxy for the photosynthetic activity of terrestrial
                      vegetation. However, the effect of spatial heterogeneity on
                      the SIF retrievals from canopy radiance derived from images
                      with medium and low spatial resolution remains
                      uncharacterised. In images from forest and agricultural
                      landscapes, the background comprises a mixture of soil and
                      understory and can generate confounding effects that limit
                      the interpretation of the SIF at the canopy level. This
                      paper aims to improve the understanding of SIF from coarse
                      spatial resolutions in heterogeneous canopies by considering
                      the separated contribution of tree crowns, understory and
                      background components, using a modified version of the
                      FluorFLIGHT radiative transfer model (RTM). The new model is
                      compared with others through the RAMI model intercomparison
                      framework and is validated with airborne data. The airborne
                      campaign includes high-resolution data collected over a
                      tree-grass ecosystem with the HyPlant imaging spectrometer
                      within the FLuorescence EXplorer (FLEX) preparatory
                      missions. Field data measurements were collected from plots
                      with a varying fraction of tree and understory vegetation
                      cover. The relationship between airborne SIF calculated from
                      pure tree crowns and aggregated pixels shows the effect of
                      the understory at different resolutions. For a pixel size
                      smaller than the mean crown size, the impact of the
                      background was low (R2 > 0.99; NRMSE < 0.01). By contrast,
                      for a pixel size larger than the crown size, the goodness of
                      fit decreased (R2 < 0.6; NRMSE > 0.2). This study
                      demonstrates that using a 3D RTM model improves the
                      calculation of SIF significantly (R2 = 0.83, RMSE = 0.03 mW
                      m−2 sr−1 nm−1) when the specific contribution of the
                      soil and understory layers are accounted for, in comparison
                      with the SIF calculated from mixed pixels that considers
                      only one layer as background (R2 = 0.4, RMSE = 0.28 mW m−2
                      sr−1 nm−1). These results demonstrate the need to
                      account for the contribution of SIF emitted by the
                      understory in the quantification of SIF within tree crowns
                      and within the canopy from aggregated pixels in
                      heterogeneous forest canopies.},
      cin          = {IBG-2},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IBG-2-20101118},
      pnm          = {2171 - Biological and environmental resources for
                      sustainable use (POF4-217)},
      pid          = {G:(DE-HGF)POF4-2171},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000604324900005},
      doi          = {10.1016/j.rse.2020.112195},
      url          = {https://juser.fz-juelich.de/record/889930},
}