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@INPROCEEDINGS{JunkerFrohn:916854,
      author       = {Junker-Frohn, Laura and Näthe, Paul and Siegmann, Bastian
                      and Atherton, Jon and Burkart, Andreas and Julitta, Tommaso
                      and Porcar-Castel, Albert and Rascher, Uwe},
      title        = {{M}onitoring the spring recovery of photosynthesis in a
                      mixed boreal forest based on spectral reflectance based
                      vegetation indices and sun-induced chlorophyll fluorescence},
      reportid     = {FZJ-2023-00148},
      year         = {2022},
      abstract     = {Boreal forests propose a specific challenge for
                      remote-sensing of phenology and photosynthetic capacity, as
                      they are characterized by mixed stands of deciduous and
                      evergreen trees. In spring, onset of photosynthetic activity
                      in deciduous trees is marked by the obvious development of
                      new leaves. Conifers retain their leaves throughout the
                      winter, and recovery of photosynthesis is associated with
                      changes of bulk photosynthetic pigments. Consequently, these
                      processes are reflected differently by canopy-level
                      measurements of spectral reflectance and sun-induced
                      chlorophyll fluorescence. Here we present results of two
                      consecutive years of tower based measurements at the Finnish
                      Hyytiälä Forestry Field Station, for which also
                      measurements of environmental conditions and gross primary
                      production (GPP) and phenological observations by digital
                      imaging were available. We hypothesized, that spectral
                      reflectance based vegetation indices vary in their
                      suitability to represent the spring recovery of
                      photosynthesis of deciduous and evergreen trees, and that
                      sun-induced chlorophyll fluorescence measurements, F760 and
                      F687, are superior due to their universal representation of
                      photosynthesis. Contrasting spring temperatures between
                      years revealed a strong effect of the timing of phenological
                      events on the correlation between vegetation indices with
                      GPP. Under cold spring conditions, when spring recovery of
                      photosynthesis was temperature constrained, a sudden
                      increase in GPP after birch budburst was represented by
                      drastic changes in NDVI, NIRvref, CCI, NIRvrad, and F687.
                      Increased GPP under cold conditions prior to birch budburst
                      exclusively attributable to conifers was only reflected by
                      F687. Under warmer spring conditions, a more gradual
                      recovery of GPP was well represented by NDVI, PRI, CCI,
                      NIRvrad and F687, with an early saturation and thus mostly
                      without distinct changes in response to birch budburst.
                      Consequently, correlations of NIRvref and PRI to GPP varied
                      considerably between years, while NDVI and CCI were unable
                      to represent GPP during cold winter and warm early summer
                      conditions. Surprisingly, F760 only increased in response to
                      adverse conditions in early summer and did not reflect the
                      spring recovery of photosynthesis, likely due to technical
                      limitations of the setup. Altogether, F687 was most suitable
                      to reflect photosynthesis of deciduous as well as evergreen
                      species, which makes it a superior measure for
                      photosynthetic capacity of mixed boreal forests.},
      month         = {Dec},
      date          = {2022-12-12},
      organization  = {Fall meeting of the Amrican
                       Geophysical Union, Chicago (USA), 12
                       Dec 2022 - 16 Dec 2022},
      subtyp        = {After Call},
      cin          = {IBG-2},
      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)6},
      url          = {https://juser.fz-juelich.de/record/916854},
}