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| Hauptseite > Publikationsdatenbank > Monitoring the spring recovery of photosynthesis in a mixed boreal forest based on spectral reflectance based vegetation indices and sun-induced chlorophyll fluorescence |
| Hauptseite > Publikationsdatenbank > Monitoring the spring recovery of photosynthesis in a mixed boreal forest based on spectral reflectance based vegetation indices and sun-induced chlorophyll fluorescence |
| Conference Presentation (After Call) | FZJ-2023-00148 |
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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.
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