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Satellite chlorophyll fluorescence measurements reveal large‐scale decoupling of photosynthesis and greenness dynamics in boreal evergreen forests
Author(s) -
Walther Sophia,
Voigt Maximilian,
Thum Tea,
Gonsamo Alemu,
Zhang Yongguang,
Köhler Philipp,
Jung Martin,
Varlagin Andrej,
Guanter Luis
Publication year - 2016
Publication title -
global change biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.146
H-Index - 255
eISSN - 1365-2486
pISSN - 1354-1013
DOI - 10.1111/gcb.13200
Subject(s) - evergreen , environmental science , primary production , atmospheric sciences , taiga , moderate resolution imaging spectroradiometer , deciduous , chlorophyll fluorescence , boreal , vegetation (pathology) , photosynthetically active radiation , population , evergreen forest , latitude , climatology , photosynthesis , satellite , ecology , ecosystem , geography , botany , biology , geology , medicine , pathology , aerospace engineering , sociology , engineering , geodesy , demography
Mid‐to‐high latitude forests play an important role in the terrestrial carbon cycle, but the representation of photosynthesis in boreal forests by current modelling and observational methods is still challenging. In particular, the applicability of existing satellite‐based proxies of greenness to indicate photosynthetic activity is hindered by small annual changes in green biomass of the often evergreen tree population and by the confounding effects of background materials such as snow. As an alternative, satellite measurements of sun‐induced chlorophyll fluorescence ( SIF ) can be used as a direct proxy of photosynthetic activity. In this study, the start and end of the photosynthetically active season of the main boreal forests are analysed using spaceborne SIF measurements retrieved from the GOME ‐2 instrument and compared to that of green biomass, proxied by vegetation indices including the Enhanced Vegetation Index ( EVI ) derived from MODIS data. We find that photosynthesis and greenness show a similar seasonality in deciduous forests. In high‐latitude evergreen needleleaf forests, however, the length of the photosynthetically active period indicated by SIF is up to 6 weeks longer than the green biomass changing period proxied by EVI , with SIF showing a start‐of‐season of approximately 1 month earlier than EVI . On average, the photosynthetic spring recovery as signalled by SIF occurs as soon as air temperatures exceed the freezing point (2–3 °C) and when the snow on the ground has not yet completely melted. These findings are supported by model data of gross primary production and a number of other studies which evaluated in situ observations of CO 2 fluxes, meteorology and the physiological state of the needles. Our results demonstrate the sensitivity of space‐based SIF measurements to light‐use efficiency of boreal forests and their potential for an unbiased detection of photosynthetic activity even under the challenging conditions interposed by evergreen boreal ecosystems.