[en] Plant phenological development is orchestrated through subtle changes in photoperiod, temperature, soil moisture and nutrient availability. Presently, the exact timing of plant development stages and their response to climate and management practices are crudely represented in land surface models. As visual observations of phenology are laborious, there is a need to supplement long-term observations with automated techniques such as those provided by digital repeat photography at high temporal and spatial resolution. We present the first synthesis from a growing observational network of digital cameras installed on towers across Europe above deciduous and evergreen forests, grasslands and croplands, where vegetation and atmosphere CO2 fluxes are measured continuously. Using colour indices from digital images and using piecewise regression analysis of time series, we explored whether key changes in canopy phenology could be detected automatically across different land use types in the network. The piecewise regression approach could capture the start and end of the growing season, in addition to identifying striking changes in colour signals caused by flowering and management practices such as mowing. Exploring the dates of green-up and senescence of deciduous forests extracted by the piecewise regression approach against dates estimated from visual observations, we found that these phenological events could be detected adequately (RMSE < 8 and 11 days for leaf out and leaf fall, respectively). We also investigated whether the seasonal patterns of red, green and blue colour fractions derived from digital images could be modelled mechanistically using the PROSAIL model parameterised with information of seasonal changes in canopy leaf area and leaf chlorophyll and carotenoid concentrations. From a model sensitivity analysis we found that variations in colour fractions, and in particular the late spring `green hump' observed repeatedly in deciduous broadleaf canopies across the network, are essentially dominated by changes in the respective pigment concentrations. Using the model we were able to explain why this spring maximum in green signal is often observed out of phase with the maximum period of canopy photosynthesis in ecosystems across Europe. Coupling such quasi-continuous digital records of canopy colours with co-located CO2 flux measurements will improve our understanding of how changes in growing season length are likely to shape the capacity of European ecosystems to sequester CO2 in the future.
Disciplines :
Environmental sciences & ecology
Author, co-author :
Wingate, L.; INRA, UMR ISPA 1391, Villenave d'Ornon
Ogée, J.; INRA, UMR ISPA 1391, Villenave d'Ornon
Cremonese, E.; Environmental Protection Agency of Aosta Valley, Italy > Climate Change Unit
Filippa, G.; Environmental Protection Agency of Aosta Valley, Italy > Climate Change Unit
Mizunuma, T.; School of GeoSciences, University of Edinburgh
Migliavacca, M.; Max Planck Institute for Biogeochemistry, Jena, Germany
Moisy, C.; INRA, UMR ISPA 1391, Villenave d'Ornon
Wilkinson, M.; Forest Research, Alice Holt, Farnham
Moureaux, Christine ; Université de Liège > Agronomie, Bio-ingénierie et Chimie (AgroBioChem) > Phytotechnie des régions tempérées
Wohlfahrt, G.; University of Innsbruck, Institute of Ecology and European Academy of Bolzano
Hammerle, A.; University of Innsbruck, Institute of Ecology
Hörtnagl, L.; University of Innsbruck, Institute of Ecology and ETH Zurich, Institute of Agricultural Sciences
Gimeno, C.; Centro de Estudios Ambientales del Mediterráneo, Paterna
Porcar-Castell, A.; University of Helsinki > Department of Forest Sciences
Galvagno, M.; Environmental Protection Agency of Aosta Valley > Climate Change Unit
Nakaji, T.; University of Hokkaido > Regional Resource Management Research
Morison, J.; Forest Research, Alice Holt, Farnham
Kolle, O.; Max Planck Institute for Biogeochemistry, Jena
Knohl, A.; Georg-August University of Göttingen > Faculty of Forest Sciences and Forest Ecology
Kutsch, W.; Johann Heinrich von Thünen-Institut - vTI > Institut für Agrarrelevante Klimaforschung
Kolari, P.; University of Helsinki > Department of Forest Sciences
Nikinmaa, E.; University of Helsinki > Department of Forest Sciences
Ibrom, A.; Risø National Laboratory for Sustainable Energy, Roskilde
Gielen, B.; University of Antwerp > Department of Biology/Centre of Excellence PLECO
Eugster, W.; ETH Zurich, Institute of Agricultural Sciences
Balzarolo, M.; University of Antwerp and University of Tuscia
Papale, D.; University of Tuscia > Department of Forest Environment and Resources
Klumpp, K.; Institut Scientifique de Recherche Agronomique - INRA > Grassland Ecosystem Research Unit
Köstner, B.; Technische Universität Dresden > Chair of Meterorology
Grünwald, T.; Technische Universität Dresden > Chair of Meterorology
Hellstrom, M.; Lund University > Department of Physical Geography and Ecosystem Science
Lindroth, A.; Lund University > Department of Physical Geography and Ecosystem Science
George, C.; Centre for Ecology and Hydrology, Wallingford
Longdoz, Bernard ; Institut Scientifique de Recherche Agronomique - INRA > UMR EEF, Nancy
Genty, B.; CEA, IBEB, SVBME, Laboratoire d'Ecophysiologie Moléculaire des Plantes, Saint-Paul-lez-Durance et CNRS, UMR Biologie Végétale et Microbiologie Environnementales
Levula, J.; University of Helsinki > Department of Forest Sciences
Heinesch, Bernard ; Université de Liège > Ingénierie des biosystèmes (Biose) > Echanges Ecosystèmes - Atmosphère
Sprintsin, M.; Jewish National Fund-Keren Kayemet LeIsrael, Eshtaol > Forest Management and GIS Department
Yakir, D.; Weizmann Institute for Science, Rehovot
Manise, Tanguy ; Université de Liège > Agronomie, Bio-ingénierie et Chimie (AgroBioChem) > Phytotechnie des régions tempérées
Guyon, D.; INRA, UMR ISPA 1391, Villenave d'Ornon
Ahrends, H.; ETH Zurich, Institute of Agricultural Sciences and Institute for Geophysics and Meteorology, University of Cologne
Plaza-Aguilar, A.; University of Cambridge > Plant Sciences
Guan, J. H.; Max Planck Institute for Biogeochemistry, Jena
Grace, J.; University of Edinburgh > School of GeoSciences
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