Interpreting canopy development and physiology using the EUROPhen camera network at flux sites
Creators: name: Wingate, Lisa
name: Ogée, Jérôme
name: Cremonese, Edoardo
name: Filippa, Gianluca
name: Mizunuma, Toshie
name: Migliavacca, Mirco
name: Moisy, Christophe
name: Wilkinson, Matthew
name: Moureaux, Christine
name: Wohlfahrt, Georg
name: Hammerle, Albin
name: Hörtnagl, Lukas
name: Gimeno, Cristina
name: Porcar-Castell, Albert
name: Galvagno, Marta
name: Nakaji, T.
name: Morison, James
name: Kolle, Olaf
name: Knohl, Alexander
name: Kutsch, Werner L.
name: Kolari, Pasi
name: Nikinmaa, Ero
name: Ibrom, Andreas
name: Gielen, Bert
name: Eugster, Werner
name: Balzarolo, Manuela
name: Papale, Dario
name: Klumpp, Katja
name: Köstner, Barbara
name: Grünwald, Thomas
name: Joffre, Richard
name: Ourcival, Jean-Marc
name: Hellström, Margareta
name: Lindroth, Anders
name: George, Charles
name: Longdoz, Bernard
name: Genty, Bernard
name: Levula, Janne
name: Heinesch, Bernard
name: Sprintsin, Michael
name: Yakir, Dan
name: Manise, T.
name: Guyon, Dominique
name: Ahrends, Hella
name: Plaza-Aguilar, Andrés
name: Guan, J.H.
name: Grace, John
Publisher: name: ETH Zurich
Year: 2015

description: 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.
description: Biogeosciences Discussions, 12 (20)
description: ISSN:1810-6277
description: ISSN:1810-6285