KeenanDarbyFeltsEtAl2014

Référence

Keenan, T.F., Darby, B., Felts, E., Sonnentag, O., Friedl, M.A., Hufkens, K., O'Keefe, J., Klosterman, S., Munger, J.W., Toomey, M. and Richardson, A.D. (2014) Tracking forest phenology and seasonal physiology using digital repeat photography: A critical assessment. Ecological Applications, 24(6):1478-1489. (Scopus )

Résumé

Digital repeat photography is becoming widely used for near-surface remote sensing of vegetation. Canopy greenness, which has been used extensively for phenological applications, can be readily quantified from camera images. Important questions remain, however, as to whether the observed changes in canopy greenness are directly related to changes in leaf-level traits, changes in canopy structure, or some combination thereof. We investigated relationships between canopy greenness and various metrics of canopy structure and function, using five years. 2008-2012. of automated digital imagery, ground observations of phenological transitions, leaf area index (LAI) measurements, and eddy covariance estimates of gross ecosystem photosynthesis from the Harvard Forest, a temperate deciduous forest in the northeastern United States. Additionally, we sampled canopy sunlit leaves on a weekly basis throughout the growing season of 2011. We measured physiological and morphological traits including leaf size, mass (wet/dry), nitrogen content, chlorophyll fluorescence, and spectral reflectance and characterized individual leaf color with flatbed scanner imagery. Our results show that observed spring and autumn phenological transition dates are well captured by information extracted from digital repeat photography. However, spring development of both LAI and the measured physiological and morphological traits are shown to lag behind spring increases in canopy greenness, which rises very quickly to its maximum value before leaves are even half their final size. Based on the hypothesis that changes in canopy greenness represent the aggregate effect of changes in both leaf-level properties (specifically, leaf color) and changes in canopy structure (specifically, LAI), we developed a two end-member mixing model. With just a single free parameter, the model was able to reproduce the observed seasonal trajectory of canopy greenness. This analysis shows that canopy greenness is relatively insensitive to changes in LAI at high LAI levels, which we further demonstrate by assessing the impact of an ice storm on both LAI and canopy greenness. Our study provides new insights into the mechanisms driving seasonal changes in canopy greenness retrieved from digital camera imagery. The nonlinear relationship between canopy greenness and canopy LAI has important implications both for phenological research applications and for assessing responses of vegetation to disturbances. © 2014 by the Ecological Society of America.

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@ARTICLE { KeenanDarbyFeltsEtAl2014,
    AUTHOR = { Keenan, T.F. and Darby, B. and Felts, E. and Sonnentag, O. and Friedl, M.A. and Hufkens, K. and O'Keefe, J. and Klosterman, S. and Munger, J.W. and Toomey, M. and Richardson, A.D. },
    TITLE = { Tracking forest phenology and seasonal physiology using digital repeat photography: A critical assessment },
    JOURNAL = { Ecological Applications },
    YEAR = { 2014 },
    VOLUME = { 24 },
    NUMBER = { 6 },
    PAGES = { 1478-1489 },
    NOTE = { cited By 65 },
    ABSTRACT = { Digital repeat photography is becoming widely used for near-surface remote sensing of vegetation. Canopy greenness, which has been used extensively for phenological applications, can be readily quantified from camera images. Important questions remain, however, as to whether the observed changes in canopy greenness are directly related to changes in leaf-level traits, changes in canopy structure, or some combination thereof. We investigated relationships between canopy greenness and various metrics of canopy structure and function, using five years. 2008-2012. of automated digital imagery, ground observations of phenological transitions, leaf area index (LAI) measurements, and eddy covariance estimates of gross ecosystem photosynthesis from the Harvard Forest, a temperate deciduous forest in the northeastern United States. Additionally, we sampled canopy sunlit leaves on a weekly basis throughout the growing season of 2011. We measured physiological and morphological traits including leaf size, mass (wet/dry), nitrogen content, chlorophyll fluorescence, and spectral reflectance and characterized individual leaf color with flatbed scanner imagery. Our results show that observed spring and autumn phenological transition dates are well captured by information extracted from digital repeat photography. However, spring development of both LAI and the measured physiological and morphological traits are shown to lag behind spring increases in canopy greenness, which rises very quickly to its maximum value before leaves are even half their final size. Based on the hypothesis that changes in canopy greenness represent the aggregate effect of changes in both leaf-level properties (specifically, leaf color) and changes in canopy structure (specifically, LAI), we developed a two end-member mixing model. With just a single free parameter, the model was able to reproduce the observed seasonal trajectory of canopy greenness. This analysis shows that canopy greenness is relatively insensitive to changes in LAI at high LAI levels, which we further demonstrate by assessing the impact of an ice storm on both LAI and canopy greenness. Our study provides new insights into the mechanisms driving seasonal changes in canopy greenness retrieved from digital camera imagery. The nonlinear relationship between canopy greenness and canopy LAI has important implications both for phenological research applications and for assessing responses of vegetation to disturbances. © 2014 by the Ecological Society of America. },
    AFFILIATION = { Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, United States; Department of Biological Sciences, Macquarie University, North Ryde, NSW, Australia; Department of Biology, Boston University, Boston, MA, United States; School of Engineering and Applied Sciences, Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, United States; Département de géographie, Université de Montréal, Montréal, QC, Canada; Department of Earth and Environment, Boston University, Boston, MA, United States; Isotope Bioscience Laboratory, Faculty of Bioscience Engineering, Ghent University, Ghent, East Flanders, Belgium; Harvard Forest, Harvard University, 324 North Main Street, Petersham, MA, United States },
    AUTHOR_KEYWORDS = { Carbon cycling; Deciduous forest phenology; Digital repeat photography; Green chromatic coordinate; Green-down; Ice storm; MODIS; Near-surface remote sensing; PhenoCam },
    DOCUMENT_TYPE = { Article },
    DOI = { 10.1890/13-0652.1 },
    SOURCE = { Scopus },
    URL = { https://www.scopus.com/inward/record.uri?eid=2-s2.0-84901824026&doi=10.1890%2f13-0652.1&partnerID=40&md5=e2dabf4387cb340fb6982b37608cd732 },
}

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