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Title: Diurnal Dynamics of Canopy-Level Solar-Induced Chlorophyll Fluorescence and Spectral Reflectance Indices in a Cornfield throughout the 2008 Growing Season

item Kustas, William - Bill
item Russ, Andrew - Andy

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 12/7/2008
Publication Date: 3/16/2009
Citation: Middleton, E.M., Cheng, Y.B., Corp, L.A., Huemmrich, K.F., Campbell, P., Zhang, Q.Y., Kustas, W.P., Russ, A.L. 2009. Diurnal dynamics of canopy-level solar-induced chlorophyll fluorescence and spectral reflectance indices in a cornfield throughout the 2008 growing season [abstract]. Sixth EARSeL SIG IS Workshop on "Imaging Spectroscopy: Innovative Tool For Scientific and Environmental Applications." 2009 CDROM.

Interpretive Summary:

Technical Abstract: With climate changes rapidly occurring, the terrestrial ecosystem and remote sensing science communities have a common goal: to employ hyperspectral information to assess vegetation productivity and down-regulation due to environmental stresses on a global scale. This is the rationale for the Fluorescence Explorer (FLEX) satellite mission concept under development by the European Space Agency. Research to complement and provide justification for the FLEX mission is being undertaken by several groups. Our Spectral Bio-Indicator Team at NASA/Goddard Space Flight Center is investigating various spectral approaches and spectral reflectance indices for retrieving and monitoring vegetation photosynthetic parameters, including light use efficiency (LUE), using remote sensing technologies. We are particularly interested in understanding how the physiological process that control chlorophyll fluorescence (ChlF), the photosynthetic reflectance index (PRI), and red-edge derivative indices are related, and how each of these changes throughout a day and season. For ChlF, we are investigating in situ passive, solar-induced fluorescence (SIF) using a modified Fraunhofer Line Depth retrieval approach in the two atmospheric oxygen bands (O2b' and O2a) centered on 687 and 760 nm (to sample the red and far-red ChlF emission peaks, respectively), the approach to be utilized by the FLEX mission. We undertook a field campaign in collaboration with USDA scientists at the Agriculture Research Service (ARS) in Beltsville, MD to examine the temporal dynamics of canopy-level SIF, LUE, PRI and other spectral reflectance parameters derived from hyperspectral and flux information collected along transects within an ARS experimental cornfield. These measurements were made in conjunction with continuous air temperature and eddy covariance flux measurements acquired from an instrumented tower. We conducted intensive diurnal measurements weekly, mid-July through early October 2008. On those intensive field days, we collected: i] canopy-level spectral data at multiple (4-11) times per day using two different field spectroradiometers: a FieldSpec Spectroradiometer (ASD Inc., Boulder, CO, USA) and an USB4000 Miniature Fiber Optic Spectrometer (Ocean Optics Inc., Dunedin, Florida, USA); ii] supporting biophysical data, one or more times per day (LAI, PAR transmission, soil reflectivity, soil moisture); iii] leaf-level gas exchange and steady state ChlF data (Fs), obtained with gas analyzers coupled to leaf chambers equipped with fluorimeters (Li-Cor, Lincoln, NE), using two separate systems set at 380 and 1000 ppm to provide ambient and maximum in situ photosynthesis and Fs; and iv] in situ leaf reflectance with the ASD on these same leaves. These tagged leaves were excised following the field measurements to obtain the following set of biophysical measurements in the laboratory: leaf spectral optical properties; photosynthetic pigment content; fresh and dry weight; SLW; C/N ratio; and actively-induced ChlF (with excitation at 530 nm). We show that canopy-level SIF, PRI, and a red-edge index varied throughout each field day, with some consistent daily trends across the season as the cornfield expressed mild drought symptoms, and progressed through phenology (i.e., vegetative and reproductive stages, and senescence). Both the PRI and SIF values tracked the daily and seasonal changes in physiological stress. For example, during the most active growth phase, the daily PRI changed from an early morning value of -0.011 to -0.022 at midday (more negative values indicate increased stress), whereas the early and mid-day values during senescence indicated greater stress, changing from -0.034 and -0.050. On those two days, the early and mid-day red SIF values (expressed as % irradiance) varied between 0.118% and 0.093% (growth phase) and between 0.065% a