Canopy level solar induced fluorescence for vegetation in controlled experiments

E.M Middletona,L.A Corp,P.K.E Campbell

doi:10.1109/igarss.2007.4423661

Abstract

Solar induced chlorophyll fluorescence (SIF) was retrieved from high resolution canopy reflectance spectra acquired with an Analytical Spectral Devices (ASD) FieldSpec spectroradiometer one meter above tree sapling canopies. Three deciduous tree species were examined during four measurement periods, spanning the two growing seasons in 2005 and 2006: tulip poplar (Liriodendron tulipifera L.), red maple (Acer rubrum L.), and sweet gum (Liquidambar styraciflua L.). The Fraunhofer Line Depth (FLD) principal was applied to above-canopy spectra for SIF retrievals at the two atmospheric oxygen (O 2 ) absorption features that occur in the chlorophyll fluorescence (ChlF) region (660-780 nm). The broader and deeper telluric feature centered at 760 nm, O 2 alpha, enables retrievals of SIF760 on the shoulder of the far-red ChlF peak. O 2 beta, although a narrower feature, is well positioned for SIF retrieval at 688 nm (SIF688) near the red ChlF peak. Canopy-level SIF688 for tree saplings varied between 1.1 and 2.9 (plusmn 0.3) mW m -2 nm -1 sr 1 while SIF760 varied between 0.5 and 1.3 (plusmn 0.15) mW m -2 nm -1 sr -1 . Typical values for the SIF red/far-red ratio (SIF688 / SIF760) ranged between 1.45 and 6.9, with the highest values observed in autumn. Contemporary canopy and leaf level measurements (e.g., fluorescence, F; reflectance; photosynthesis) were obtained. Leaf-level ChlF obtained from laboratory measurements of actively induced and solar-corrected fluorescence (SCF) emissions at the ChlF peaks (SCF685, SCF740) was determined for leaves of these tree species and three other species (corn, Zea mays L; green pepper, Capsicum annuum L; and soybean, Glycine max L.). For the trees, leaf-level estimates were determined at the SIF wavelengths (SCF688, SCF760) and compared with canopy field retrievals (SIF688, SIF760). The red ChlF values from leaf and canopy (SCF688 vs. SIF688) were similar, but SCF760 was ~4 times higher than SIF760; consequently, considerably (~5X) higher red/far-red ratios (SIF688/760) resulted for tree canopies as compared to individual tree leaves. The 2006 tree leaf and canopy SIF measurements were also compared with leaf-level SIF estimates derived using FluorMOD, a radiative transfer model that simulates F properties of foliage, utilizing our supporting leaf and environmental measurements as model inputs. The preliminary FluorMOD red and far-red SIF estimates were higher than our field measurements and higher than the laboratory estimates (except for far-red F). SIF of tree foliage was also compared with similar estimates made over corn crops under N treatments. These results will assist in determining the expected SIF intensity of vegetation's near- surface signal associated with O 2 alpha and O 2 beta.

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