Submitted to: International Society for Optical Engineering
Publication Type: Proceedings
Publication Acceptance Date: 9/19/2005
Publication Date: 11/30/2005
Citation: Middleton, E.M., Corp, L.A., Daughtry, C.S., Campbell, P.K., Butcher, L.M. 2005. Deriving chlorophyll flurescence emissions of vegetation canopies from high resolution field reflectance spectra. In: Proceedings of SPIE, International Society for Optical Engineering, September 19-22, 2005, Bruges, Belgium. doi: 10.1117/12.631159. Interpretive Summary: The productivity of terrestrial ecosystems must be monitored periodically so that the impacts of human and environmental disturbances and climate changes can be reliably assessed. However, timely, direct measurements of many ecosystems are nearly impossible. Numerous reflectance indices have been developed that are related to foliage density and total chlorophyll content. Unfortunately, none of these indices has been consistently related to photosynthesis (CO2 uptake). However, actively induced chlorophyll fluorescence is a well-documented indicator of photosynthetic function at the leaf and plant level and has been used to differentiate N status of foliage in laboratory and field studies. In terrestrial vegetation, chlorophyll fluorescence occurs in the red and far-red spectrum, with peaks at 685 and 735 nm. Chlorophyll fluorescence represents energy that was absorbed by chlorophyll but then discarded (emitted) as fluorescence when it could not be used for C fixation. Excess energy is also discarded as heat. Chlorophyll fluorescence is a direct indicator of plant physiological stress. Chlorophyll fluorescence measured from aircraft and satellite platforms could improve ecosystem productivity estimates. But, use of lasers to actively induce chlorophyll fluorescence over landscapes is not a feasible technology due primarily to eye safety issues. Alternatively, there is evidence that information on chlorophyll fluorescence can be extracted from high spectral resolution reflectance data obtained under ambient solar conditions. However, determination of solar-induced fluorescence for canopies has proven challenging because it is a relatively weak signal compared to reflected radiation. Moreover, few instruments are capable of remotely detecting the SIF signal directly and fewer still that have successfully ascribed the signal to vegetation stress. Corn and 3 species of trees were grown with a wide range of nitrogen fertilizer rates at the USDA-ARS Beltsville Agriculture Research Center in Beltsville, MD. Solar-induced fluorescence intensities were derived directly from high resolution reflectance spectra in narrow-bands associated with atmospheric oxygen absorption features centered at 688 and 760 nm. The red/far-red solar-induce fluorescence ratio successfully discriminated foliar pigment ratios altered by N application rates in corn. This study has relevance to future passive satellite remote sensing approaches for monitoring C dynamics from space.
Technical Abstract: Fluorescence of foliage in the laboratory has proven more rigorous than reflectance for correlation to plant physiology. Especially useful are emissions produced from two stable red and far-red chlorophyll fluorescence (Chlorophyll fluorescence) peaks centered at 685"10 nm and 735"5 nm. Methods have been developed elsewhere to extract steady state solar induced fluorescence (SIF) from apparent reflectance of vegetation canopies/landscapes using the Fraunhofer Line Depth (FLD) principal. Our study utilized these methods in conjunction with field-acquired high spectral resolution canopy reflectance spectra obtained in 2004 and 2005 over corn crops and small tree plots of three deciduous species (red maple, tulip poplar, sweet gum). Leaf level measurements were also made of foliage which included Chlorophyll fluorescence, photosynthesis, and leaf constituents (photosynthetic pigment, carbon (C), and nitrogen (N) contents). As part of ongoing experiments, measurements were made on N application plots within corn (280, 140, 70, and 0 kg N/ha) and tree (0, 37.5, 75, 112.5, 150 kg N /ha) sites at the USDA/Agriculture Research Service in Beltsville, MD. SIF intensities for Chlorophyll fluorescence were derived directly from canopy reflectance spectra in specific narrow-band regions associated with atmospheric oxygen absorption features centered at 688 and 760 nm. The red/far-red SIF ratio (SIFratio) derived from these field reflectance spectra successfully discriminated foliar pigment ratios altered by N application rates in both corn crops. This ratio was also positively correlated to the C/N ratio at leaf and canopy levels, for the available corn data (e.g., 2004). No consistent N treatment or species differences in SIF were detected in the tree foliage, but additional 2005 data are forthcoming. This study has relevance to future passive satellite remote sensing approaches to monitoring C dynamics from space.