Passive fluorescence measurements during SIFLEX
Abstract
The goal of the participation of the LURE team to the SIFLEX campaign was %to provide clear evidence of quantitative absolute measurements of natural fluorescence andfluorescence flux evaluation by passive methods in the Fraunhofer lines. More specifically, to quantify the fluorescence flux in A and B absorption bands of atmospheric oxygen. The measurements have been focus on: Testing specific passive fluorescence instrumentation recently developed at LURE for assessing the sun induced fluorescence radiance from vegetation. Investigating diurnal and seasonal change of fluorescence and its behaviour with respect to plant condition, pigment content, global radiation amount and its correlation against accumulated biomass during growing period, Cross-correlate the fluorescence signal with hyperspectral reflectance and thermal IR radiance. A special attention has been paid to diurnal cycle and seasonal variations. Comparing fluorescence fluxes with CO2 fluxes measured by the already existing gas exchange equipment of the site. Instrumentation PMFD (Passive Multiwavelength Fluorescence Detector) was the main instrument used to asses fluorescence fluxes. The instrument was based on the Fraunhofer line principle, applied in the atmospheric oxygen A and B bands (760nm and 687nm, respectively). The output parameters were two stationary fluorescence flux at 687nm and 760nm and the NDVI (NDVI = (R760-R687) / (R760+R687)) index. Two additional channels of the instrument are devoted to measure reflectance variations at 531nm and 570 nm in order to calculate the PRI (PRI = (R531-R570) / (R531+R570)) index. This instrument was installed on the main tower, 10 m above the crown of the trees and maintained in the same viewing direction during the campaign (towards the north direction). The zenith angle was set to about 50-65 degrees. The footprint of the instrument contained the crowns of several trees. BD was a second passive detector operating solely in the atmospheric oxygen A band. It was used to complete the measurements of the PMFD instrument. It was installed on a tripod at 2 m above the ground and focused on a small tree in the vicinity of the tower. The instrument was used to explore fluorescence fluxes in correlation with a small lidar (LASERPAM) which measured the stationary fluorescence on the same footprint. In addition to the instrumentation for passive fluorescence measurements two lidar systems (FIPAM &LASERPAM) to better characterise the diurnal cycle of the sun induced fluorescence of the Scott pine forest (see accompanying presentation of Ounis et al. this meeting). A commercial LICOR 6400 IRGA system was also operated for punctual gas exchange measurements. More than 1 Gb of data as been collected which would require several months of work to be correctly evaluated. One original aspect of the campaign was the nature of the target constituted of the crowns of several Scott pines in different planes. The low density of the trees and the distribution of the needles on the volume of the target generated a situation very far for the usual approximation of the "big leaf" of a dense canopy. As a consequence the 760 nm radiation, which is well transmitted and well reflected generated unpredictable variations upon changes of solar radiation. More reliable results seems to be obtained using the 687 nm channel, as a consequence of the strong absorption of this wavelength. A rough examination of the PRI data showed interesting correlation, along the time series with the maximum electron flow of the photosystem II determined by variable fluorescence measurements.
- Publication:
-
Remote Sensing of Solar-Induced Vegetation
- Pub Date:
- 2002
- Bibcode:
- 2002ESASP.527E..20M