Remote sensing of sunlight-induced chlorophyll fluorescence and reflectance of Scots pine in the boreal forest during spring recovery

A measurement campaign to assess the feasibility of remote sensing of sunlight-induced chlorophyll fluorescence (ChlF) from a coniferous canopy was conducted in a boreal forest study site (Finland). A Passive Multi-wavelength Fluorescence Detector (PMFD) sensor, developed in the LURE laboratory, was used to obtain simultaneous measurements of ChlF in the oxygen absorption bands, at 687 and 760 nm, and a reflectance index, the PRI (Physiological Reflectance Index), for a month during spring recovery. When these data were compared with active fluorescence measurements performed on needles they revealed the same trend. During sunny days fluorescence and reflectance signals were found to be strongly influenced by shadows associated with the canopy structure. Moreover, chlorophyll fluorescence variations induced by rapid light changes (due to transient cloud shadows) were found to respond more quickly and with larger amplitude under summer conditions compared to those obtained under cold acclimation conditions. In addition, ChlF at 760 nm was observed to increase with the chlorophyll content. During this campaign, the CO2 assimilation was measured at the forest canopy level and was found remarkably well correlated with the PRI index

TELEDETECTION DE LA FLUORESCENCE DES COUVERTS VEGETAUX INDUITE PAR LASER (APPLICATION DES TECHNIQUES DE CORRELATION TEMPORELLE MICROSECONDE ET NANOSECONDE)

NOUS AVONS REALISE TROIS SYSTEMES FLIDAR (FLUORESCENCE LIGHT DETECTION AND RANGING) POUR LA TELEDETECTION DE LA FLUORESCENCE DE LA VEGETATION AU NIVEAU FOLIAIRE ET A L'ECHELLE DU COUVERT VEGETAL. LE DE-FLIDAR PERMET DE MESURER SIMULTANEMENT LA FLUORESCENCE CHLOROPHYLLIENNE ET LA FLUORESCENCE BLEUE-VERTE, SUR UN COUVERT VEGETAL. IL A LA PARTICULARITE D'UTILISER DEUX LONGUEURS D'ONDE D'EXCITATION, A 355 ET 532 NM, ET TROIS CANAUX DE DETECTION. SA PORTEE EST ESTIMEE A 300 M. NOUS AVONS AUSSI REALISE DEUX MICRO-FLIDARS (LE LASER-PAM ET LE FIPAM) POUR DES MESURES A FAIBLE ET MOYENNE DISTANCE AU NIVEAU FOLIAIRE. ILS MESURENT SIMULTANEMENT LA FLUORESCENCE CHLOROPHYLLIENNE ET L'ECLAIREMENT DES FEUILLES SUR LA MEME SURFACE. LE LASER-PAM EST A L'ORIGINE UN FLUORIMETRE COMMERCIAL (PAM 101, HEINZ WALZ) REALISANT DES MESURES PAR CONTACT, QUE NOUS AVONS MODIFIE POUR EFFECTUER DES MESURES A DISTANCE DE 0.3 A 2 M. LE FIPAM EST UN INSTRUMENT CONCU SPECIFIQUEMENT POUR DES MESURES A DISTANCE, JUSQU'A 6 M. IL A LA PARTICULARITE D'UTILISER LA MEME SOURCE D'EXCITATION POUR LA MESURE ANALYTIQUE ET LA SATURATION DE LA PHOTOCHIMIE. A L'AIDE DE CES INSTRUMENTS, NOUS AVONS MIS EN EVIDENCE TROIS PARAMETRES IMPORTANTS POUR LE SUIVI A DISTANCE DE L'ETAT PHYSIOLOGIQUE DE LA VEGETATION : _ LA FLUORESCENCE STATIONNAIRE DE LA CHLOROPHYLLE (FS) : PAR SIMPLE CORRELATION AVEC L'ECLAIREMENT AU COURS D'UN CYCLE DIURNE, ELLE FOURNIT UNE BONNE INDICATION SUR LA FERMETURE DES STOMATES, ET DONC SUR LE STRESS HYDRIQUE. _ LE DOUBLE RAPPORT DES EMISSIONS DE FLUORESCENCE DE LA CHLOROPHYLLE EXCITEE 355 ET 532 NM (DER) : IL MESURE LA QUANTITE DE CHLOROPHYLLE PAR UNITE DE SURFACE, SUR UNE LARGE GAMME, SANS ETRE AFFECTE PAR LES VARIATIONS DE LUMIERE. _ LE RAPPORT DES EXCITATIONS DE FLUORESCENCE, A 355 ET 532 NM, (FER) : IL MESURE LE CONTENU DES COMPOSES PHENOLIQUES PRESENTS DANS L'EPIDERME FOLIAIRE, ET DONC, FOURNIT UNE INDICATION SUR L'ETAT DE LA VEGETATION : DEFICIENCE MINERALE, EXPOSITION AUX UV, ATTAQUE PARASITAIRE.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Data-based investigation of the effects of canopy structure and shadows on chlorophyll fluorescence in a deciduous oak forest

International audienceAbstract. Data from satellite, aircraft, drone, and ground-based measurements have already shown that canopy-scale sun-induced chlorophyll fluorescence (SIF) is tightly related to photosynthesis, which is linked to vegetation carbon assimilation. However, our ability to effectively use those findings are hindered by confounding factors, including canopy structure, fluctuations in solar radiation, and sun–canopy geometry that highly affect the SIF signal. Thus, disentangling these factors has become paramount in order to use SIF for monitoring vegetation functioning at the canopy scale and beyond. Active chlorophyll fluorescence measurements (FyieldLIF), which directly measures the apparent fluorescence yield, have been widely used to detect physiological variation of the vegetation at the leaf scale. Recently, the measurement of FyieldLIF has become feasible at the canopy scale, opening up new opportunities to decouple structural, biophysical, and physiological components of SIF at the canopy scale. In this study, based on top-of-canopy measurements above a mature deciduous forest, reflectance (R), SIF, SIF normalized by incoming photosynthetically active radiation (SIFy), FyieldLIF, and the ratio between SIFy and FyieldLIF (named Φk) were used to investigate the effects of canopy structure and shadows on the diurnal and seasonal dynamics of SIF. Further, random forest (RF) models were also used to not only predict FyieldLIF and Φk, but also provide an interpretation framework by considering additional variables, including the R in the blue, red, green, red-edge, and near-infrared bands; SIF; SIFy; and solar zenith angle (SZA) and solar azimuth angle (SAA). Results revealed that the SIF signal is highly affected by the canopy structure and sun–canopy geometry effects compared to FyieldLIF. This was evidenced by the weak correlations obtained between SIFy and FyieldLIF at the diurnal timescale. Furthermore, the daily mean SIF‾y captured the seasonal dynamics of daily mean F‾yieldLIF and explained 58 % of its variability. The findings also revealed that reflectance in the near-infrared (R-NIR) and the NIRv (the product of R-NIR and normalized difference vegetation index (NDVI)) are good proxies of Φk at the diurnal timescale, while their correlations with Φk decrease at the seasonal timescale. With FyieldLIF and Φk as outputs and the abovementioned variables as predictors, this study also showed that the RF models can explain between 86 % and 90 % of FyieldLIF, as well as 60 % and 70 % of Φk variations under clear-sky conditions. In addition, the predictor importance estimates for FyieldLIF RF models revealed that R at 410, 665, 740, and 830 nm; SIF; SIFy; SZA; and SAA emerged as the most useful and influential factors for predicting FyieldLIF, while R at 410, 665, 705, and 740 nm; SZA; and SAA are crucial for predicting Φk. This study highlighted the complexity of interpreting diurnal and seasonal dynamics of SIF in forest canopies. These dynamics are highly dependent on the complex interactions between the structure of the canopy, the vegetation biochemical properties, the illumination angles (SZA and SAA), and the light conditions (ratio of diffuse to direct solar radiation). However, such measurements are necessary to better separate the variability in SIF attributable to radiation and measurement conditions from the subtler variability attributable to plant physiological processes

Canopy chlorophyll fluorescence applied to stress detection using an easy-to-build micro-lidar

International audienceLEDFLEX is a micro-lidar dedicated to the measurement of vegetation fluorescence. The light source consists of 4 blue Light-Emitting Diodes (LED) to illuminate part of the canopy in order to average the spatial variability of small crops. The fluorescence emitted in response to a 5-μs width pulse is separated from the ambient light through a synchronized detection. Both the reflectance and the fluorescence of the target are acquired simultaneously in exactly the same field of view, as well as the photosynthetic active radiation and air temperature. The footprint is about 1 m 2 at a distance of 8 m. By increasing the number of LEDs longer ranges can be reached. The micro-lidar has been successfully applied under full sunlight conditions to establish the signature of water stress on pea (Pisum Sativum) canopy. Under well-watered conditions the diurnal cycle presents an M shape with a minimum (Fmin) at noon which is Fmin > Fo. After several days withholding watering, Fs decreases and Fmin < Fo. The same patterns were observed on mint (Menta Spicata) and sweet potatoes (Ipomoea batatas) canopies. Active fluorescence measurements with LEDFLEX produced robust fluorescence yield data as a result of the constancy of the excitation intensity and its geometry fixity. Passive methods based on Sun-Induced chlorophyll Fluores-cence (SIF) that uses high-resolution spectrometers generate only flux data and are dependent on both the 3D structure of vegetation and variable irradiance conditions along the day. Parallel measurements with LEDFLEX should greatly improve the interpretation of SIF changes

Continuous Monitoring of Canopy Level Sun-Induced Chlorophyll Fluorescence During the Growth of a Sorghum Field

International audienceA field platform dedicated to fluorescence measurements (INRA, Avignon, France) was used to monitor the fluorescence emission of a sorghum field during its growing period. The measurements were performed continuously at the canopy level, from seeding to maturity. A passive instrument based on three spectrometers was used to monitor the evolution of fluorescence fluxes and vegetation indexes such as Photochemical Reflectance Index (PRI) and Normalized Difference Vegetation Index (NDVI). Fluorescence fluxes were retrieved from radiances, using the filling-in of the atmospheric oxygen absorption bands, at 687 and 760 nm. In parallel, leaf fluorescence spectra, canopy height, and leaf chlorophyll contents were acquired during the growth. Both PRI and NDVI indexes varied with the development of the sorghum field: we observed that NDVI was more sensitive during the early stage of the growth. However, NDVI saturates before the PRI index. Fluorescence fluxes at 687 nm (Fs687) and 760 nm (Fs760) showed an overall increase: Fs687 increased more rapidly at the beginning of growth but trends to saturate while Fs760 still increase. During the growth, the Fs687/Fs760 ratio at the canopy level is found lower than at leaf level. At canopy level, the ratio decreased when the leaf chlorophyll content increases. A decrease was also observed at leaf level with a lower extend. This more important decrease of the fluorescence ratio at canopy level is attributed to a reabsorption of red fluorescence (Fs687) during its transfer through the canopy layers. In the context of forthcoming large-scale remote sensing application, the modification of the leaf level fluorescence emission by the structure of the canopy observed in this article is one of the major issues that must be addressed to interpret the fluorescence signal

Gross Primary Production of a Wheat Canopy Relates Stronger to Far Red Than to Red Solar-Induced Chlorophyll Fluorescence

"Programme National de Teledetection Spatiale" through "Plateforme de test pour capteurs de fluorescence satellitaires ou avionnes" project CNES through "Terre, Ocean, Surfaces Continentales, Atmosphere" (TOSCA) program (LASVEG) CNES through "Terre, Ocean, Surfaces Continentales, Atmosphere" (TOSCA) program (ACTIPASS)Sun-induced chlorophyll fluorescence (SIF) is a radiation flux emitted by chlorophyll molecules in the red (RSIF) and far red region (FRSIF), and is considered as a potential indicator of the functional state of photosynthesis in remote sensing applications. Recently, ground studies and space observations have demonstrated a strong empirical linear relationship between FRSIF and carbon uptake through photosynthesis (GPP, gross primary production). In this study, we investigated the potential of RSIF and FRSIF to represent the functional status of photosynthesis at canopy level on a wheat crop. RSIF and FRSIF were continuously measured in the O-2-B (SIF687) and O-2-A bands (SIF760) at a high frequency rate from a nadir view at a height of 21 m, simultaneously with carbon uptake using eddy covariance (EC) techniques. The relative fluorescence yield (Fyield) and the photochemical yield were acquired at leaf level using active fluorescence measurements. SIF was normalized with photosynthetically active radiation (PAR) to derive apparent spectral fluorescence yields (ASFY687, ASFY760). At the diurnal scale, we found limited variations of ASFY687 and ASFY760 during sunny days. We also did not find any link between Fyield and light use efficiency (LUE) derived from EC, which would prevent SIF from indicating LUE changes. The coefficient of determination of the linear regression between SIF and GPP is found to be highly variable, depending on the emission wavelength, the time scale of observation, sky conditions, and the phenological stage. Despite its photosystem II (PSII) origin, SIF687 correlates less than SIF760 with GPP in any cases. The strongest SIF-GPP relationship was found for SIF760 during canopy growth. When canopy is in a steady state, SIF687 and SIF760 are almost as effective as PAR in predicting GPP. Our results imply some constraints in the use of simple linear relationships to infer GPP from SIF, as they are expected to be better predictive with far red SIF for canopies with a high dynamic range of green biomass and a low LUE variation range

Running interval training and estimated plasma volume variation.

International audienceAbstract The effect of endurance interval training (IT) on haematocrit (Ht), haemoglobin (Hb) and estimated plasma volume variation (PVV) in response to maximal exercise was studied in 15 male subjects (21.1±1.1 years, control group (CG) n=6and training group (TG), n=9). Training group participated in interval training 3-times a week for 7-weeks. A maximal graded test (GXT) was performed to determine maximal aerobic power (MAP) and maximal aerobic speed (MAS) both before and after the training program. To determine Ht and Hb and lactate concentrations, blood was collected at rest, at the end of GXT and after 10- and 30-min of recovery. Maximal aerobic power and maximal aerobic speed increased significantly (p<0.05) after training only in training group. Haematocrit determined at rest was significantly lower in training group than control group after the training period (p<0.05). Interval training induced a significant increase of estimated plasma volume variation at rest for training group (p<0.05) whereas there were no changes for control group. Hence, significant relationships were observed after training between plasma volume variation determined at the end of the maximal test and maximal aerobic speed (r= 0.60, p<0.05) and maximal aerobic power (r= 0.76, p<0.05) only for training group. In conclusion, seven weeks of interval training lead to a significant increase in plasma volume that possibly contributed to the observed increase of aerobic fitness (maximal aerobic power and maximal aerobic speed)

A Field Platform for Continuous Measurement of Canopy Fluorescence

International audienceThis paper presents a field platform for continuous measurement of fluorescence at the canopy level. It consists of a 21-m-high crane equipped for fluorescence measurements. The crane is installed in the middle of the fields dedicated to agricultural research. Thanks to a jib of 24 m and a railway of 100 m distance, fluorescence measurements can be performed at nadir viewing over various field crops. The platform is dedicated to the development and test of future passive or active airborne and space-borne vegetation sensors. A new fully automatic instrument, called TriFLEX, has been installed at the end of the jib. TriFLEX is designed for passive measurement of fluorescence in the oxygen A and B absorption bands. It is based on three spectrometers and allows for continuous measurements with a repetition rate of about 1 Hz. The data products are the radiances of the target, the fluorescence flux at 687 and 760 nm, and several vegetation indexes, including the photochemical reflectance index and the normalized difference vegetation index. A new algorithm for fluorescence retrieval from spectral bands measurement is described. It improves upon the well-known Fraunhofer line discriminator method applied to passive fluorescence measurement by taking into account the spectral shape of fluorescence and the reflectance of vegetation. A measurement campaign of 38 days has been carried out in summer 2008 over a sorghum field. The evolution of the signals showed that the crop was suffering from stress due to lack of water. After several rainy days, a reversion of the water stress was observed