Photosynthesis is the process by which plants harvest sunlight to produce sugars from carbon dioxide and water. Unused absorbed photosynthetically-active radiation is released by plants primarily as heat with a small fraction being re-emitted as fluorescence in the 660 – 900 nm wavelength range. Thus, the fluorescence signal is related to the photosynthetic process and offers thereby a potential to constraint the gross primary production (GPP) related to photosynthesis which can help to develop a more mechanistic understanding of the ecosystem carbon exchange. Although this fluorescence signal is small (about 1% of the reflected sunlight), the fluorescence signal can be measured from satellites based on in-filling of solar lines, and global fluorescence dataset are now available from multiple satellite missions including GOME-2, GOSAT, OCO-2 and Sentinel-5 Precursor.

The Copernicus Anthropogenic CO2 Monitoring Mission is the space segment of a proposed operational capacity to monitor fossil fuel CO2 emissions which has the overarching objective to provide the European and international community with the appropriate information and capacity for assessing the effectiveness of the Paris Agreement. It should support policymakers to assess the effectiveness of CO₂ emission reduction strategies of man-made CO2 emissions and the overall CO2 budget at country and regional/megacity scales. The space system will measure images of total column CO2 from a multiple (low Earth orbiting) satellite constellation each with a minimum across-track swath width of 200 km providing global coverage every 5-days with a ground pixel size of 4 km2. The satellite will also measure the vegetation fluorescence signal which is needed as a correction for the CO2 retrieval procedure but which will also provide valuable carbon cycle information that can be used in synergy with the CO2 observations.

In this this presentation, we will describe the expected performance of the fluorescence retrieval obtained from the Copernicus Anthropogenic CO2 Monitoring Mission. We will discuss the implication of the improved coverage and resolution obtained from this mission compared to existing missions. We will also discuss different sources of potential biases in the fluorescence retrieval, in particular from instrument calibration uncertainties and we will investigate how how well these biases can be corrected using soundings over non-vegetated areas.

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Remote sensing of Sun-induced fluorescence (SIF) signal emitted from the core of photosynthetic machinery is a direct opportunity to track and diagnose the actual functional status of vegetation. In fact, SIF is also considered as an indicator of plant photosynthetic activity and its physiological status. This makes SIF a highly relevant and purely attractive research interest for scientific communities. The global space agencies like ESA and NASA have considered plant fluorescence observation missions (i.e. FLEX mission by ESA, OCO-2 mission by NASA) as one of their key project to understand terrestrial ecosystems and vegetation closely.
Peatlands are one of the vital ecosystems of the earth which is due to its carbon storage, biodiversity, and distinctive vegetation species. But due to huge heterogeneity, internal diversity, steep environmental gradients, high spatial and spectral variability, it is always a challenge to understand peatland vegetation functionality through remotely sensed vegetation indices (VIs) and SIF signals. Previous studies of SIF were mostly restricting their observation on homogenous ecosystems like croplands, grasslands, and forest areas. Hence, considering this knowledge gap this study will be the first to demonstrate the degree of relationships between VIs and SIF from heterogeneous ecosystems and peatland vegetation communities using novel airborne HyPlant data collected during the SWAMP campaign in Poland.
Imaging spectrometer ‘HyPlant’ (the airborne demonstrator for the ESA FLEX mission) was flown for the first time over extremely heterogeneous peatland and surrounding ecosystems during the SWAMP campaign. The campaign was funded by COST Action OPTIMISE, EUFAR, and ESA under the FLEX-EU mission framework at Rzecin peatland in Poland and was held on 11th July 2015. Rzecin (POLWET) peatland is located in central-western Poland (Wielkopolska region) within Noteć Forest (52°45′ N latitude,16°18′E longitude, 54 m asl). The entire Rzecin peatland and its surroundings are enriched with diverse vegetation groups of vascular and non-vascular plant communities. The target of this study is to explore a comparative analysis of SIF signals and VIs and their relationships obtained from different vegetation groups at ecosystem level (forest, grassland, peatland) and from diverse peatland plants at plant communities level. The VIs and Spectral Fitting Method (SFM) retrieved SIF maps (O2A and O2B) were validated (VIs: R² = 0.79-0.91; SIF: R² = 0.87-0.92) in support of measured ground spectra before any analysis. A clear positive trend with a good agreement (R² = 0.55-0.91) and statistically significant (p<0.01) relations have been observed between SIF bands (O2A and O2B) and greenness indices (SR, NDVI) whereas, a negative trend with a good agreement (R² = 0.40-0.71) and statistically significant (p<0.01) relation have been found between PRI and SIF at large scale vegetation group. Due to huge diversity and low SNR (Signal to Noise Ratio), no such agreements were established between SIF bands and VIs at smaller scale peatland plant communities, except for some isolated homogenous patches of meadows. Filling the knowledge gap, this work provides a new proof that the degree of relationship between SIF bands and several VIs are highly dynamic and complex with hierarchical scales (large scale vegetation groups to small-scale plant communities) for heterogeneous ecosystems.

Keywords: Solar induced fluorescence, Vegetation indices, HyPlant, Peatland, Heterogeneous ecosystems

Acknowledgment
The Research was co-funded by the COST Action ES1309 OPTIMISE, FP7 European Facility for Airborne Research (EUFAR), FLEX-EU ESA (Contract No. 4000107143/12/NL/FF/If CCN3) and European Space Agency which supported the SWAMP training course (6-16th of July 2015) and airborne campaign on 11th of July 2015. Analyses of the data were supported by the Polish National Research Centre within the project No 2016/21/B/ST10/02271 (Sun Induced florescence and photosynthesis of peatland vegetation response to stress caused by water deficits and increased temperature under conditions of climate manipulation experiment).

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Variations in sun-induced chlorophyll fluorescence (F) are closely related to dynamics of non-photochemical quenching (NPQ) - a fast-responding regulatory mechanism that helps keeping the energy balance of the light-absorbing complexes stable - which can be tracked to some extent from remote sensing data using the Photochemical Reflectance Index (PRI). Recent studies have shown that F and PRI are characterised by noticeable directional variations, controlled by leaf and canopy structure (LAI, gap fraction and leaf inclination), pigments and sun-observation geometry that influence the light distribution, scattering and absorption inside the leaf and the canopy.
Although in previous studies F and PRI were used for monitoring the photosynthetic activity, their anisotropic behaviour was not yet fully characterised. Therefore, the aim of this research is to contribute to the understanding of F and PRI directionality based on the field spectroscopy data.
Multi-angular datasets of F and hemispherical–conical reflectance factor (HCRF) were collected in summer 2018 over five different vegetated targets with a hyperspectral device (FloX, JB Hyperspectral Devices UG) coupled with a goniometer, allowing to vary view zenith angles (VZA) and view azimuth angles (VAA). Spectral measurements were acquired with a field of view of 25°. The VZA varied from 0° to 45° in steps of 15° while the VAA varied from 0° to 315° in steps of 45°. These repeatable cycles were sampled continuously during a day from 9 am to 6 pm to characterise F and PRI anisotropy at different times of the day.
We observed that in homogenous fully developed canopies multi-angular F760 and F687 have a distinct dome-like shape in the solar principal plane, with the maximum values occurring in the hotspot direction and minimum values in the forward scatter direction. On the contrary, multi-angular F760 and F687 of row crops show a more complex hemispherical distribution.
To compare directionality of F and HCRF, we computed Pearson Coefficient of Correlation for each plane separately to distinguish effects of varying view geometry on this relationship. Overall, we observed strong positive correlation between F and HCRF at the corresponding wavelength (F760/R750 and F687/R680) in the solar principal plane, which was stable during the whole day.
PRI exhibited strong directional responses for all homogeneous canopies, with the highest values retrieved at VZAs corresponding to the forward scatter direction, and the lowest at the hotspot in solar principal plane, showing that PRI variations can be related to sunlit and shaded canopy fraction. The obtained dataset of multi-angular F and PRI of five canopy types considerably contributes to our understanding of the anisotropic behaviour of the signals due to varying canopy structure.

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Biocrusts are a combination of autotrophic and heterotrophic organisms including mosses, lichens, liverworts, cyanobacteria and algae that colonize arid and semi-arid areas of the Earth. Besides playing a critical role in the stabilization and development of drylands, biocrusts contribute appreciably (~7%) to the global carbon fixation. Nowadays, drylands cover about 45% of the Earth’s land surface, and are expected to expand by up to 23% by the end of this century. For these reasons, understanding biocrusts functioning and quantifying their imprint on the global carbon dynamics is key to establish ecosystem feedbacks to the climate system. In this framework, Remote Sensing (RS) of sun-induced chlorophyll fluorescence (F) might be a promising tool for obtaining new insights into biocrusts functional activity.
In this contribution we aimed at characterizing the F emission of biocrusts samples characterized by different composition of lichens and mosses. In particular, we aimed at: i) quantifying the F emission of biocrusts at both the red and far-red peak; ii) relating the measured F signal to the fractional cover of the different biocrust components in order to understand their relative contribution.
To achieve this, we acquired high-spectral resolution radiance measurements on 37 biocrust samples collected in a dryland area located in central Spain (Aranjuez). The spectral measurements were collected under direct solar irradiance using the FLoX instrument (JB Hyperspectral Devices, Germany). The core of this system is a QEPro (Oceanoptics, USA) spectrometer covering the red and near-infrared regions (650-800 nm), allowing to detect F at both the red and far-red peak. The samples were measured before and after watering in order to determine the effect of biocrusts activation on the measured F signal. F was retrieved using spectral fitting methods at the red and far-red emission peaks. Optical vegetation indices were also calculated.
Results showed that, when active (i.e., after watering), biocrusts emit F with a typical two peaks spectrum. Respect to vascular plants, that are typically characterized by a higher emission in the far-red, they showed a higher signal in the red. This distinctive behavior is related to the different structure and chlorophyll content of biocrusts compared to vascular plants and might be exploited to detect and monitor active biocrusts from remote. Results further showed that moss-dominated biocrusts emit higher F compared to lichen-dominated ones.
Jointly, this analysis conducted at the ground level highlighted the potential of using F to obtain new insights into biocrusts functional state from remote, demonstrating the possibility of passively retrieving the faint F signal emitted by active biocrusts. This provides a great opportunity to improve the understanding of dryland ecosystems functioning from the local to the global scale.

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The mountains of South Siberia are located in the center of the Asian continent and stretches sublatitudinally for three thousand kilometers. Their elevations reach more than 3000 meters. The mountains are the barriers for intercepting moisture-bearing air masses that create favorable conditions for growing of water-loving tree species Siberan pine (Pinus sibirica Du Tour) and Siberian fir (Abies sibirica Ledeb.). These tree species are shade tolerant and named by darkleaf conifers in Russian geobotanic classifications. From the 1970s scientists observed damage and decline of these darkleaf coniferous forests at middle-to-high elevations. The damage was expressed by necroses of branches with generative organs, by the linear and radial increment decrease, by chloroses, necroses and premature needle falls, the occurrence of stem rot.
There exist several hypotheses to explain this phenomenon:
- decline of water-loving darkleaf conifers is associated with climate change (aridization). We found from the temperature and rainfall trends for the last 50 years did not change enough across the highlands to cause die-back of Siberian pine and fir;
- anthropogenic environmental pollution (various gas emissions, acid rainfall). Specific investigations did not detect increased concentrations of technogenic elements in needles, soils and snow over the mountains;
- trees are affected by phytopathogens (Armillaria mellea, Heterobasidion annosum, etc.);
- trees are affected by invasive insect pests.
Foresters and ecologists are skeptical about diseases and insect attacks to be the primary causes and consider them as consequences of already weakened trees;
- recently, the hypothesis emerged that dark coniferous forest die-backs over the southern Siberian mountains may be associated with the ozone layer depletion caused by tropical volcano eruptions. The increased shortwave ultraviolet radiation (UV-B) that follows the depleted concentration of stratospheric ozone in turn may damage the plant photosynthetic and reproductive apparatus. We hypothesise that the increased ozone concentration in the near-surface air layer may additionally amplify the damage and die-back of the middle-to-high mountain forests.

Fig. Daily observations trends of Total Ozone on the four Siberian WOUDC stations

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Quantifying and understanding the relationship between biodiversity and ecosystem functioning has attracted much attention during the last decade. Functional diversity is an important component of biodiversity, which characterizes the variability of those plant traits functionally relevant for growth, reproduction, and survival, within a community. Changes in functional diversity can influence ecosystem dynamics and stability. In particular, evidence is accumulating that functional diversity strongly determines ecosystem productivity, making it highly relevant to monitor its variations at different spatial and temporal scales. In the last few decades, surface reflectance data were used with different degrees of success to monitor changes in functional diversity from remote.
In this contribution, for the first time the potential of sun-induced chlorophyll fluorescence in providing information on functional diversity of terrestrial ecosystems is investigated.
This was achieved exploiting high resolution airborne images acquired with the FLEX airborne demonstrator HyPlant. The HyPlant sensor consists of two modules: one hyperspectral imaging spectrometer covering the visible, near-infrared and shortwave infrared and a second imager specifically designed for fluorescence estimation covering the wavelength range 670 to 780 nm with 1,024 spectral bands with a Full Width at Half Maximum (FWHM) of 0.25 nm and a sampling interval of 0.11 nm.
The sensor was flown over forested areas characterized by different species composition and stand age. Different approaches to evaluate the functional diversity based on reflectance and fluorescence data were then assessed.
The heterogeneity across the study area was measured using two different metrics related to the diversity: the Shannon Entropy index (Shannon's H), which is one of the simplest, and widely used, remote sensing-based diversity measures for landscape heterogeneity assessment which takes into account the relative proportion of each pixels’ value and the Rao’s Q diversity which explicitly accounts for distances among pixels’ numerical values.
First of all, HyPlant raw data were georectified using the CaliGeo software (Specim Ltd, Oulu, Finland) and atmospherically corrected using the ATCOR-4 atmospheric radiative transfer code to obtain top-of-canopy radiance and reflectance. Maps of sun-induced chlorophyll fluorescence at the far-red peak were then successfully obtained (r²=0.73, p<0.001, compared to top-of-canopy ground-based measurements acquired synchronously with the overflight) using the Spectral Fitting Method retrieval approach.
The entropy metrics were then calculated on maps of functional plant traits, fluorescence and traditional vegetation indices (e.g., Normalized Difference Vegetation Index – NDVI) commonly used in ecology. The Shannon’s index was overall characterized by high values all over the images and the diversity patterns were not easily inferable. On the contrary, Rao’s Q diversity performed better in characterizing the spatial heterogeneity of the investigated forests. Notably, the functional diversity patterns obtained calculating Rao’s Q diversity on the fluorescence map better related to the functional diversity map used as a reference compared to those obtained applying the entropy metric on the NDVI maps. In particular, NDVI strongly underestimates the functional diversity across the study area and particularly in the juvenile stands, where the low entropy based on NDVI calculation shows that NDVI cannot grasp the variability of the functional diversity.
Thus, the results obtained in this study show that sun-induced fluorescence can be used as an indicator of functional diversity in terrestrial ecosystems opening new perspectives for studying the link between biodiversity and ecosystem productivity at different spatial and temporal scales.

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