Carbon exchanges between the surface and the atmosphere mediated by plant photosynthesis are a key component of the terrestrial carbon balance, as photosynthesis is the mechanism for atmospheric carbon uptake by terrestrial vegetation and assimilation at the surface as accumulated biomass. Vegetation photosynthesis is highly variable according to environmental factors, particularly when plants are exposed to variable stress conditions, and such variability is enhanced under climate change and human pressure. A better quantitative estimation of the actual carbon assimilation by plants is needed to improve the accuracy of current terrestrial carbon models and to improve the predictability of such models towards future scenarios.
While Earth Observation (EO) techniques have been used extensively to model and quantify terrestrial vegetation dynamics, by determining the structure and functioning of plants, a direct estimate of actual photosynthetic activity by vegetation is only possible by quantifying not only the actual absorbed light by plants but also how the absorbed light is internally used by the plants. In fact, only a fraction of such absorbed light is used for photosynthesis, but such fraction is highly variable in space and time as a function of environmental conditions and regulation factors. Measuring chlorophyll fluorescence together with the total absorbed light, in addition to other key plant variables, provides a unique opportunity to quantify vegetation photosynthesis and its spatial and temporal dynamics by satellite observations.
The Fluorescence Explorer (FLEX) mission was selected in 2015, by the European Space Agency (ESA), as the 8th Earth Explorer within the Living Planet Programme, with the key scientific objective of a quantitative global mapping of actual photosynthetic activity of terrestrial ecosystems, with a spatial resolution adequate to resolve land surface processes associated to vegetation dynamics. FLEX will also provide quantitative physiological indicators to account for vegetation health status and environmental stress conditions, to better constraint global terrestrial carbon models.
Spatial coverage is driven by the need to globally observe all terrestrial vegetation, while optimal observation time around 10:00 is driven by the diurnal cycle of photosynthetic processes. The spatial resolution of 300 m is driven by the need to resolve land processes at appropriate scales relevant for the identification and tracking of stress effects on terrestrial vegetation, covering at least several annual cycles. The targeted uncertainty for photosynthesis derived from instantaneous measurements ranges from about 5% in unregulated conditions up to 30% in case of highly variable regulated heat dissipation, in line with model requirements and improving current capabilities provided by other techniques.
Given the four main pathways for light absorbed by plants (photochemistry, constitutive heat dissipation, chemically regulated heat dissipation and fluorescence) FLEX measurements include not only fluorescence emission, but also vegetation temperature and estimates of regulated heat dissipation, which is highly variable and drives the relation between fluorescence and photosynthesis. In addition, FLEX has also to acquire all the necessary information to determine vegetation conditions to properly interpret the photosynthesis variability, and all the necessary information needed for appropriate cloud screening, compensation for atmospheric effects and proper analysis of the measured signals.
For the retrieval of vegetation fluorescence, very high spectral resolution (better than 0.3 nm) is needed, with also very high signal-to-noise ratio given the weakness of the fluorescence signal as compared to the background reflected radiance. To be able to accomplish such objective, the FLEX mission carries the FLORIS spectrometer, with a spectral sampling in the order of 0.1 nm, specially optimized to derive, spectrally resolved, the overall vegetation fluorescence spectral emission in the full range 650-780 nm, and also measuring the spectral variability in surface reflectance in the range 500-650 nm indicative of chemical adaptations in regulated heat dissipation.
FLEX is designed to fly in tandem with Copernicus Sentinel-3. Together with FLORIS, the OLCI and SLSTR instruments on Sentinel-3 provide all the necessary information to retrieve the emitted fluorescence, and to allow proper derivation of the spatial and temporal dynamics of vegetation photosynthesis from such global measurements. OLCI and SLSTR data also help in the compensation for atmospheric effects and the derivation of the additional biophysical information needed to interpret the variability in fluorescence measurements.
The science products to be provided by the FLEX mission are not restricted to the basic chlorophyll fluorescence measurements, but include also the estimates of regulated and non-regulated heat dissipation, needed to quantify actual photosynthesis. Together with canopy temperature and other variables characterizing vegetation status, such as chlorophyll content and fraction of light absorbed by photosynthetic pigments, Level-2 products include instantaneous photosynthesis rates and estimates of vegetation stress based on ratios between actual versus potential photosynthesis and variable PSI/PSII contributions tracking photosynthesis dynamics. Level 3 products are derived by means of spatial mosaics and temporal composites, giving also as a temporal product the activation / deactivation of photosynthesis, growing season length and related vegetation phenology indicators. Finally, by means of data assimilation into advanced dynamical models of FLEX time series and ancillary information, Level-4 products are also provided, including time series of Gross Primary Productivity (GPP) and more advanced dynamical vegetation stress indicators.
Such science products can be directly used by vegetation dynamical models, climate models and different applications. In particular, with the increase population and food demand, usage of agriculture will need optimization of crop photosynthesis in such variable conditions, and FLEX is expected to contribute not only to the carbon science but also in associated applications. Efforts are put in place to guarantee proper cal/val activities and dedicated validation network for FLEX products. Particular efforts are in place to provide each product with realistic and properly estimated uncertainties, and also to propagate the derived uncertainties from the original satellite data until the final high-level products, accounting in each step for the covariance matrices associated to each set of intermediate variables, and using a combination of Montecarlo and analytical error propagation tools along the whole FLEX data processing chain.
The availability of validated ready-to-use high level science products will allow an extensive scientific usage of FLEX data, with also a high potential for derived applications. The open availability of FLEX products, and the accessibility through open exploitation platforms where users can themselves validate the products or even make changes in the algorithms to optimize the products towards their specific needs, is intended to offer a large versatility in FLEX exploitation approaches. FLEX is also expected to be used in conjunction with many other sources of EO data, including high spatial resolution time series from Sentinel-2. The FLEX Level-2 products are already provided in the same geographical grid as Sentinel-2 products to facilitate such exploitation developments. Usage of common global multi-resolution spatial grids for high-level products, and compatibility of data formats, are also taken into account to maximize the inter-operability of FLEX products with other EO products in global data assimilation approaches and multiple applications.