## 1 Introduction
A commonly-adopted approach integrating ecosystem productivity and RS is based on the theory of light use efficiency (LUE) [15] which states that a relatively constant relationship exists between productivity (expressed as Gross Primary Production (GPP) or Net Ecosystem Exchange (NEE)) and absorbed radiation at the canopy level:  
\[\mathrm{C~{}uptake}=\varepsilon\times(\mathrm{fAPAR}\times\mathrm{PAR})= \varepsilon\times\mathrm{APAR} \tag{1}\]  
where \(\varepsilon\) is the photosynthetic light use efficiency expressing the carbon sequestration efficiency per amount of the absorbed solar energy (\(\mathrm{\mu mol~{}CO_{2}\cdot\mathrm{\mu mol^{-1}}}\) APAR); PAR is the incident photosynthetically active radiation (\(\mathrm{\mu mol\cdot m^{-2}.s^{-1}}\)); fAPAR is the fraction of PAR absorbed by the vegetation canopy (-), and APAR is the amount of PAR absorbed by the aboveground green biomass (\(\mathrm{\mu mol\cdot m^{-2}.s^{-1}}\)). Productivity in herbaceous ecosystems such as grasslands and crops is strongly related to the seasonal variation of aboveground green biomass and is mostly driven by APAR, while \(\varepsilon\) is often assumed as a constant in the models [16; 17]. The fAPAR term was demonstrated to be directly linked to canopy chlorophyll content, and in the Monteith model can be estimated using spectral vegetation indices (SVIs) [18; 19], which are mathematical combinations of vegetation surface reflectance at two or more bands [20]. Productivity is usually retrieved using SVIs, including in their formulation a NIR band characterized by a high reflectance due to leaf and canopy scattering, and a VIS band where absorption by the chlorophyll pigments is predominant (e.g., the Normalized Difference Vegetation Index (NDVI), and other Chlorophyll-related indices [20]). The adopted models include an incident PAR component [16; 21; 22], although some other studies [17; 23] demonstrated the suitability of SVIs to estimate productivity without PAR information. The European Space Agency (ESA) satellite Sentinel-2 (S2) recent mission has opened unprecedented opportunities for BPRs monitoring. The S2 twin satellites are equipped with a Multispectral Instrument (MSI) measuring reflected radiance in 13 spectral bands ranging from VIS and NIR to shortwave infrared (SWIR) with a high spatial resolution. The red-edge domain, which is on the borderline between chlorophyll absorption (in the red wavelengths) and leaf/canopy scattering in the NIR wavelengths [24], is well covered by S2 (B5, B6). S2 allows the calculation of both \"traditional\" VIS-NIR SVIs (which are greenness-related, as chlorophyll absorption takes place mostly in the VIS wavelengths up to 700 nm [25]) and NIR indices based on NIR bands \(\geq\)740 nm. Such band combinations can provide important information related to canopy light scattering and canopy structure, and therefore canopy structural controls on photosynthesis [26; 27]. However, can NIR-based canopy structure observations be an indicator of productivity? To explain this, we cite [PERSON] [25], who highlighted the paradox that \"the physiological activity of vegetation is often more strongly related to reflectance at wavelengths that are not used in photosynthesis than to those that are\". The nature of the \"paradoxical\" relationship between the NIR spectral response and ecosystem physiology is related to the complex scattering dynamics of leaf, stem, and canopy-level structural traits, which appear to co-vary with traits related with physiology. Such dynamics have only been partially explored, and more studies are needed to investigate the links between ecosystem physiology, canopy structure, and NIR spectral responses [25; 28]. Assuming that leaf biochemistry and canopy structure variability are linked to different strategies to cope with different environmental resource limitations [25], spectral heterogeneity can track relative diversity, which incorporates species richness, biochemical properties, and canopy structure [13; 29].