## 4 Discussion  
### Chlorophyll and Structural Controls on Ecosystem Function  
Analyzing the relative contributions of different spectral domains for predicting productivity of different grassland ecosystems, both structural and biochemical controls were highlighted (Figure 2). Numerous studies demonstrated how the red-edge spectral domain [45] can be used for monitoring chlorophyll-related canopy traits (i.e., fAPAR and LAI) and ecosystem productivity of herbaceous ecosystems [16; 17; 18; 19; 24; 47; 48].  
Band combinations investigated by these authors included at least one band <740 nm, in the region of the spectrum which is sensitive to chlorophyll absorption.  
Indices calculated with NIR band combinations are expected to be influenced mostly by canopy structure [25; 28; 48]. As canopy chlorophyll is usually calculated as the product of foliar chlorophyll and LAI [36], the correlations between canopy chlorophyll content and NIR-based indices may be indirect and mostly driven by factors related to canopy structure and architecture. As demonstrated by [PERSON] [25], variables such as cellular leaf anatomy, leaf clumping, and leaf angle distribution (which are contributing to the whole canopy architecture and, in turn, to scattering and reflectance in the NIR region) co-vary with traits strongly related to photosynthesis. The underlying mechanisms of such co-variations have only been partially explored and need more research.  
Within model 1, the R\({}^{2}\) values between some SVIs and NEE showed variations among the three different flights (e.g., NDVI performance was much better in flight 3 in respect to the other two flights). Such variations very likely result from uncertainties which are typically associated with both EC flux [49; 50; 51] and spectral measurements [52; 53], and from the constrained number of tower observations per flight available in the study. However, distinctive results were shown by both model 1 and model 2. The good performance of NIR-based indices is pointing out a substantial canopy structural control on photosynthesis. The correlations between NIR-based indices and productivity were often stronger than the ones observed with traditional VIS-NIR SVIs, as shown by [PERSON] et al. [26] and [PERSON] et al. [27].  
Figure 4: Map of the productivity (expressed as MTCI, on the left) and optical diversity (expressed as CV, on the right) values for the 30 60 \(\times\) 60 m ROIs located on the different grasslands associations. The spectral and spatial resolutions of the image used to process the map were achieved resampling the AisaEAGLE hyperspectral imagery and simulate the resolutions which can be obtained using Sentinel-2 data to calculate all the investigated SVIs. Darker green colors correspond to higher levels of productivity (expressed as MTCI, on the left) and optical diversity (expressed as CV, on the right).  
But how strong are structural controls on productivity? Can we expect relevant structural controls even in ecosystems where leaf chlorophyll content is strongly varying? According to the results of [PERSON] et al. (2016), who compared the photosynthetic activity of a soybean chlorophyll deficient mutant (with approximately 80% less chlorophyll than green varieties) and a wildtype, the leaf and canopy structure still appeared to be the key drivers for light absorption and photosynthesis dynamics.