## 3 Results
### The Relationships between GPPEC and Vegetation Indices from MODIS, Landsat, and Sentinel-2 Images  
For both sugarcane plantations, we assessed in terms of GPP dynamics the biophysical response of the vegetation indices. Figures 4 and 5 show the agreement during the cane growing seasons between the GPPEC and EVI at both plantations.  
At the Brazil site, the GPPEC had a sturdier linear relationship with EVILS-S2 from the Landsat and Sentinel-2 images (R\({}^{2}\) = 0.74) than with EVIMODs from the MODIS image (R\({}^{2}\) = 0.67). The difference between Landsat/Sentinel-2 and MODIS can be attributed to the MODIS pixel containing information from neighboring fields with different planting and harvesting dates. Figure 4 illustrates the differences in the crop management of sugarcane fields around the EC site. In February 2016 and August 2016 (Figure 4a,b), within the 500 m MODIS pixel, the south field (February) and the plantation west of the EC tower  
Figure 4: (**Left panel**): The relationships between gross primary production (GPPEC) and the enhanced vegetation index (EVI) derived from moderate-spatial-resolution images (MSR, MODIS) and high-spatial-resolution images (HSR, Landsat and Sentinel-2) within the growing season. (**Right panel**): 3 m 4-band planet Scope and 5 m RapidEye Ortho tile surface reflectance true-color images of vegetation cover in the FAYS Brazil study area. The red circle represents the area used to obtain the time-series of HSR data.  
(August) had bare soil landcover, while the study field had green vegetation. In addition, the effects of radiation changes and cloud cover on the relationship between the \(\mathrm{GPP_{EC}}\) and the vegetation index are supported by Figure 4d, where green vegetation is visible, but the estimated \(\mathrm{GPP_{EC}}\) was below 5 g C/m\({}^{2}\).  
At the USA site, \(\mathrm{GPP_{EC}}\) also had a stronger correlation with \(\mathrm{EVI_{LS-S2}}\) (R\({}^{2}\) = 0.76) than \(\mathrm{EVI_{MODIS}}\) (R\({}^{2}\) = 0.71) (Figure 5). The sugarcane fields around the EC study field had different planting and harvest dates (Figure 5a-d), which affected the data analysis of the relationship between \(\mathrm{GPP_{EC}}\) and \(\mathrm{EVI_{MOD}}\). Wind conditions influenced the tower's fetch footprint and varied depending on the season and the landcover type [105; 106]. May and June 2018 had high EVI values (0.628) (Figure 3d), and the field was green in June (Figure 5a), but there was no vegetation cover on some of the fallowed sugarcane fields nearby, resulting in a low \(\mathrm{GPP_{EC}}\) of 3.7 g C/m\({}^{2}\). Similar situations were observed on 22 September 2018 (Figure 5b), the date with the highest EVI of 2018 (0.72); October 2019 (Figure 5c); and September 2020 (Figure 5d), for which there were high EVI values (>0.5), relatively low \(\mathrm{GPP_{EC}}\) values (<5.45 g C/m\({}^{2}\)), and a harvested field contiguous with the green sugarcane plantation EC site. The sugarcane fields adjacent to the EC site were managed differently and could introduce uncertainty in the MSR data products (i.e., \(\mathrm{EVI_{MOD}}\)) as some of the fields were within the 500 m MODIS pixel centered on the EC tower.  
The slightly lower R\({}^{2}\) values at the Brazil site when compared to the USA site were partly attributed to the effect of cloud coverage, increase in shadow during the rainy season, fetch footprint changes throughout the seasons, and influence of the crop management (rotation and harvest) of neighboring sugarcane fields, as evinced by the moderate differences in the vegetation indices (Figure 3a,c).