## 3 Results  
### The Seasonal Dynamics of Climate, Vegetation Indices, and GPP at the Two Sugarcane Tower Sites  
Figure 2 shows that the seasonal dynamics of the daily mean air temperature, PAR, and rainfall at the two sugarcane sites over several years. The Brazil site has wet and dry seasons, and the daily mean air temperature is above 15\({}^{\circ}\)C over the year (Figure 2a). The USA site has a temperate climate and a moderately cold winter (Figure 2b). The seasonal dynamics of the PAR, air temperature, and rainfall were similar, with the peaks in mid-summer at the site (Figure 2b). The land surface phenology (LSP) includes the start of the growing season (SOS), the end of growing season (EOS), and the growing season length (GSL). The seasonal dynamics of the vegetation indices (NDVI, EVI, and LSWI) at the two sites reveals the LSP metrics in terms of the canopy structure and the process of crop development during the growth season (Figure 3a,b). At the Brazil site, the vegetation indices started to green up and rise rapidly in May 2005, reaching their maximum value in December 2005, then suddenly dropped to \(<\)0.4 (NDVI), \(<\)0.1 (EVI), and \(<\)0 (LSWI) by May 2006 (Figure 3a). The sugarcane plants were harvested while the sugarcane plant leaves were still green. Using the criteria of EVI \(>\) 0.1 and LSWI \(>\) 0, the VI-based start of the growing season (SOS) was in mid-May 2005. Using the criteria of EVI \(<\) 0.1 and LSWI \(<\) 0, the end of the growing season (EOS) was in May 2006. The growing season length (GSL) was approximately 12 months. New sugarcaneplants were planted in 2006, and the NDVI, EVI, and LSWI increased again in October 2006 and reached their peak by February in 2007 (Figure 3a). At the USA site, the vegetation indices started to rise rapidly in late April, reached their maximum value by August, and gradually dropped to \(<\)0.4, \(<\)0.1, and \(<\)0 by December 2017, respectively (Figure 3b). Using the same criteria (EVI \(>\) 0.1, LSWI \(>\) 0; EVI \(<\) 0.1, LSWI \(<\) 0), the VI-based SOS and EOS of sugarcane at the USA site were April and December, respectively, and the GSL was about 8 months. The differences in the VI-based SOS, EOS, and GSLVI between these two sites are largely determined by the air temperature and management practice (e.g., harvest time) (Figure 2). The seasonal dynamics of NEEEC and GPPEC at the two sites reveal the phenology of sugarcane croplands from the perspective of ecosystem function (Figure 3c,d). At the Brazil site, the GPPEC rose steadily in May 2005, when the GPPEC was larger than 1 g C m\({}^{-2}\) day\({}^{-1}\); it had a maximum level in December 2005 and decreased to zero by May 2006 (Figure 3c). Using the criterion of GPP \(>=\) 1 g C/m\({}^{2}\)/day, the GPP-based SOS of the sugarcane was in mid-May in 2005 and the EOS was in May 2006, which corresponds well with the growing season delineated by the vegetation indices. The GPPEC started to increase again after the onset of the rainy season in October 2006 [18] and reached the maximum in February of 2007 (Figure 3c). At the USA site, the GPPEC rose rapidly in April (GPPEC larger than 1 g C m\({}^{-2}\) day\({}^{-1}\)), had a maximum value in August, and decreased to zero by December (Figure 3d). Using the criterion of GPP \(\geq\) 1 g C/m\({}^{2}\)/day, the GPP-based sugarcane growing season at the USA site ranged from April (SOS) to December (EOS) (Figure 3d), which corresponds well with the growing season by the vegetation indices (Figure 3b). The growing season length (GSL) of the sugarcane, defined as the number of days with a GPPEC larger than 1 g C m\({}^{-2}\) day\({}^{-1}\), lasted approximately 12 months at the Brazil site and 8 months at the USA site.