# Estimation of Gross Primary Productivity (GPP)
## Results
### Describing SOS and EOS Using VIs

Table 4: Distance, in days, to the 1:1 line between the start of the season (SOS) and the end of the season (EOS) derived from gross primary productivity (GPP) and VIs (see VIs definitions in Table 1) and for each filtering method (HANTS—Harmonic Analysis of Time Series; SavGol—Savitzky–Golay; and Polyfit—Polynomial). The shortest distances are in bold. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Period** & **Year** & **Threshold** & **MTCI** & **EVI** & **NDVI** & **ChlRedEdge** & **MCARI** & **PSSR** & **gNDVI** & **SIPI** \\ \hline SOS & 2016 & First derivative & 7.54 & 28.28 & 38.18 & 37.01 & 12.73 & 20.98 & 43.84 & 34.18 \\  & & Percentage 10\% & 1.18 & 4.24 & 5.19 & 4.95 & 1.65 & 2.83 & 6.60 & 47.61 \\  & & Percentage 20\% & 1.89 & 7.54 & 9.19 & 8.49 & 1.41 & 4.95 & 11.79 & 33.94 \\  & & Percentage 10\% & **0.00** & **0.00** & **0.00** & **0.00** & 6.36 & **0.00** & **0.00** & 46.67 \\  & & Percentage 20\% & **0.00** & **0.00** & **0.00** & **0.00** & 19.09 & **0.00** & **0.00** & 37.24 \\  & 2017 & First derivative & 11.55 & 6.36 & 5.42 & 2.36 & 8.01 & 19.56 & 9.19 & 42.90 \\  & & Percentage 10\% & 4.24 & 7.78 & 1.18 & 2.36 & 7.78 & 16.73 & 4.24 & 45.73 \\  & & Percentage 20\% & 4.71 & 6.84 & 1.41 & 2.12 & 7.31 & 19.80 & 4.48 & 34.41 \\  & & Percentage 10\% & **0.00** & 3.77 & **0.00** & **0.00** & 2.36 & 13.20 & **0.00** & 51.62 \\  & & Percentage 20\% & **0.00** & **0.00** & **0.00** & **0.00** & **0.00** & 5.66 & **0.00** & 38.89 \\  & 2018 & First derivative & 9.66 & 14.78 & 18.56 & 16.63 & 10.37 & 15.83 & 21.28 & 39.66 \\  & & Percentage 10\% & 1.89 & 5.97 & 4.26 & 4.43 & 4.76 & 8.68 & 6.13 & 42.63 \\  & & Percentage 20\% & 3.06 & 2.36 & 3.06 & 2.36 & 3.30 & 1.65 & 2.59 & **17.21** \\  & & Percentage 10\% & **0.00** & **0.00** & **0.00** & **0.00** & **0.00** & **0.00** & **0.00** & 45.73 \\  & & Percentage 20\% & **0.00** & **0.00** & **0.00** & **0.00** & **0.00** & **0.00** & **0.00** & 37.48 \\ \hline EOS & 2016 & First derivative & 20.03 & 50.68 & 46.20 & 42.43 & 47.85 & 31.82 & 47.38 & 70.00 \\  & & Percentage10\% & 17.21 & 17.68 & 21.68 & 20.98 & 15.32 & 15.56 & 22.86 & 78.25 \\  & & Percentage20\% & 17.91 & 15.32 & 26.16 & 25.22 & 17.21 &Bar plots in Figure 6 show the difference between the SOS/EOS derived from GPP and the SOS/EOS derived from VIs for each year averaged for filtering methods. A positive difference illustrated an earlier SOS/EOS derived from VI time-series, while a negative difference was indicative of a delay in SOS/EOS. SIPI predicted more than one month later the SOS of GPP and about two months earlier the EOS of GPP. All VIs (except SIPI and MCARI) estimated the SOS earlier than GPP in 2016. MTCI showed the smallest difference with the SOS of GPP. This tendency to predict earlier the SOS of GPP was also evident for all VIs, except for MTCI, MCARI and SIPI, in 2018. PSSR predicted about 2 days earlier the SOS of GPP. For the 2017 growing season, the year of the copping, all VIs, except ChlRedEdge and gNDVI, delayed the SOS of GPP. NDVI showed the smallest difference with the SOS of GPP. Clearly, all VIs (except SIPI and MCARI in 2016, and SIPI in 2017) predicted later the EOS than the GPP for all years. The EOS predicted by EVI and MCARI, showed a smaller difference with the EOS of GPP in 2017 and 2018. PSSR differed little to EOS of the GPP in 2016. The VIs performed differently in predicting daily GPP of the SRC plantation across the three growing seasons (Figure 6 and Supplementary material Figure S1). In addition, performances in predicting GPP also varied greatly across the years. MTCI performed best in predicting GPP for the 2016 and 2017 growing seasons (Figure 6) and for all three growing seasons together (R\({}^{2}\) = 0.52, RMSE = 22.53 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\)). In contrast, ChlRedEdge performed slightly better for the 2018 dry growing season (R\({}^{2}\) = 0.59, RMSE = 22.86 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\)).