## Results
### Comparison of Growing Season Length (GSL)-Derived VIs and GPP  
The growing season length (GSL: defined as the difference between PTDs of RD and UD) derived from GPP shows no statistically significant difference with GSL calculated from CamNIRv (Figure 8). By contrast, GSLs derived from GCC, CamNDVI, and CamRVI is statistically significantly different from GSL derived from GPP with mean absolute error (MAE) of 12.2, 10, and 15.5 days, respectively  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Season Summary**} \\ \hline  & \multicolumn{2}{c}{**GCC**} & \multicolumn{2}{c}{**CamNDVI**} & \multicolumn{2}{c}{**CamNIRv**} & \multicolumn{2}{c}{**CamRVI**} \\ \hline
**Stats** & **Green-Up** & **Dry-Down** & **Green-Up** & **Dry-Down** & **Green-Up** & **Dry-Down** & **Green-Up** & **Dry-Down** \\ \hline N & 24 & 30 & 24 & 30 & 24 & 30 & 24 & 30 \\ MAE (day) & 9.4 & 6.3 & 17.4 & 4.6 & 7.0 & 7.4 & 20.5 & 5.0 \\ RMSE (day) & 12.8 & 8.6 & 20.4 & 5.5 & 11.0 & 8.9 & 22.7 & 6.3 \\ \(p\)-value & *** & *** & *** & ns & ns & *** & *** & * \\ \hline \hline \end{tabular}  
* Statistics are computed using the phenological transition dates (PTDs, defined in Table 2) of the Green-up period (UD, SOS\({}_{\text{irs}}\), and POS1) and the Dry-down period (EOS\({}_{\text{irs}}\), RD, and POS2). \(N\), number of observations; MAE, mean absolute error; RMSE, root mean squared error. \(p\)-values are as follows: ***\(p\leq\) 0.001,
* 0.01 \(<p\leq\) 0.05, ns for \(p>\) 0.05.  
\end{table}
Table 4: Comparison between phenological transition dates (PTDs) derived from PhenoCam-based green chromatic coordinate (GCC), normalized difference vegetation index (CamNDVI), near-infrared reflectance of vegetation index (CamNIRv), ratio vegetation index (CamRVI), and PTDs derived from GPP in four Mediterranean experimental sites 1.  
Figure 7: Barplots of the differences between phenological transition dates (PTDs) extracted from different PhenoCam-based vegetation indexes (green chromatic coordinate (GCC), normalized difference vegetation index (CamNDVI), near-infrared reflectance of vegetation index (CamNIRv), ratio vegetation index (CamRVI)) and gross primary productivity (GPP). The error bars represent the standard error of the PTDs derived from different experimental sites, and hydrological years. The statistically significant differences were tested using Wilcoxon signed-rank tests (when PTDs were not normally distributed) and paired Student’s \(t\) tests (when PTDs were normally distributed). \(p\)-values are as follows: ***\(p\leq\) 0.001, *** 0.001 \(<p\leq\) 0.01, * 0.01 \(<p\leq\) 0.05, ns for \(p>\) 0.05. Please see the definition of each PTD in the Table 2.  
(Figure 44). We also compare the GSLs derived from GPP and VIs with the GSL defined as the difference between EOS\({}_{\text{trs}}\) and SOS\({}_{\text{trs}}\), which is widely used in the remote sensing field. The results (Figure 45) also agree with the above results.  
In general, the GSL calculated from GPP is larger than the ones derived from the VIs (Figures 8 and 44). Most of GSLs range from 240 to 270 days, with the exception of GSL derived from ES-Abr sites, which is the driest site among the four studied (Figure 44). GSLs derived from different VIs have good correlation between each other (\(r>\)0.8; Figure 44). Specifically, GSL derived from GCC has the least difference (MAE: 4.8 days) between the ones derived from CamNDVI. GSL derived from CamNDVI and CamRVI has a difference of 5.8 days. Contrastively, GSL derived from GCC, CamNDVI and CamRVI has a relatively high difference (>8 days) between the GSL derived from the CamNIRv (Figure 44).