## 2 Materials and Methods
### Phenological Transition Dates (PTDs) Extraction  
The Mediterranean climate is characterized by rainy late autumn-winters, and warm dry summers. Typically, the studied tree-grass ecosystems are dry and covered with senescent grasses in summer, while they increase in greenness in the late autumn (after the onset of the rainy season) and spring. Considering the characteristics of the phenological cycle described above, we decided to conduct the analysis using the concept of \"hydrological years\", which is, here, defined from 1 August to 31 July (Figure 2).  
Figure 2: Conceptual figure of the seasonal dynamic of green chromatic coordinates (GCC), PhenoCam-based normalized difference vegetation index (CamNDVI), near-infrared reflectance of vegetation index (CamNIRv), ratio vegetation indexes (CamRVI), or daily maximum gross primary productivity (GPP), as well as their phenological transition dates (PTDs) for a “hydrological year”. The black circles represent the original vegetation indexes value (CamGCC, CamNDVI, CamNIRv, CamRVI, or GPP). Results of the smoothing procedure and its uncertainty are shown by red circles and gray area, respectively. The vertical dashed lines represent the PTDs and the corresponding names are shown: UD, SOts, POS1, POS2, EOSts, RD. Two periods are most focused upon in this study: the Green-up period during autumn to winter (green rectangle which including UD, SOts, and POS1) and Dry-down period in late spring to summer (light-red rectangle that including POS2, EOSts and RD). The time interval between UD and RD is defined as the growing season which indicated in the figure. Detailed description of PTDs and the other phenological terms are reported in the Table 2.  
In this study, we developed a PTD extraction method for PhenoCam-based VIs in seasonally dry tree-grass ecosystems. The methodology of PTDs extraction is composed by the following steps:  
1. Data were smoothed using the spline method [20; 36]; PTDs were extracted using the derivatives of smoothed seasonal cycle [61] and applying thresholds (i.e., 50%) of amplitude of VIs [62]. As the start and end of the season are extremely important to characterize the phenology, we defined two sets of PTDs in the start (UD, SO\({}_{\text{trs}}\); Table 2) and end of season (RD, EOS\({}_{\text{trs}}\); Table 2) for intercomparison and better characterizing the phenology. These two sets of PTDs are derived based on different perception and methodology. UD and RD are retrieved as the intersection dates between steepest slope and minimum value in the Green-up and Dry-down periods, respectively [61]. In contrast, SO\({}_{\text{trs}}\) and EOS\({}_{\text{trs}}\) are retrieved by using the thresholds of 50% amplitude [62]; i.e., they are defined when 50% of amplitudes are reached in the Green-up and Dry-down periods, respectively. Other extracted PTDs and the phenological periods analyzed in this study were summarized in Figure 2 and Table 2. The detailed procedures and corresponding code related to the extraction of PTDs are provided in Appendix B.
2. Uncertainty of extracted PTDs was assessed by extracting PTDs repeatedly (100 times) from an ensemble of time series constructed by summing original data and random noise as described by [PERSON] et al. [36].