## 4 Discussion
### Combing Different PhenoCam-Based VIs to Represent Physiological Phenology  
The relationship of VIs derived from PhenoCam imagery with ecosystem-scale carbon fluxes in semi-arid systems is recently receiving more attention. However, to our knowledge, previous studies focused mainly on the relationship between GPP and GCC, and here, we move forward to include other potential VIs that can be derived from PhenoCams. We observed varying performance among PhenoCam-based VIs in tracking physiological phenology as measured by GPP. During Green-up period, the PTDs derived from CamNIRv agreed the best with the PTDs derived from GPP (Figure 6 and Table 4). Apart from CamNIRv, the UD and SOStns derived from the GCC, CamNDVI, and CamRVI are statistically delayed more than the ones derived from the GPP (Figure 7). Our results about the differences between GPP and GCC are contrasting to previous studies [20; 32; 75], whichmostly focused on temperate deciduous forests, evergreen needleleaf forests or grasslands, while the relationship between GCC and GPP is comparable to a study focused on a grassland understory of a \"open forest savanna\" in Australia [30]. One possible explanation for the discrepancy between previous studies [20; 32; 75] and this study, can be related to the patterns of re-greening of the vegetation in autumn after the onset of the rainy season. At the beginning of the Green-up period, the canopy of the grass is characterized by a relevant amount of dry biomass (\(\sim\)38% senescent grasses as measured between October to November 2014-2015). Therefore, the new emerging grasses contribute to the photosynthetic activities, but not so much to the measureable greenness. For this reason, the GPP increases systematically earlier than GCC. However, there is evidence in the literature that this systematic delay is dependent on the greening patterns and mechanisms of vegetation phenology, therefore, it is vegetation type-dependent [20; 32; 75]. During the Dry-down period, the PTDs derived from CamNDVI are closer to the ones derived from GPP. The PTDs of EOStrs, and RD derived from GCC and CamNIRv are more advanced than the ones derived from GPP. Dry biomass starts to accumulate in the senescent period at the top of the grass canopy with still a certain amount of living biomass at the bottom. As mentioned above, GCC is sensitive to the changes of color [28; 29; 34; 75], hence, PTDs derived from GCC are anticipated to advance more than the PTDs derived from GPP at Dry-down period. However, more investigations are needed to explain why CamNIRv is also more advanced than GPP at the Dry-Down period. By contrast, PTDs derived from CamNDVI and CamRVI have no statistically significant differences compared to the ones from GPP, which implies the potential to use CamNDVI or CamRVI to represent GPP in the Dry-down period. For the timing of max structural and physiological phenology, we did not observe the systematic differences between PhenoCam-based VIs and GPP for the POS1 (Figure 7). However, the POS1 in our study is not comparable to the timing of the maximum value in other studies, as the POS1 in our study is in the winter time, while others are in the late spring or summer [32; 75]. By contrast, the POS2 extracted from GPP is delayed compared to the POS2 extracted from VIs during the spring period (Figure 7), which agrees with previous studies [32; 75]. As a key factor controlling net uptake of carbon dioxide [81; 82], accurate estimates of GSL has rendered substantial interest, since it has distinct impacts on ecosystem function [83]. In this study, we also study the GSL as extracted from GPP and PhenoCam-based VIs. GSL derived from CamNIRv is most representative of the GSL derived from GPP (Figures 8, A4 and A5). NIRv is claimed to explain a large fraction of the variance of GPP, and has better representation than NDVI on monthly to annual time scales [41].