## 4 Discussion
### Utilizing Different PhenoCam-Based VIs to Represent Structural Phenology  
We found distinct differences in the PTDs extracted from GCC and CamNDVI, two widely used indexes from PhenoCam (Figure 5). Overall, the PTDs derived from the GCC anticipate the ones derived from the CamNDVI, which is in line with previous studies [28; 29; 34; 75]. In the Green-up period (UD, SOStns, and POS1), changes in CamNDVI are more gradual than the changes in GCC (Figure 5) [29], which is likely due to the fact that GCC is more sensitive to color changes of leaves under low value of leaf area index (LAI; LAI < 2 [28]). [PERSON] et al. [28] found that the initial increase of GCC during the early growing season is caused by the rapid changes in leaf area and leaf chlorophyll content (Chl). With the increase of foliar biomass associated with shoot elongation and the formation of new leaves [28; 76] after the early growing season, CamNDVI, which is more sensitive to the changes of canopy structure, increases continuously while GCC has no substantial changes [34]. Previous studies also found that GCC becomes saturated when nitrogen content or Chl only reaches half of the maximum value at Green-up periods [35; 75]. By contrast, NDVI continues to increase with the maturation of leaves [77] and the increase of LAI in the canopy [35; 75]. This likely explains why there is a big difference in the timing of the first peak (POS1) of greenness between GCC and CamNDVI (Figure 5).  
Similar to the Green-up period, during the Dry-down period, GCC is more sensitive to color changes in leaves from green to yellow, while CamNDVI is a better proxy for variation of LAI and biomass [28; 29; 34]. During the Dry-down, the total green LAI of grasses start decreasing, but there is a large amount of dry and senescent biomass presented in the top canopy [78]. As such, we observed an earlier decline of GCC than CamNDVI during the Dry-down period [34] (Figure 5). The complementary information provided by GCC and CamNDVI point towards the need of using both indices to monitor structural changes of the canopy [29; 34], as well as the need for investigating different VIs that can be computed from PhenoCam.  
To our knowledge, this study is the first attempt to compute the CamNIRv and CamRVI, and to compare them with the more widely used GCC or CamNDVI [34; 41]. There is a systematic difference in PTDs extracted from CamNIRv as compared to those derived from other VIs, particularly for PTDs from the Green-up period. The PTDs derived from CamRVI tend to be later than those from CamNDVI and other VIs. This result is in agreement with [PERSON] et al. [79], which also showed that satellite-based NDVI increases faster than RVI with the increase of LAI in maize and soybean fields.  
Our results confirmed that PhenoCam-based VIs provide complementary information that can be used to monitor phenology of structure (biomass, greenness). The systematic differences observed between VIs are consistent with results reported in the literature [34; 75] and obtained with spectroradiometers [29; 34] or satellites [80]. Future studies are needed to analyze the systematic differences between PhenoCam-based VIs (e.g., the comparison between NIRv and other VIs). For instance, studies that combine physiological measurements and plant traits collected in the field with PhenoCam data [75; 77], in parallel with the use of radiative transfer models (e.g., [PERSON] et al., [28]) can provide valuable information to better understand the difference between VIs and different aspects of vegetation phenology.