## 4 Results  
SOS and EOS Estimated from Vegetation Indices Based on DLF and HGAM at Flux Sites and Their Validation  
The SOS and EOS derived from the DLF and HGAM models varied with the time series observations. Among the 60 experiments, the HGAM method successfully extracted the SOS and EOS within the effective time range (referring to 46 eight-day periods in a year) in all cases. However, the DLF method only extracted the SOS and EOS within the effective time range in 45 out of the 60 experiments. The calculated effective extraction rate for the HGAM was 100%, while, for DLF, it was 75%. Notably, among the 18 experiments using GCC, DLF failed to provide valid results in 12 cases. Therefore, despite the different vegetation indices used in phenology extraction of different ecosystems, the HGAM consistently produced valid results and exhibited better robustness compared to DLF. Specific dates of SOS and EOS in the DOY with an 8-day interval (the same as used below) are summarized in Table 2. In most years, the SOS of ENF (US-Ho1) showed a closed range of 14.42-19.91, and the EOS displayed a relatively wider range from 28.09 to 45.55 at US-Ho1. Several anomaly SOS and EOS values were derived from GCC by the DLF model (SOS = 9.57 and 5.31, and EOS = 15.01 and 13.90 in 2013 and 2015, respectively) or from NDVI by the HGAM model (SOS = 6.72 in 2014). At the two DBF sites, the ranges of both SOS and EOS were quite comparable, showing the SOS to be within 14.02-17.58 at US-Ha1 and 10.68-16.72 at US-MMS and the EOS to be within 32.59-43.61 at US-Ha1 and 34.96-41.16 at US-MMS. Most SOS and EOS values at US-Ne2 site fell within the range of 19.83-23.21 and 27.96-34.51, with the exception of SOS = 7.97 and EOS = 18.55 obtained from NDVI by the DLF model in 2011. The estimates of phenological stages from the three vegetation indices by means of the DLF and HGAM models had strong comparability at US-Twt. The SOS and EOS ranged from 16.72 to 25.36 and from 28.36 to 38.12, respectively. The growth period of the wetland (US-Myb) demonstrated the early start (averaged SOS \(<\) 12.01) and the late end (averaged EOS \(>\) 36.96) of the growth period, implying a longer growing season of vegetation at the US-Myb wetland site. On the contrary, the growth period of grassland was the shortest of all PFTs of this study. Most LOS values were less than twelve 8-day intervals at US-Var. We used phenology camera photos to evaluate the accuracy of the key phenological events. The photos acquired at the SOS and EOS extracted from NDVI, EVI, and GCC using the DLF and HGAM models were downloaded from PhenoCam. A red rectangular box enclosing the vegetation area in the foreground was set in each photo for visual inspection. We took the visual interpretation of US-MMS (in 2014) and US-Myb (in 2013) as examples to represent the woody plant and herbaceous plant. At US-MMS, green leaves were less visible in the photos where the acquired date of SOS was estimated from NDVI_DLE, NDVI_HGAM, EVI_HGAM, and GCC_HGAM, while many new green leaves had emerged in the photo whose acquired date of SOS was estimated from EVI_HGAM. The EOS date derived from the NDVI_HGAM, GCC_HGAM, and NDVI_DLF was obviously lagging behind, which could be seen by the large number of yellow leaves or chlorophyll-degraded leaves in these photos (Figure 3a). The photo witnessed the forest growing stage when the camera date was derived from the SOS of the EVI_DLF, and the EOS estimated from the EVI_DLF and EVI_HGAM matched the time that the vegetation started to turn yellow in the camera images well. Overall, the SOS and EOS obtained by EVI_HGAM were more compatible with the vegetation growth stages, and their dates were very close to those in the research conclusions of [PERSON] et al. [59]. With regards to the restored wetland of US-Myb in 2013, the SOS was estimated to be in the 12 th 8-day interval from EVI_DLF, NDVI_HGAM, and EVI_HGAM, while it was estimated from GCC_HGAM to be in the 15 th 8-day interval.