## 4 Discussion
### Discrepancies between VPP Estimates from the Regression Models and MODIS-LSP  
Comparing MODIS-LSP and VPP estimates using the same threshold (15% of the amplitude), we found considerable differences that varied across PFTs. The discrepancies (RMSD) ranged from 10 to 25 days between SOG and MODIS-SOS, and from 11 to 62 days between EOG and MODIS-EOS, depending on PFTs. This is confirmed by previous studies that indicated significant temporal lags between MODIS EVI-derived LSP and VPP [11; 59], and between LSP from in situ measured green indices and VPP [11; 60]. MODIS-LSP showed good agreement and synchronization with VPP with high correlations and low random differences (RMSDb) for DBF, which is consistent with previous studies [7; 11; 16; 49]. Results from these studies indicate that the photosynthetic activity is closely related to greening and coloring of leaves for deciduous forests [7; 11; 16]. Leaf emergence is triggered by cumulative temperature at the start of season and leaf senescence is controlled by minimum temperature at the end of the season [61]. Hence, strong linkages and synchronizations among photosynthetic activity, leaf activity, and variations in temperature result in a close relationship between LSP and VPP for deciduous forests. We found that the bias between MODIS-LSP and VPP varied among PFTs and climatic regions. Spring photosynthetic phenology occurred later than EVI-derived spring phenology, and autumn photosynthetic phenology occurred earlier than EVI-derived autumn phenology for DBF. Previous studies showed that the SOG occurs approximately one week later than the EVI-derived SOS, and EOG occurs approximately two weeks earlier than the EVI-derived EOS for DBF [11; 49]. Physiological capacity, such as maturation of chlorophyll and Rubisco enzymes, does not end after leaf expansion [11; 62]. The start of photosynthetic capacity in beech trees during spring occurred with an observed lag after leaf expansion [11; 16; 63; 64]. Furthermore, for some deciduous trees leaves in the inner part of the canopy start changing their color earlier than at the top part of canopy [11; 65], which results in disagreement between remote sensing observations and photosynthetic capacity. All of these physiological phenomena explain the temporal lags between VPP and MODIS-LSP for deciduous forests. However, the large negative bias between SOG from the regression models and the MODIS-SOS for ENF indicates that the SOG occurred earlier than the MODIS-SOS, which is consistent with results from [66; 67]. Vegetation-indices-derived SOS occurred 50 days late relative to the timing of 5% GPP [66], and 25 days later in high-latitude regions for ENF [67]. For evergreen forests, the positive bias between EOG and MODIS-EOS for ENF indicates that the EOG occurred later than the MODIS-EOS. Photosynthetic phenology in evergreen forests has no correlation with vegetation-indices-derived LSP because the VPP of evergreen forests is more related to climatic factors than canopy greenness and physiological activity is decoupled from changes in greenness [16]. Physiological activity is likely to start early in spring and continue to the end of the season if climatic conditions are still favorable [11]. The greenness of evergreen understory vegetation is barely detectable by optical remote sensing, and the carbon uptake contributed by evergreen understory vegetation will make the SOG occur earlier than the SOS and the EOG occur later than the EOS, which also results in lags and discrepancies between LSP and VPP [11]. The discrepancies between VPP and the MODIS-LSP for MF were more complex than those for ENF and DBF. The temporal lags between VPP and the MODIS-LSP for MF are similar to those of DBF at high latitudes and evergreen forest at low latitudes. When the fraction of snow cover is low during the snowmelt process, greenness can be detected by remote sensing data; however, photosynthesis is weak due to unsuitable temperatures.