## 4 Discussion
### Discrepancies between VPP Estimates from the Regression Models and MODIS-LSP
This may be a reason for the fact that the SOG of MF occurred later than the MODIS-SOS in subarctic regions. Spatial variations in bias and RMSD between VPP and MODIS-LSP were relatively high across the same PFT. The bias between MODIS-LSP and VPP also varied across climatic regions. The bias was larger in tropical regions than in boreal and temperate regions for forest types. Similar findings were also obtained by [66], who showed that the bias between SOG and the MODIS-SOS for ENF in the Mediterranean region with a humid and warm climate was greater than that in the boreal and temperate regions. Joint controls of climatic forcing and canopy greenness result in differences between VPP and MODIS-LSP across climatic regions. Discrepancies between VPP and MODIS-LSP for evergreen and deciduous forests in subarctic, boreal, and temperate regions were relatively low, due to temperature being a major climatic factor influencing both VPP and MODIS-LSP for deciduous forests in these regions [68]. The MODIS-SOS was detected at the time of recovery of pigments in old leaves and the emergence of new leaves for evergreen forest when the temperature was suitable for the start of photosynthesis [66]. Additionally, snow cover has an important influence on spring and autumn phenology [69], and the date of snowmelt may result in a shortening of the temporal lags between SOG and the MODIS-SOS in high-latitude regions [42; 66], as leaves emerge directly after snowmelt [45]. In tropical regions, variations in photosynthetic carbon uptake primarily depend on climatic forcing rather than canopy greenness [16; 70], and this variation is not reflected by remote sensing data [16; 71]. This causes the random difference between VPP and the MODIS-LSP to be relatively high for evergreen forests in tropical regions. Spatial variations in discrepancies between VPP and MODIS-LSP for non-forest types are relatively smaller but more complex than those for forest types, and primarily occur in CRO and GRA in arid tropical and temperate Asian regions due to the extreme climatic conditions [72] and human land use management [73]. The bias between VPP and LSP in arid tropical regions was larger than those in temperate North America and the European Mediterranean region for CRO and GRA. Water and heat stress result in a decoupling of vegetation greenness and photosynthetic capacity [74], which causes the discrepancy between VPP and MODIS-LSP in arid regions, especially for evergreen plants. High variation in MODIS-SOS and MODIS-EOS for CRO was observed in northern Africa (an arid tropical region), with SOS ranging from June to October and EOS ranging from June to November [56]. The MODIS-LSP is more similar to results from [56] than VPP estimates, implying that there may be large uncertainty in VPP estimates for CRO in arid tropical regions. A large bias between SOG and MODIS-SOS for GRA in arid tropical regions was also observed. The SOG (DOY 144-151) for GRA was more similar to the spring phenology (early June, approximately DOY 151-160) from [56] in northern Africa than the MODIS-SOS (DOY 160-190). Both averaged EOG (DOY 315-324) and MODIS-EOS (DOY 309-330) for GRA were similar to the results (DOY 300-330) from [56] in northern Africa. This comparison shows that the SOG and EOG estimates for GRA in arid tropical regions were relatively reliable. The averaged SOG (DOY 100) was comparable to the averaged MODIS-SOS (DOY 96) for temperate Asia and both were within the range from DOY 90 to 120 for CRO [54]. The averaged EOG (DOY 279) for CRO occurred later than the averaged MODIS-EOS (DOY 252) for temperate Asia and was closer to the results of [54] (DOY 270 to 300) than that of MODIS-EOS. Agricultural land management is a major reason for the large random differences between MODIS-LSP and VPP for CRO in temperate Asia [54]. In addition, the cropping patterns have a clear difference between temperate Asia and North America [54; 75].