## 5 Discussion
### Uncertainty of the MODIS Time-Share Two-Stage CI Product  
In this study, the MODIS time-share two-stage CI product is derived from the MODIS 8-day CI product, which is strongly influenced by the quality of the input data (MCD43A1/A2 and MCD12Q1) and the inversion algorithm. The MODIS 8-day CI product used in this study is the C6 version, which uses the enhanced and more accurate C6 MCD43 product compared with the C5 version [73], so the C6 version of the MODIS 8-day CI product has a greater proportion of inversion of the main algorithm and a lower proportion of filling values, which determines the high accuracy of the MODIS time-share two-stage CI product to some extent. However, since the basic structural characteristics of the vegetation canopy are determined by the IGBP classification schemes in MCD12Q1 as input data, the error of the land cover types also affects the quality of the MODIS 8-day CI product [23]. Furthermore, the quality of MODIS CI products is influenced by factors such as sparse vegetation cover, high background reflectance [20; 23; 35], and surface shading due to topographic relief [20; 21; 24]. Therefore, special attention should be given to these factors when MODIS time-share two-stage CI products are used.  
In the current version, the MODIS time-share two-stage CI product is related mainly to the MODIS LSP (MCD12Q2) product in the vegetation phenological cycle, which divides a vegetation cycle into LOS and LFS using the \"greenup\" and \"dormancy\" bands. The MCD12Q2 product estimates phenological parameters based on the 2-band enhanced vegetation index (EVI2). In theory, the CI characterizes the degree of clumping in the vegetation canopy; however, the EVI/NDVI mainly characterize the changes in the greenness of vegetation leaves [74; 75]. Despite the probable differences in the vegetation phenological cycles identified in this study, the assumption that the foliage clumping state indicated by the CI is synchronously related to the foliage greenness indicated by the EVI/NDVI may unavoidably generate uncertainties to some degree for the MODIS time-sharing two-stage CI product [36]. Previous studies have analyzed the timing of maximum clumping and maximum greenness of vegetation leaves, indicating that the timing of maximum clumping generally precedes that of maximum greenness by approximately 0.73 months (22 days) in the Northern Hemisphere [36]. This implies that such a difference has a marginal influence on the uncertainties of this time-share two-stage CI product [37; 38].  
Although the MCD12Q2 product derives vegetation phenological parameters based on the EVI, which represents the greenness of vegetation leaves, in this study, we selected the MODIS CI product with an 8-day temporal resolution as the input data. Therefore, when phenological stages are divided based on MCD12Q2, the 22-day time difference between maximum aggregation and maximum greenness has a negligible effect on the accuracy of the MODIS time-share two-stage CI product. However, we will also aim to explore more detailed seasonal variation characteristics of CI based on this conclusion in future research.  
In addition, the filling values in the MCD12Q2 product may generate uncertainties, especially for the filling values in the MODIS time-share two-stage CI product. Owing to technical constraints, retrieving the values of MCD12Q2 in arid and semiarid ecosystems has faced challenges [76; 77; 78], especially in Australia, where vegetation phenology is highly correlated with rainfall [78; 79]. This has resulted in substantial missing remote sensing information on vegetation phenology in this region, leading to numerous filling values in the MODIS time-share two-stage CI product. Indeed, how to consider and reduce the uncertainties generated by the upper-level input product is a challenge for all remote sensing quantitative products.