## 6 Conclusions  
As a significant vegetation structural parameter, the seasonal variation in CI plays an important role in LAI mapping and various ecosystem models. However, a phenologically simplified time-sharing two-stage CI product helps to further extend the CI applications mentioned above. In this study, we estimated the key phenological parameters from CI and NDVI time series as a comparison based on the discrete Fourier transform and the improved dynamic threshold method, respectively, and confirmed that the CI exhibits seasonal variations. Additionally, we obtained the SOS and EOS on a global scale via a pixel-by-pixel approach from the MODIS LSP product (MCD12Q2, V06I) and developed the MODIS time-share two-stage CI product based on the quality bands (QA) of the MODIS 8-day CI product. The main conclusions are as follows:  
1. The study shows that the CI exhibits an approximately inverse trend of phenological variation compared with the NDVI. The optimal thresholds for estimating phenological parameters are not only related to phenological stages but also vary with vegetation indices and land cover types. Additionally, the optimal thresholds for the SOS generally range from 40% to 80%, whereas those for the EOS range from 80% to 90%, with errors remaining within acceptable limits. 2. The accuracy evaluation results of the MODIS time-share two-stage CIs using the field-measured CIs indicate that the time-share two-stage CI is highly accurate (RMSE = 0.06, bias = 0.01), although a slight overestimation is generally observed. 3. Globally, based on the MODIS time-share two-stage CI product, the CI generally shows distinct seasonal variations, with the \(\mathrm{CL}_{\mathrm{LOS}}\) being smaller than the \(\mathrm{CL}_{\mathrm{LFS}}\) across all land cover types. Needleleaf forests presented the smallest CI values, and the difference between the \(\mathrm{CL}_{\mathrm{LOS}}\) and \(\mathrm{CI}_{\mathrm{LFS}}\) was insignificant. Compared with needleleaf forests, broadleaf forests, with slightly higher CI values, present more pronounced seasonal variations. In addition, Osh presented the largest CI values, whereas CSh, GL, PWe, CL, UB, and CVM presented relatively high CI values. Broadleaf forests, MF, Wsa, and Sav had relatively low CI values. 4. Compared with the LFS stage, the quality of the MODIS time-share two-stage CI product is better in the LOS stage, where the QA values are basically 0 and 1, accounting for more than 90% of the total, which is significantly greater than that in the LFS stage (~60%). As research has advanced, the seasonal variation characteristics of CI have been realized and gradually clarified, which should not be overlooked in studies, especially from subsequent CI applications. The MODIS time-share two-stage CI product in this study provides seasonal, large-scale, pixel-by-pixel CI values for reference and comparison in potential studies of CI derived from VIIRS and other data. The product presents the seasonally simplified variations in CI in a concise fashion, providing new insights for the derivation of CI phenology and opportunities for subsequent CI applications. In addition, exploring the extent to which CI characterizes the relationship between leaves and other canopy structures in a localized area, as well as investigating more detailed seasonal variation characteristics of CI, will be valuable in the future. Conceptualization, G.G. and Z.J. ; data curation, G.G., J.G. and X.Z. ; formal analysis, G.G. and Z.L. ; funding acquisition, Z.J. and X.Z. ; methodology, G.G., Z.J. and Z.L. ; software, G.G. ; supervision, S.C. and X.D. ; validation, G.G. and C.W. ; visualization, G.G. and Z.T. ; writing--original draft, G.G. ; writing--review and editing, [PERSON], A.D. and F.Y. All authors have read and agreed to the published version of the manuscript.