Spatiotemporal trends of rubber defoliation and refoliation and their responses to abiotic factors in the northern edge of the Asian tropics

Hevea brasiliensis, a species native to evergreen broadleaf forest in Amazon tropical regions, exhibits a concentrated period of defoliation and refoliation after introduced to the northern edge of Asian tropics. However, up to date, spatiotemporal patterns of rubber phenology and the underlying mechanism remain unclear. In this study, we first investigated the optimal vegetation indices in monitoring four key rubber phenology metrics (i.e., Start of Defoliation (SOD), End of Defoliation (EOD), Start of Refoliation (SOR), and End of Refoliation (EOR)). Then the trends of the four phenology metrics from 2003 to 2022 in the northern edge of Asian tropics were explored. Finally, the phenological responses to climatic and topographical factors were also investigated. Results indicated that the kernel normalized difference vegetation index performed best in extracting SOD and EOD while the near-infrared reflectance of vegetation performed best for SOR and EOR. SOD exhibited an annual delay of 0.14 days, whereas EOD, SOR, and EOR showed significantly advance by 0.11, 0.27, and 0.52 days, respectively. The four phenological metrics generally delayed with increasing elevation and slope, with 0.13 days/50 m and 0.21 days/° for SOD, 0.43 days/50 m and 0.24 days/° for EOD, 1.10 days/50 m and 0.31 days/° for SOR, and 0.94 days/50 m and 0.36 days/° for EOR. Temperature and humidity were found to jointly regulate SOD and SOR, while humidity predominantly influenced EOD and EOR. This study contributes to a deeper understanding of the mechanism underlying rubber phenology and its response to future climate change. Trends of rubber defoliation and refoliation and their responses to abiotic factors ● Four rubber phenological dates are spatiotemporally mapped using four rebuilt VIs. ● NIRv performs best for refoliation while kNDVI performs best for defoliation dates. ● Refoliation is significantly advanced while trend of defoliation is not significant. ● Four phenological dates show delayed trend with arising elevation and slope. ● Temperature and humidity co-control start dates while humidity controls end dates.