## 4 Discussion
### Phenological Metrics of SOS, EOS, and LOS
The 90-day pre-SOS rainfall accumulation recorded for these two periods was almost half of the 10-year average (see 90-day pre-SOS rainfall plot in Figure A1 in the Appendix A). In 2016/17 and 2018/19, there was an advancement in the SOS dates. It was most notable in 2016/17, where the SOS occurred almost 30 days earlier than the mean SOS date. This can possibly be due to high rainfall and temperature, as the rainfall accumulation recorded in this season was nearly double the average 90-day period before the SOS date (see 90-day preSOS rainfall plot in Figure A1 in the Appendix A). In studying vegetation phenology in Southeast Asia, [PERSON] et al. [84] concluded that rainfall could be the most dominant factor influencing phenological changes in this region, particularly on rainfed cropland. A study conducted on rubber trees has reported that high rainfall during the refoliation period delayed the entire leaf ageing process [9]. Meanwhile, the recorded temperature was the highest amongst all the seasons (see 90-day pre-SOS temperature plot in Figure A1 in the Appendix A). [PERSON] et al. [9] found that temperature was a significant climatic variable that influenced rubber phenology. The advancement in 2018/19 is thought to be associated with a rubber leaf disease outbreak, whose cause is under investigation. This outbreak in South Sumatra is reported to have begun in late 2017 [85]. The disease was evident until the present (2021), with high and low levels of severity. The year 2018 recorded high levels of disease severity and could thus have caused premature SOS. The NDVI plots of the growing season during the disease outbreak showed a drop in the value (see NDVI plot for 2018/19 and 2017/18 in Figure A2 in the Appendix A). There were noticeable delays in EOS in 2011/12 and 2014/15. Based on the 90-day of pre-EOS, these two periods had high rainfall accumulation (see 90-day preEOS rainfall plot in Figure A1 in the Appendix A), causing wet conditions and fewer sunshine hours. Rubber is known to respond to sunlight duration, and reduced sunlight hours leads to a delay in the phenophase [5]. The advance in EOS can be seen in three consecutive years from 2015/16 to 2017/18. These advances can also be linked to the disease outbreak, which is believed to shorten the EOS period. Previous studies have also reported that fluctuations in phenological metrics can be caused by non-ecological factors such as oil spills [21] and soil moisture [64]. The EOS advance in 2017/18 could also be due to low rainfall accumulation during the dry season. A more pronounced dry season will shorten the defoliation period [81]. The standard deviation, variance, and dispersion index values have shown some fluctuation in SOS and EOS datasets. Meanwhile, there is no fluctuation seen in LOS. Generally, the rubber season in the study area commences around mid-August (DOY 230) and ends around early-July (DOY 183) in the following year. Each season spans an average of 318 days. This implies that rubber trees in the study area remain leafless for approximately 47 days (between 6 and 7 weeks). A defoliation period normally takes 2 to 3 weeks before new shoots emerge [86; 87], whereas the refoliation period takes approximately 4 to 5 weeks [9]. In general, defoliation (EOS) and refoliation (SOS) is a sequential process that is typically one to two months apart [88].