## 4 Discussion  
### Phenological Metrics of SOS, EOS, and LOS  
The main aim of this study was to evaluate the influence of rainfall and average temperature on rubber phenological metrics. We first extracted three important phenological metrics, SOS, EOS, and LOS, from the MODIS NDVI data. We computed the overall mean of SOS, EOS, and LOS for the study area on a pixel basis from these data. Both the SOS \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Phenological** & **Intercept** & **Temperature** & \\
**Model** & **Value** & **Coefficient** & **\(p\)-Value** \\ \hline SOS & 925.21 & \(-25.26\) & \(<\)0.001 \\ EOS & 562.59 & \(-13.78\) & \(<\)0.001 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Multiple linear regression parameters for SOS and EOS models. Figure 9: Linear mixed models of (**a**) SOS and (**b**) EOS. Linear mixed models of (**a**) SOS and (**b**) EOS. The grey lines represent best linear unbiased predictors (BLUPs) of individual pixels. The length of each line varies depending on the availability of data for each pixel across years of the study period. The thick black line shows the mean fixed effect. (DOY 150-270) and EOS (DOY 120-240) range falls within the normal range of wintering dates in South Sumatra, within the dry season. This indicates that the SOS and EOS are representative of defoliation and refoliation periods in the study area. The rubber defoliation and refoliation process occur during the dry season [79]. The spatial distribution map of mean SOS and EOS varies from pixel to pixel. This can be due to two reasons: (1) uneven distribution of mature plantations or trees age [80]; and (2) different rubber clones planted [81]. The year 2010 was chosen as the start of study duration because it is the year when most trees reached maturity in the study area. The plantations are said to reach maturity when 80% of total trees have a circumference over 45 cm [82] which means there is still a mixture of mature and immature trees on the ground. Additionally, immature trees are unlikely to experience wintering. This will alter the SOS/EOS dates to some extent. On the other hand, some clones are more sensitive to meteorological factors, which can impact the wintering period. For example, while comparing five rubber clones, [PERSON] et al. [5] found that RRIM 600 and GT1 defoliated and refoliated one to two weeks earlier than clones Yunyan 277-5, Yunyan 34-4, and PR 107. These two clones also have shorter wintering periods compared to the other three clones. It is a common practice in rubber plantations to plant different clones and stagger the planting time [83]. This can explain the inter-variability illustrated in Figure 6.  
We observed substantial fluctuations in SOS and EOS in specific years during the study period. Similar findings were reported by [PERSON] et al. [9]. Their study focused on the refoliation period from 2003 to 2011 using ground data. They found that the year 2005, 2009, and 2010 showed an early start of refoliation with rapid development, while 2008 also had an early start but slow growth. The other years (2003, 2004, 2006, 2007, and 2011) displayed a late onset of refoliation with rapid development. The fluctuations in SOS and EOS were also noticed in other croplands, such as in the salt marsh ecosystem. [PERSON] et al. [21] observed a delay in SOS for 2000, 2010, and 2013. They concluded that this could possibly be due to dieback, late spring, and hurricane Isaac. The salt marshes of southeast Los Angeles also experienced advances in EOS for 2005 and 2008, where storms were identified to have played a contributing role. The advancement and delay in both phenological metrics likely reflect environmental climatic anomalies or extreme weather events. In our study, the SOS was found to have been delayed in 2014/15 and 2015/16. This could be attributed to the reduction in rainfall seen in both years based on the recorded climatic data.