## 2 Materials and Methods
### Drought Efforts on Rubber GPP  
Figure 8 shows the correlation heatmap results between the standardized precipitation evapotranspiration index (SPEI) values at different time scales and vegetation indices (such as SIF, EVI, NDVI, NIRv, and PAR) for four sites (DR, XR, SR, and NR). For the DR site, SPEI12 and SPEI24 show relatively high positive correlations with SIF, EVI, NDVI, and NIRv, with values around 0.5, indicating that prolonged drought significantly impacts vegetation indices. These correlations are statistically significant at the 0.05 level (\(p<0.05\)). For the XR, NR, and SR sites, SPEI03 shows the highest correlations with vegetation indices, with values around 0.7, and these correlations are also statistically significant at the 0.05 level (\(p<0.05\)). At all four sites, PAR is negatively correlated with vegetation indices, with the strongest negative correlation observed with PSEI01. These negative correlations are statistically significant at the 0.05 level (\(p<0.05\)), confirming the reliability of these results. This indicates that the lower the drought index for the month is, the higher the photosynthetically active radiation (PAR) will be. Figure 7: Annual GPP comparisons of model-predicted GPP, flux observations, and MOD17A3 HGF data. Figure 9 shows the time series of GPP (in gray) and SPEI12 (green representing wet periods, orange representing drought periods). At the DR site, GPP shows an upward trend, with the annual GPP lows increasing over time. Several significant drought periods are highlighted in orange at this site, particularly the prolonged droughs of 2003-2005 and 2007-2009, along with a few shorter drought periods. During the 2003-2005 drought, the eight-day GPP minimum was the lowest from 2001 to 2020, indicating a negative impact of drought on primary productivity. At the XR site, the eight-day GPP minimum values were generally higher than those at DR, with five significant drought events, the most severe occurring around 2019-2020. During this period, the GPP lows were also the lowest in recent years, reflecting the negative impact of drought events on GPP. At the NR site, several major drought periods occurred, mainly around 2004, 2007, and 2015. There were also a few minor and shorter drought events. During these major drought periods, GPP fluctuations were more pronounced during high-value periods. At the SR site, the timing of drought occurrences was similar to those at NR, with several prolonged severe drought periods coinciding. However, the GPP cyclic trend at this site was not evident, possibly due to satellite observations being affected by cloud cover, warranting further analysis. Nonetheless, significant GPP reductions were observed during drought periods at other sites, highlighting the sensitivity of rubber plantations to drought conditions. Figure 8: Correlation between drought indices and remote sensing indices at four flux stations. Significant correlations are indicated with * (\(p<0.05\)). Figures 10 and 11 show the deviations in GPP under drought conditions at four different sites across various seasons and time scales (SPEI03, SPE12). Table 3 shows the proportion of deviations due to drought relative to the average value. Considering the sample size and representativeness, periods with an SPEI less than \(-1.0\) are defined as drought periods. Based on the phenological characteristics of rubber forests and seasonal climatic features, the year at each site is divided into three periods: rainy season (May\(-\)November), cool dry season (December\(-\)February), and warm dry season (March\(-\)April). Figure 9: Time series of eight-day GPP and the SPEI12 drought index for four sites (DR, XR, NR, and SR) from 2001 to 2020. The green area represents the wet stage, the yellow area indicates the drought stage, and the grey line corresponds to the GPP values over an eight-day interval. Figure 11: Deviations of GPP under long-term drought conditions during different seasons.