# Effects of Precipitation and Fire on Land Surface Phenology in the Brazilian Savannas (Cerrado)
## 3 Results
### Relationships Between Precipitation and NDVI-Derived LSP Metrics from the MODI3 QI Product  
Variations in MODIS NDVI throughout the 2007/2008 seasonal cycle (October 2007 to September 2008) are presented in Figure 6 for grassland and woodland areas (\(n\) = 500 pixels per vegetation class). These variations are representative of the patterns observed in the other cycles. The transition from grasslands to woodlands was characterized by increasing NDVI values with increasing vegetation cover, along with a smaller difference between the minimum and maximum NDVI observed during the dry and rainy seasons, respectively. For both vegetation types, the NDVI curves generally reproduced the seasonal rainfall pattern. The 2007/2008 growing season began with the onset of the rainy season from September to October 2007, leading to a rise in NDVI values in tandem with rainfall until May 2008. From June to the middle of September 2008, the NDVI declined as the water deficit increased during the dry season (Figure 6). In the 2007/2008 cycle, the highest NDVI values were observed in January (around DOY 017), while the lowest values occurred in September (around DOY 250) at maximum water deficit. Figure 5: Accumulated precipitation during the rainy season of each studied seasonal cycle, highlighting the occurrence of wet (blue), regular (gray), and dry (red) growing season cycles at Emas National Park (ENP). Rainfall patterns were identified through a boxplot analysis of long-term precipitation data (1983–2023) collected by a local meteorological station. Severely dry or wet cycles were not detected in the MODIS period of analysis (2002/2003 to 2022/2023). Figure 6: Average MODIS NDVI variations in areas of woodlands and grasslands at the EMAS National Park (ENP) during the 2007/2008 seasonal cycle (October 2007 to September 2008). The local dry and rainy seasons are indicated at the top of figure. The mean and standard deviation values, calculated from the 21 seasonal cycles (2002/2003 to 2022/2023), revealed substantial variability in most of the MODIS NDVI-derived LSP metrics (Table 2). Compared to grasslands, woodland areas exhibited lower mean values of SOS and higher mean values of EOS, resulting in a slightly longer growing season, as indicated by LOS. However, the observed nine-day difference in LOS was smaller than the 16-day interval used to generate the MODIS MOD13Q1 composite product. The mean values of BL, LDV, LSI, TMS, VES, and VSS generally increased with vegetation cover towards the woodlands, while average AMP and SSI followed an opposite trend. No significant differences in RDES and RIBS were observed between the two broad classes of vegetation. After correlating the measured accumulated precipitation during the rainy season of each cycle with the corresponding MODIS LSP metrics, we found the highest positive correlations with metrics such as LSI, SSI, RIBS, EOS, and AMP, particularly over woodland areas (Figure 7). They were statistically significant at the 0.05 level. Examples of the relationships between rainy season cumulative precipitation and LSI in grassland areas, as well as SSI in woodland areas, are shown in Figures 8a and 8b, respectively. Both metrics exhibited a linear increase with rising precipitation across cycles, with Pearson's correlation coefficients of +0.575 for LSI in grasslands (Figure 8a) and +0.585 for SSI in woodlands (Figure 8b). In comparison to dry cycles, wet cycles prolonged the growing season, increased the amplitude of NDVI values relative to the baseline, and resulted in changes in the areas beneath the growing season NDVI curves. After selecting two opposite cycles from Figure 5 (the dry 2006/2007 and the wet 2019/2020), unaffected by any of the three large fire events (2005, 2010, and 2021), we observed significant statistical differences in the LSP metrics highly correlated with precipitation.