## 3 Results  
### Vegetation Dynamics and Relationships with Drought  
As indicated by the decreasing PDSI, the Californian climate has become drier over the past 30 years, where the relative drying on the south coast has been stronger than the rest of the state (Figure 3). An extreme water deficit can exert negative impacts on ecosystem health, as the observed decline in vegetation greenness. Due to the persistent drying (Figure 3) and frequent wildfires (Figure 1) in Southern California, the natural vegetation might be expected to display general browning. Pixel-wise Theil-Sen's slopes of NDVI represent the greening or browning of the vegetation (Figure 4). The burned regions showed the most abrupt NDVI declines at the rate more than 0.009/yr (Figure 4). However, browning was not constrained to burned sites. Besides the fire disturbances, the severe browning along the Californian south coast could mainly be attributed to climate disturbances. The spring (MAM) and summer (JJA) images show the greatest NDVI declines. The most severely browned regions include west Riverside County (more than 0.009/yr in spring), southwest Kern County (more than 0.007/yr in spring), south Los Angeles County (0.003-0.005/yr in spring and summer), and the western Transverse Ranges (more than 0.005/yr in all seasons). Figure 4: Theil-Sen’s slopes (NDVI trend, change per year) for seasonal NDVI during 2000–2017 with 95% Mann–Kendall significance. Light gray refers to insignificant trends. Dark gray lines indicate burned areas during 2000–2017. White areas refer to the data gaps caused by snow and cloud. DJF: December–February; MAM: March–May; JJA: June–August; SON: September–November. Many areas displayed significant NDVI declines throughout the year, while a few regions at the highest elevations showed an NDVI increase. Figure 3: Self-calibrated PDSI monthly time-series for **(a)** the whole of California and **(b)** the south coast region since 1980. The dark blue lines refer to the linear trends, with 95% confidence indicated by light grey. The black lines refer to the 3-year running mean of PDSI. PDSI showed an accelerated declining trend during the past four decades. The Californian south coast displayed stronger drying than the entire state. The 2012–2016 drought was the most severe drought during this period. Though all the five major vegetation types showed stronger NDVI decline (Table 1), there were some seasonal differences between them. CSS (\(-\)0.0083/yr) and grassland (\(-\)0.0078/yr) showed greater NDVI declines in spring than the other seasons and vegetation communities. Chaparral showed strong browning across all four seasons. The fact that chaparral and forest did not display apparent seasonal variations in the NDVI trends might be related to their deep-rooted features. CSS and grassland communities displayed less browning than chaparral and forest in summer and fall. The chaparral communities (\(-\)0.0078--0.0082/yr) and the developed-land vegetation (\(-\)0.0033--0.0027/yr) showed the largest and smallest NDVI declines, respectively, throughout the year (see Table 1). The significant correlations and regressions between NDVI anomalies and PDSI for all seasons in nearly all pixels suggest that the vegetation was very sensitive to drought in this region (Figures 5 and 6). The strongest correlations were found in the summer months (June-August), while the highest regression slopes were shown in winter (December-February). The Mediterranean climate can account for this seasonal pattern (i.e., most of the precipitation and vegetation growth occurs in winter, and all the natural plants must endure water scarcity in summer). In consequence, moderate magnitudes of the correlation (0.47-0.62) and regression coefficients (0.0109-0.0204) between NDVI and PDSI for the natural vegetation were observed for spring (March-May) and fall (September-November) as transitional periods (Tables 2 and 3).