## 4 Discussion
In addition, as PDSI reflects the variations of many climate variables (e.g., temperature, precipitation, evaporation, soil moisture, and wind speed), there is still no ready-to-use downscaling method for PDSI. Moreover, [17] analyzed how spatial resolution changes affect the relationship between PDSI and NDVI, and they found that the NDVI-PDSI correlations and regression coefficients did not change much when the data resolution was adjusted from 4 km to 500 m. Besides the spatial resolution issue, uncertainties may arise from the differential temporal resolution of PDSI and NDVI data. To match the PDSI time step, we adjusted the MODIS NDVI data set from 16-day to monthly. The higher frequency of the NDVI observations can capture more vegetation dynamics. However, as this study was focused on climatological analysis, andas the three droughts in Southern California during 2000-2017 were all prolonged droughts (2-4 years), the degraded temporal resolution of MODIS NDVI should only have little influence on the results included in this paper. Thus, we assume the uncertainties and biases induced by the different data resolutions cannot largely change the conclusions presented here. In addition, the lagged relationship between hydrological variables (precipitation, soil moisture, and drought) and vegetation growth also brings uncertainties to this study. Compared to the newly developed drought indices, such as the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPE), PDSI is limited by its fixed time scale of one month. However, the PDSI of a given month can represent an accumulation of drought over the previous few months and is highly correlated with the top 1 m soil moisture [103; 104]. SPI is calculated solely based on precipitation and does not take temperature influence into account. SPEI is better than PDSI in reflecting the variations of soil moisture and evapotranspiration in drought and is superior for its varying time scales [103]. As a traditional drought index, PDSI has long been used for drought measurement and has advantages in providing comparable results with other related studies, e.g., [105; 106; 105; 107]. As [107] suggested, no single drought index can capture all aspects of soil moisture drought. An interesting work in the future would be investigating the vegetation responses to drought and climate change using other drought indices and varying time scales. Previous studies (e.g., [108; 109; 110; 111]) have suggested that the sprawling development pattern in Southern California has been an important driver of contemporary chaparral conversion to herbaceous cover, both through the direct removal and fragmentation of habitat, but also through its indirect role in driving annual grass expansion associated with shortened fire intervals. As we have excluded the burned areas in this study, the NDVI change analyses should not be affected by the wildfires. As the SWAP land cover map is a recently updated (in 2015) database, the areas newly occupied by residents were identified as developed lands, instead of as natural vegetation. Thus, the NDVI trends and vegetation sensitivity for the developed lands in this study may include some signals of type conversion from natural vegetation communities to urban vegetation. However, as our study is limited to the MODIS era (after 2000), the uncertainties due to vegetation type conversions are restrained and should not alter the main conclusions. Relative differences in the drought and temperature sensitivities of the five wildland vegetation types and their related seasonal changes in NDVI also reflect their different phenological characteristics. For example, the annual Californian grassland and summer deciduous or dimorphic CSS are well-adapted to the summer drought of the Mediterranean climate; as they die back, they become dormant or reduce photosynthesis when the soil moisture significantly decreases in summer [40]. In the cool and humid winter, the two communities reach maximum biomass quickly.