## 1 Introduction
Despite the uncertainty in precipitation, the potential impacts of \"global-change-type drought\" (simultaneous drought and heat waves; [26]) for vegetation health have been widely documented and cannot be ignored [27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. The co-occurrence of heat and drought events can impose considerable strains on plant functions [37]. For example, rising temperatures can increase carbohydrate consumption, reduce photosynthesis, and force greater stomatal closure [38]. The major vegetation types in the coastal zone of Southern California are chaparral, coastal sage scrub, grassland, and forest-woodland [39]. Not only do these vegetation types contain many endemic plant and animal species, but they also provide important ecosystem services to the people who live in the region [40]. These services include maintaining slope and soil stability, clean water resources, recreation, and visually attractive landscapes [41]. However, due to the appreciable rise of California's population [42], significant areas of the natural land-covers in California have been converted to developed lands [43; 44]. Although many Mediterranean climate plants are well adapted to short-term droughts, additional heat stress can significantly compound the effects of drought stress. Many species tend to close their stomata to reduce water loss in drought, which makes heat stress much more severe when transpirational cooling is reduced [37]. Furthermore, high temperatures may accelerate the demography of insect populations and exacerbate mortality during drought [45]. However, some evidence has been presented indicating reduced vulnerability of some vegetation communities to hotter drought [30]. For instance, warmer conditions may promote tree growth in cold regions [46]. Some species have developed compensatory physiological mechanisms to decrease the combined drought and heat stress; for example, reducing embolism by xylem refilling [47], down-regulation of respiration [48], and acclimation to recurrent droughts [49]. Furthermore, compared to natural vegetation communities, vegetation in developed lands (which may be continuously irrigated during drought) may be less affected by recent and future droughts and warming. Thus, understanding which vegetation communities are vulnerable to upcoming hotter droughts and which communities are resistant to them is crucial for climate change adaptation. A coarse-scale analysis of the state-wide aridity and vegetation response to the 2012-2016 drought indicated that this drought was the most severe in the southern regions of California [17]. Moreover, [17] suggests that a hydroclimatic dipole has contributed to the stronger drying of Southern California than the north region. A detailed study of the impacts of the recent drought on the vegetation greenness of Southern California can thus provide insights into the relative sensitivities of different vegetation types or geographic sub-regions to prolonged warming and drying anticipated in the 21 st century. These insights are important for developing environmental management policies for the future. A recent study by [50] revealed that the favorable soil water conditions of the deep-rooted chaparral makes them less sensitive to short-term drought events, but particularly vulnerable to prolonged droughts, as they are not adapted to soil water depletion, such as that which occurred in the 2012-2016 California drought. By contrast, the shallow-rooted coastal sag scrub (CSS) displayed a low sensitivity to the recent multi-year drought, indicating a better adaptation to persistent dry conditions [50]. The differential behavior of the two shrublands during drought is a result of long-term environmental adaption. However, as the local climate rapidly warms, it is of high significance to further investigate how these vegetation communities will respond to drought under both a warmer and drier climate.