## 1 Introduction  
Globally, CMIP5 climate models consistently project future warming and drying in most Mediterranean climate regions through the poleward-expanding subtropical subsidence [1; 2; 3]. Previous studies have suggested that the impacts of climate fluctuations on ecosystem processes become less predictable as climate extremes increase in frequency [4; 5]. The resulting ecosystem changes can have many implications. As vegetation distributions change, so will habitat characteristics, potentially to the detriment of some species. Mediterranean climate regions are diversity hotspots, with large numbers of endemic and threatened species dependent upon specific habitat types [6]. Conversion of woody shrub cover to grasslands can lead to slope instability and increased landslides and erosion [7]. Winter and spring droughts in a Mediterranean climate zone could induce anomalies in ecosystem-atmosphere CO\({}_{2}\) fluxes and reduce both gross primary productivity (GPP) and ecosystem respiration significantly [8; 9]. There has been increasing concern about a shift in Mediterranean climate ecosystems from \"near-neutral\" to a long-term carbon source, due to climate extremes or other disturbances in the near future [9; 10; 11]. California is a biodiversity hotspot and home to more species of plants and animals than any other state in the U.S. [12]. The mild temperatures and winter rain of the Mediterranean climate provides favorable conditions for such great species richness in the state [13]. Although the vegetation is drought-adapted, increasing aridity and drought intensity can be a threat to Mediterranean climate vegetation communities and potentially lead to land cover changes, such as the replacement of woody vegetation by grassland [14]. The recent exceptional California drought (from 2012-2016) was characterized by prolonged precipitation deficits, exacerbated by extremely warm temperatures and associated high rates of evapotranspiration [15]. This produced particularly large negative deviations in the Palmer Drought Severity Index (PDSI), and this drought may have been the most severe drought in the last 1200 years [15]. An analysis of the role of elevated temperatures associated with greenhouse gases in contributing to the severity of the drought concluded that there has been a significant imprint of anthropogenic climate change in producing drought [16]. The recent Californian drought cannot be viewed in isolation but, instead, as occurring in the context of a longer-term trajectory towards warmer and more arid conditions over the U.S. Southwest and Southern California over the past century [17; 18; 19; 20]. Since entering the 21 st century, Southern California has experienced three prolonged droughts (i.e., 2002-2003, 2007-2009, and 2012-2016). A prolonged drought is defined as a consecutive period of more than one year (12 months) with PDSI values \(\leq-2.0\) (moderate drought) [21]. The Californian droughts have been shown to be more and more severe and persistent. Thus, drought damage to the Californian Mediterranean ecosystems has been hypothesized to increase in the future. The 2012-2016 drought, occurring in the context of a multi-decadal trend of increasing temperature, can be seen as an example of the type of drought that will characterize the future. In addition to direct effects of drought, in terms of vegetation health and mortality, high temperatures and aridity produce increased pathogen impacts and fire danger [22; 23]. According to NOAA's climate change assessment [24], the annual temperature of the U.S. Southwest is projected to increase by 1.8-3.4 \({}^{\circ}\)C over the 2041-2070 period. However, over California, the global climate models (GCMs) have displayed poor consensus on future precipitation changes, with some models trending drier, and others wetter [25]. Moreover, it is still poorly understood whether a slight increase in precipitation, which is projected to occur in some areas of California, can offset the negative effects of the hotter droughts.