# Monitoring Changes in Water Use Efficiency to Understand Drought Induced Tree Mortality  
Sparkle [PERSON]  
1 Department of Biological Sciences, Florida International University, 11200 S.W. 8 th Street, Miami, FL 33199, USA; [EMAIL_ADDRESS]; Tel. : +1-305-348-1988 Rocky Mountain Research Station, United States Forest Service, 240 W Prospect Rd., Fort Collins, CO 80526, USA 123  
WUE connotes ecosystem resistance by signaling severe changes in resource availability that lead to shifts in ecosystem function. Here, resistance is the capacity to absorb a disturbance (i.e., drought) and maintain the same function (i.e., productivity) and sensitivity (i.e., WUE) [8; 10]. Studies have shown that WUE increases with aridity [8; 11; 12; 13; 14; 15; 16; 17] and that water limited ecosystems tend to have higher WUE [11; 12; 18]. Across all systems, if a drought becomes severe enough, a breakdown in ecosystem resistance can lead to a reduction in WUE [8; 14; 15; 17; 19] and ecosystem type conversions [8; 17]. A recent study evaluated drought resistance across California ecosystems by quantifying deviations in WUE in 2014 (WUE\({}_{\text{Severe Drought}}\)) from WUE under normal climate conditions (WUE\({}_{\text{BASELINE}}\)) [8]. Here, we aim to link shifts in WUE under drought conditions (i.e., \(\Delta\text{WUE}=\text{WUE}_{\text{Severe Drought}}-\text{WUE}_{\text{BASELINE}}\)) to tree mortality rates to understand at what point severe drought alters the structure and function of forests. Extended drought has been linked directly to tree mortality in tropical and temperate forests [1; 20; 21; 22] and is indirectly linked to tree mortality by triggering insect or pathogen outbreaks [23; 24; 25; 26; 27]. Drought often makes trees more vulnerable to cambium-eating insects and this vulnerability often results in episodes of high tree mortality [23; 28]. Small changes in mortality rates can profoundly affect forest structure, composition, and function [29; 30; 31], making it essential that we improve our understanding of the conditions that enhance tree mortality rates and how mortality varies across the landscape. The primary objective of this research is to use spatial and temporal patterns in WUE to understand drought induced tree mortality. In California, a prolonged drought occurred from 2012-2016 and high tree mortality rates were observed across forested ecosystems [5]. To enhance our understanding of how forested ecosystems respond to drought and how carbon dynamics change with major shifts in climate and extreme events, we evaluate shifts in WUE across California forests. Here, temporal shifts in WUE is thought to reflect drought induced changes in ecosystem structure and function. We hypothesize that forests with lower WUE\({}_{\text{BASELINE}}\) and \(\Delta\text{WUE}\) will have higher maximum tree mortality rates. The frequency of drought years is also likely to be an important determinant of tree mortality. Studies have shown that a low WUE\({}_{\text{BASELINE}}\) is indicative of low drought resilience [8] and that extended droughs often lead to enhanced tree mortality rates [4]. In California forests, more trees die in years with below normal precipitation, and increases in tree mortality are often evident only for multi-year droughs (2-5 years) [4]. Forests of California are of particular interest because they include the largest and oldest trees on Earth. California relies on forested watersheds to support water provisioning, carbon storage, timber products, ecotourism, and recreation. This is the first study to examine drought induced shifts in WUE using satellite imagery and relate it to tree mortality across forested ecosystems in California. This research is essential to enhancing our understanding of how forested ecosystems respond to drought and how carbon dynamics change with major shifts in climate and extreme events.