# Monitoring Changes in Water Use Efficiency to Understand Drought Induced Tree Mortality
## 4 Discussion  
A critical link between carbon and water cycles in terrestrial ecosystems, WUE is an effective way of assessing ecosystem response to drought [8; 13; 48; 50; 64; 65]. Our results show that drought frequency, WUE\({}_{\text{BASELINE}}\), and \(\Delta\)WUE captured variation in post drought maximum tree mortality rates in California forests. As hypothesized, forested areas with lower WUE\({}_{\text{BASELINE}}\) were the most vulnerable to high post drought tree mortality. Surprisingly, the effects of \(\Delta\)WUE varied and were dependent on severe drought frequency. Previous research suggested that a lower \(\Delta\)WUE was indicative of a decline in ecosystem resilience under drought conditions [8]. While this may be true, in forested ecosystems a \(\Delta\)WUE > 0 was associated with the highest tree mortality rates when  
Figure 7: Estimates of maximum tree mortality (TPH) across California forests. The regression tree model of tree mortality showed that areas with high drought frequencies (years) and a low WUE\({}_{\text{BASELINE}}\) (<2.3 g C/mm H\({}_{2}\)O) exhibited elevated tree mortality rates. Where WUE\({}_{\text{BASELINE}}\) was <2.3 g C/mm H\({}_{2}\)O drought frequency and \(\Delta\)WUE (g C/mm H\({}_{2}\)O) were essential in understanding changes in tree mortality rates. Latitude and longitude are shown along the axes. drought frequencies where high. Combined with high drought frequencies (>5 years) in recent years, substantial increases in drought induced tree mortality were detected in areas where bark beetles were active. Here, changes in WUE were in response to extreme prolonged drought and were important for understanding patterns in forest dieback. Mortality rates have been increasing since 1983 in California forests [66]. This increase in mortality is thought to be a response to the gradual temperature-driven rise in water stress, pests and pathogens [66]. The steady gradual increase in mortality has been concentrated in small trees [66] and until the most recent drought (2012-2015), climate conditions remained below thresholds that could cause large-scale forest die-back. In the 2012-2016 drought, the cumulative rainfall deficit was described as a one in a 1000-year event [37], and resulted in severely reduced snowpack, soil moisture, groundwater, and reservoir stocks [5]. This increase in water deficit, combined with high air temperatures and insect infestations, generated a large pulse of tree mortality in California [5]. Unlike the gradual increase in tree mortality since 1983, the 2012-2016 drought led to mortality in large trees [5]. Approximately 10.6 million ha of forest containing up to 888 million large trees experienced measurable loss in canopy water content [5]. Severe canopy water losses of greater than 30% occurred over 1 million ha, affecting up to 58 million large trees [5]. Drought affects virtually every plant process and the magnitude of the effect depends on drought severity and duration as well as the plant's developmental stage [28; 67]. One of the most sensitive and immediate responses to water stress at the cellular level is a reduction in growth processes (i.e., cell division and enlargement) [67]. Photosynthesis itself declines only slowly until water stress becomes moderate to severe [67; 68]. Monitoring changes in WUE allows us to track changes in photosynthesis by monitoring shifts in greenness and measuring how greenness scales with water availability. Although photosynthesis declines with drought conditions, carbon uptake per mm of ET often becomes more efficient, resulting in an increase in WUE. Our results indicate that a low WUE\({}_{\text{BASELINE}}\) at the ecosystem scale was linked to enhance mortality rates and that the interaction of the frequency of severe drought and \(\Delta\)WUE were important for understanding high rates of drought induced tree mortality. Substantial evidence indicates that drought stress promotes insect outbreaks, which are typically preceded by unusually warm, dry weather [28].