# Responses of Terrestrial Evapotranspiration to Extreme Drought: A Review
## 3 Evapotranspiration Feedbacks to Drought

Table 1: Global terrestrial ET from different data sources. ing to a rapid decline in ET [82]. Plant mortality is typically caused by a combination of several factors, for instance, diseases related to pests and pathogens, climate stress, and wildfires [83]. [PERSON] et al. [84] developed three hypotheses of plant mortality during droughts: (i) hydraulic failure, where large water losses result in xylem cavitation; (ii) carbon starvation, where stomatal closure prevents carbon uptake in order to reduce water losses; and (iii) biotic attack, in which changes in insects and pathogen populations in warmer temperatures cause amplified plant damage and mortality. Plant mortality has a long and lasting effect on the ecosystem and community scale. Furthermore, the feedback of ET may amplify the severity of the drought because (i) a higher ET due to atmospheric conditions drying may exacerbate soil moisture depletion [12]; (ii) a higher ET may also amplify runoff and storage anomalies during drought episodes [12]; and (iii) SM negative anomalies can exacerbate precipitation anomalies, which may act to amplify droughts [85]. The timing, severity (duration and intensity), and frequency of droughts may also have an impact on the results of ET's response to drought. In general, increasing drought severity (with longer duration and higher intensity) tends to hasten soil moisture depletion and plant mortality, resulting in a lager reduction in ET. The timing of the drought matters for the degree to which ET is affected by the drought. For example, [PERSON] et al. [86] demonstrated that droughts in the early growing season have induced the greatest reductions in tree growth. In addition, [PERSON] et al. [87] also indicated that extreme droughts that occur during the dry season have more lasting and severe effects compared to droughts that occur during the wet season. Furthermore, there are several studies suggesting that drought resistance is probably decreased by recurrent droughts [88; 89], but there are also arguments that it is elevated [90]. The response of terrestrial ET to elevated CO\({}_{2}\) is expected to increase the uncertainty of the ET changes during droughts because elevated CO\({}_{2}\) tends to increase water use efficiency which may offset the effects of drought to some extent [91; 92]. By regulating the structure and physiology of vegetation, increasing CO\({}_{2}\) concentrations exert two opposite impacts on ET [93]. On the one hand, elevated CO\({}_{2}\) decreases the stomatal conductance, resulting in less water being transpired per unit leaf area and potentially increasing water use efficiency [94; 95; 96]. On the other hand, several field experiments reported that the vegetation leaf area may increase contributing to a rising leaf biomass, resulting in higher ET that may partly offset the decreased stomatal conductance [95; 97]. Both the increased and decreased responses of ET to elevated CO\({}_{2}\) have been reported for different species and climates [25; 26; 27]. However, the extent to which the effects of extreme drought on ET can be offset by elevated CO\({}_{2}\) fertilization still remains uncertain and debatable. [PERSON] et al. [98] found that elevated CO\({}_{2}\) reversed the negative effects of water constraints in semiarid grasslands using FACE experiments. However, [PERSON] et al. [99] found that rising VPD caused a large terrestrial GPP decline in the late 1990s which offset the impacts of the elevated CO\({}_{2}\) fertilization. The duration and intensity of droughts and ecosystem categories are considered to determine the impacts of elevated CO\({}_{2}\) on drought resistance [100].