# Responses of Forest Carbon Cycle to Drought and Elevated CO2
## 4 Interaction of Drought and CO2 Fertilization  
Plant responses to elevated CO2 and warming is reflected in the increase of photosynthetic rates and water use efficiency. Recent studies [49; 50; 70; 71] suggest that the CO2 fertilization effect on WUE partly eliminates water limitation during drought events, however, few reports have given explicit quantitative magnitude of the WUE effect. The extent to which extreme events such as drought that negatively affect the ability of vegetation to sequester carbon, and the degree to which the impact of drought can be offset by rising CO2 remains unresolved and debatable. The ecosystem categories and drought intensity and duration are considered to determine the effects of higher water use efficiency on drought resistance. Figure 1 shows the fluxes of carbon and water under elevated CO2 in different levels of drought intensity, exhibiting the responses of forests to diverse droughts. Reduced stomatal conductance under elevated CO2 may be able to limit water consumption at leaf level (Figure 1b), but the increased evapotranspiration leaf area attributed to rising biomass stimulated by elevated CO2 may offset the effect of CO2-induced stomatal response [72]. The sensitivity coefficient of ecosystem evapotranspiration (ET) to atmospheric CO2 reflects whether biomass growth or lower stomatal conductance dominates ecosystem ET [65]. Ecosystem evapotranspiration response to elevated CO2 may lead to soil moisture and atmospheric humidity variations, which is classified as an indirect response of ecosystems to elevated CO2 [49]. This response varies greatly among ecosystems and species, such that there is not yet a consistent pattern that can describe it. Evidence from gas exchange chamber experiments suggest that elevated CO2 benefits to water efficiency will initially help resist desiccation. This benefit will gradually disappear under continued drought and heat, as VPD-induced stomatal closure will contribute to loss of CO2 assimilation and increased respiration with leaf temperature leading to CO2 loss (Figure 1c) [6]. The increase in air temperature and desiccation may reverse the CO2-induced WUE increase. [PERSON] et al. [73] suggest that the potential possibilities of stomatal closure led to ecosystem water use efficiency decreasing during drought. Although warming and elevated CO2 are expected to benefit plant growth, warming-induced water stress will restrict the positive effects of CO2 fertilization on photosynthesis [70]. [PERSON] et al. [74] indicate that the terrestrial GPP in the late 1990s experienced a continuous large-scale decline, while the vapor pressure deficit increased sharply during the same period, which offset the effect of CO2 fertilization. [PERSON] et al. [70] analyzed temperate and boreal forests in Ontario, Canada, by isotopic and dendrochronological methods, and showed that rising CO2 did increase the water use efficiency, but forest tree growth experienced a widespread decline due to water limitation induced by warming, which suggests that the increase in WUE cannot offset the growth decline under warm drought. A positive case that CO2 fertilization and warming reverses the negative effect of water limitation was found by [PERSON] et al. [50], who conducted FACE experiments in a semi-arid grassland with C4 grasses. However, their experiments only suggest the effective response of WUE to resist desiccation in grassland and did not cover other vegetation types. Furthermore, drought-induced tree mortality is an essential driver of forest carbon loss. [PERSON] et al. [75] indicated that high temperature accelerates the process of plant hydraulic failure and death and the mitigative effect of elevated CO2 is negligible, but their study is limited to tree seedling mortality. Therefore, the combined effects of drought and CO2 fertilization depend on the forest's ability to resist water limitation, which leads to whether the biomass growth is stimulated by atmospheric CO2 or lost because of drought-induced tree mortality [18].