# Water-Use Characteristics and Physiological Response of Moso Bamboo to Flash Droughs
## 4 Discussion
### Response of Evaportranspiration and Canopy Stomatal Conductance to Drought  
Canopy conductance (gc) and VPD are the main variables limiting transpiration [14]. Vapor pressure deficit reflects the level of atmospheric evaporative demand for water vapor [55]. The gc controls the transpiration by regulating the exchange of water and vapor between the atmosphere and soil in response to extreme drought conditions [41]. Stomatal behavior is constrained by ambient air and soil conditions [14]. When soil water moisture decreases, gc is usually more sensitive to VPD [55]. We analyzed the relationship between ET, gc, and VPD to investigate how the water exchange between leaves and the atmosphere is controlled by stomata during their response to flash drought conditions when soil water is limited.  
Linear regression analysis was conducted between ET and VPD. The results showed a strong linear correlation between ET and VPD (\(R^{2}>0.50\), \(p<0.01\); Figure 6a-c). Evaportranspiration increased with increasing VPD up to 2.0 kPa, and the value at which ET increased slowly or did not further increase was probably determined by the stomata. Canopy conductance decreased at an exponential rate with increasing VPD (Figure 6d-f). When VPD \(>\) 2 kPa, gc tended to be 0. This suggests that the response of ET to VPD is consistent with the response of gc to VPD.  
It can be inferred that this threshold (VPD = 2 kPa) may be a reference value useful for judging the flash drought affecting the Moso bamboo forest in this study site. When the transpiration rate was higher and water resources were limited, the stomatal regulation mechanism of Moso bamboo was triggered to decrease transpiration by adjusting stomatal conductance through changing the width of the stomatal opening in response to the changing atmospheric and soil water conditions ([PERSON] _et al._, 2016).  
The sensitivity of gc to VPD reflects the drought resistance and water-use strategies of plants and determines their growth. When VPD is higher, the carbon assimilation rate and photosynthetic yield is also higher ([PERSON] _et al._, 2016). The threshold of VPD response to ET observed in our study was consistent with results reported by [PERSON] et al. (2018); [PERSON] et al. (2018), and [PERSON] et al. (2019). However, it was higher than that reported from temperate coniferous forests, hardwood forests, and poplar forests (VPD \(<\) 0.6 kPa) ([PERSON] _et al._, 2018). This suggests that Moso bamboo is more drought resistant than these other types of forest.  
To compare the effects of meteorological and biological factors on evapotranspiration, and to determine the contribution rate of each factor to Moso bamboo forest evapotranspiration in the study area, the relationships between VPD, LAI, gc, and ET were analyzed by multiple stepwise regression analysis. The optimal ET model was obtained as follows: ET = 0.325 VPD\(-\)0.024 gc + 0.028 LAI + 0.271 (\(R^{2}\) = 0.56, \(p<\) 0.01). Evapotranspiration is a function of VPD, gc, and LAI. Partial regression coefficients showed that VPD, gc, and LAI explained 32.5%, 2.4%, and 2.8% of the changes in ET, respectively. It was found that VPD had the most significant effect on evapotranspiration and for every 1 kPa increase in VPD, ET would increase by 0.33 mm per unit area of Moso bamboo forest.  
Figure 6: (**a-c**) Relationships between the vapor pressure deficit (VPD) and actual evapotranspiration (ET) from 2012–2014, respectively; solid lines are regression lines. (**d–f**) the response of canopy conductance (gc) to VPD.