# Water-Use Characteristics and Physiological Response of Moso Bamboo to Flash Droughs
## 4 Discussion  
### The Response of SWC and LAI of Moso Bamboo Forests to Flash Drought  
Soil moisture is a direct source of water for plant growth [12], and it reflects soil water availability for tree transpiration [37]. Soil moisture is jointly controlled by rainfall, ET, and runoff [38]. The influence of soil water on ET varies with rainfall amount and ET is most clearly linked to soil water when rainfall is limited [39]. Rainfall was rare and soil water sources tended to be more depleted during the flash drought periods (10 July to 17 August 2013). Soil volumetric water content at 5-50 cm depth decreased  
Figure 4: The dynamics of daily actual evapotranspiration (ET, mm), potential evapotranspiration (PET, mm) and leaf area index (LAI, m\({}^{2}\)\(\cdot\)m\({}^{-2}\)) in the Moso bamboo forest from 2012 to 2014. (**a**) Daily ET and PET, green line represents the ET and the orange, short dashed line represents the PET, respectively; (**b**) daily LAI. rapidly and was significantly lower than that in SWC_100 cm (Figure 2f). The root hairs of Moso bamboo are usually distributed above 40 cm soil depth [40], and there is generally less soil evaporation in a closed canopy forest [41; 42]. Therefore, canopy transpiration of Moso bamboo may be the main reason for SWC_5-50 cm decreasing rapidly during the flash drought periods, which in turn, will affect the growth of Moso bamboo. According to the MODIS LAI product, the average LAI also significantly decreased to 4.87 m\({}^{2}\cdot\)m\({}^{-2}\) in August 2013, which was lower by 0.26 m\({}^{2}\cdot\)m\({}^{-2}\) LAI observed in July 2013. It also significantly decreased by 0.31 m\({}^{2}\cdot\)m\({}^{-2}\) and 0.44 m\({}^{2}\cdot\)m\({}^{-2}\) compared with the same period in 2012 and 2014, respectively (Figure 5a-c). The maximum LAI generally occurred in the growing season, peaking in August in 2012 and 2014, but the maximum value in June 2013. The decrease in LAI was probably due to the physiological dehydration of Moso bamboo caused by insufficient SWC during this period. [PERSON] et al. [24] found that the aboveground biomass of Moso bamboo decreased during drought periods, and other previous studies also reported defoliation of plants during drought periods [43]. The decrease in SWC was accompanied by a decrease in ET, especially from 16 July to 16 August. The daily ET decreased from 5.73 mm\(\cdot\)day\({}^{-1}\) to 1.51 mm\(\cdot\)day\({}^{-1}\) during the flash drought periods (Figure 4a). Linear regression analysis was conducted between ET and LAI. The results showed a significant linear correlation between ET and LAI (\(R^{2}>0.58\), \(p<0.01\), Figure 5d-f). The leaf surface is the site of transpiration and photosynthesis, and its area generally determines the capacity of the canopy to capture energy and the physiological potential of water vapor exchange [37]. Therefore, it can be inferred that the rapid decrease in daily ET in August was related to Moso bamboo defoliation [37; 44]. Further analysis revealed that the SWC_5 cm and SWC_50 cm decreased by 0.14 m\({}^{3}\cdot\)m\({}^{-3}\) and 0.18 m\({}^{3}\cdot\)m\({}^{-3}\), respectively, in August, which were at the lowest level in summer and remained stable until the  
Figure 5: The relationship between (**a–c**) leaf area index (LAI) and (**d–f**) actual evapotranspiration (ET) from 2012–2014. rainfall on 18 August (Figure 2f). Therefore, SWC_50 cm \(<\) 0.17 m\({}^{3}\cdot\)m\({}^{-3}\) may be the inducing factors of bamboo deciduous. Soil water content determines forest water use [12]. Owing to the storage capacity of the soil being limited to two weeks [45], the bamboo needed water from antecedent soil water stores to maintain high levels of transpiration [45; 46]. During long-term extreme drought conditions, soil water is depleted and there is no replenishment from rainfall resulting in physiological dehydration of plants. Furthermore, transpiration loss can be reduced by defoliation [47]. Previous studies have reported that the mean sap flow decreased steadily when soil moisture had decreased [48].