4.1. Tree water use strategy and response to drought
Soil water is the primary water source for plants in semiarid regions where the ground water is usually deep; this is very much the case on China’s Loess Plateau. Because of the paucity of precipitation, plants in drylands usually develop deep root systems to acquire water from deep in the soil or from the groundwater (Gao et al., 2018b). It is predicted that the frequency and intensity of droughts will increase in drylands as a result of climate change (Zhang et al., 2019). The occurrence of extreme droughts increases potential water demand, with intensive soil evaporation and canopy transpiration. Here the extreme spring and summer drought in 2015 reduced soil water storage, particularly in the shallow soil layers (Figure 4). The decrease in shallow water availability increased soil water use from the deep layers during droughts (Figure 5 and 6), supporting the findings of Gao et al. (2018b) based on a stable isotopic method. The shift in water use to exploit deep water sources indicates that jujube trees have a flexible water use strategy. This is often ascribed to dimorphic rooting systems, meaning that deep roots become active as shallow roots experience reversible embolism due to severe water stress (Williams and Ehleringer, 2000; West et al., 2012; Gao et al., 2018a). Deep soil water has been considered to be the critical water resource for plants coping with extreme droughts, allowing them to avoid hydraulic failure and mortality, and this applies not just to drylands (Grossoird et al., 2017) but also to subtropical and tropical regions (Choat et al., 2018). However, for drylands where ground water is far beyond the rooting depth, use of deep soil water could result in serious soil desiccation (Jia et al., 2019) which would undermine ecosystem sustainability.
In our study, the peak daily transpiration under the control was gently higher than that recorded for jujube trees without mulching at another study site on the Loess Plateau (Ma et al., 2019). This is because our site has a 6.7% higher annual mean precipitation as well as higher air temperature compared to the site investigated by Ma et al. (2019). Despite the increase in deep-layer water use during the extreme drought in 2015 (Figure 6), the prolonged drought significantly (P <0.01) reduced the amount of transpiration in the blossom & young fruit (BYF) period (Figure 8) and the daily transpiration rate (Figure 7). This can be attributed to the decrease in water availability over the whole soil profile during the BYF period. However, Schmidt-Walter et al. (2014) found that water availability did not greatly restrict transpiration rate in a full-grown mature poplar plantation; we think it likely that they did not consider soil water content in layers below 120 cm. In contrast, the moderate autumn drought did not limit the transpiration rate and was actually associated with enhanced transpiration during the fruit swelling (FRS) period (Figure 7 and 8). This could be explained by the high SWS in the 0-280 cm profile, indicating that there was abundant water provision for transpiration (Figure 8), prior to the occurrence of drought (Figure 4). At the annual scale, there was lower transpiration during the extreme drought year and significantly higher transpiration during the moderate drought year compared to the normal year (Figure 8). This may imply that moderate rather than extreme drought may favor transpiration by trees. However, the jujube trees in this study were not mature and were increasing in size during the study period; the jujube trees in 2016 had a larger canopy basal diameter and tree height than in 2014 and 2015, which would contribute to the higher transpiration in 2016.
The jujube trees tended to exhibit isohydric behavior in response to the prolonged droughts, with moderate variation in mid-day leaf water potential during the study period (Figure 10). In contrast to ansiohydric behavior, which could place plants at a great risk of dieback or mortality during extreme droughts (McDwell et al., 2008; Luo et al., 2016), isohydric behavior usually results in a conservative water use strategy and allows plants to avoid hydraulic failure (Schmidt-Walter et al., 2014). In our study, specifically, no dieback was observed in the jujube trees under any treatment after the extreme drought in 2015. Nevertheless, an in-depth investigation is needed to understand the effects of extreme droughts on the hydraulic system of jujube trees, as this is critical to understanding the mechanism of drought impacts (Choat et al., 2018) and to predict how jujube trees will respond to future extreme climates.