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.