1. Introduction
Drylands cover more than 40% of the Earth’s land surface and 72% of
them are in undeveloped regions that are home to 2.5 billion people, or
38% of the world’s total population (Huang et al., 2017). Water
scarcity impedes the sustainable development of agriculture, economy and
society in global drylands (Vohland and Barry, 2009; Schwaerzel et al.,
2020). It is projected that climate change is amplifying the intensity
and frequency of droughts, causing greater stress on water availability
particularly in arid and semiarid areas (Thornton et al., 2014) where it
could cause massive tree mortality (Choat et al., 2018). The
productivity and sustainability of agroecosystems in drylands during
prolonged droughts greatly depend on each crop’s capacity to exploit
water resources (Grossiord et al., 2017), and on soil management to
increase infiltration and minimize evaporation (Zhao et al., 2009).
Understanding the seasonal and annual patterns associated with plants’
water use strategies is critical if we are to disentangle how plants
acclimate and respond to varying water situations. The seasonal water
sources usually vary depending on plant species, wetness, and topography
(Allen et al., 2019). Generally, plants can modify their water sources,
transpiration rate, or leaf water status under changing conditions in
order to cope with drought (Gow et al., 2018). It has been demonstrated
that tree species usually shift water use to deep-layer soil water (Gao
et al., 2018a; Tang et al., 2018) or groundwater (Barbeta et al., 2016)
during prolonged drought, by concentrating more fine roots at greater
depths in order to avoid hydraulic failure (Wang et al., 2020). In fact,
plants have evolved complex water use strategies in order to adapt to
different climates and water regimes, and these include isohydric and
anisohydric behavior (McDwell et al., 2008; Luo et al., 2017).
Schmidt-Walter et al. (2014) found that the transpiration of a mature
poplar (Populus spp. ) plantation was strongly controlled
by the stomata, with the plants exhibiting isohydric behavior and a
conservative water use strategy in response to drought at two sites with
a subhumid climate in Germany. In contrast, Luo et al. (2016) found that
two evergreen tree species (Osmanthus fragrans andCinnamomum camphora ) both exhibited anisohydric behavior in
response to an extreme summer drought at a humid site in central
southern China. Nevertheless, the information about the water use
strategies employed by trees in response to real-world droughts in
drylands remains limited. Furthermore, there appears to be few
integrated studies considering water source, transpiration and leaf
water potential for a given species.
A variety of mulching and terracing measures have been introduced in
drylands to reduce water loss through evaporation and runoff. It has
been clearly demonstrated that mulching has a great effect on water
balance, in terms of increasing soil water content and soil temperature,
reducing soil evaporation, and enhancing transpiration and water use
efficiency (e.g., Zhang et al., 2007; Singh et al., 2011; Liu et al.,
2014). However, little is known about how mulching influences seasonal
water use within soil profiles. For hilly regions in drylands, various
types of terraces have been used to reduce runoff and to promote
infiltration (Arnáez et al., 2015; Wei et al., 2016; Chen et al., 2017).
However, just terracing without mulching the soil surface could increase
soil evaporation during inter-rainfall periods (Li et al., 2018). A
combination of terracing and mulching may be a better alternative to
either terracing or mulching alone on dryland hillslopes. However, the
impact of such a combination on plant water use strategy has not been
examined.
The Loess Plateau (LOP), of which almost 100% can be classified as
dryland, is one of the most undeveloped regions in China due to severe
soil erosion. Therefore, the Chinese government launched the “Grain for
Green” project to reduce both soil erosion and rural poverty. This
project encouraged farmers to plant jujube trees (Ziziphus jujubaMill.) on hillslopes to deliver both ecological and economic benefits
(Gao et al., 2018b). The planting area and output value of jujube trees
exceeded 54.5×104 hm2 and 9.4
billion RMB in 2013 (Wang et al., 2016) and supported the livelihood of
more than 1 million farmers. However, the sustainability of jujube trees
is greatly hindered by water scarcity and the increasing frequency of
extreme droughts. Therefore, the main objectives of this study are (1)
to investigate the impacts of mulching and terracing combinations on
soil moisture and the water use strategy of jujube trees in a semiarid
plantation, and (2) to apply the results to address the issues of
extreme drought in real world situations.