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.