Seasonal water use patterns of rainfed jujube trees in stands of different ages under semiarid Plantations in China

Flexible water use patterns are of great importance for planted trees if they are to acclimate to more uncertain future conditions in water-limited regions. This study aims to understand seasonal water use patterns of rainfed jujube trees in stands of various ages. A two-year experiment was conducted at four plantations of different ages (4-, 8-, 15-, and 22-years-old), in which the stable isotope technique was combined with three analytical methods. Soil moisture in the shallow (0-20 cm), middle (20-60 cm), and deep (60-200 cm) layers served as potential water sources for the jujube trees. The results showed that the 4-year-old trees mainly used soil moisture from shallow and middle layers. The 8-year-old trees shifted flexibly their water source between the three layers. However, the 15- and 22-year-old trees often ignored moisture in shallow layers and primarily used water from subsurface layers, obtaining more soil moisture from the middle layer in periods with more rainfall while more soil moisture from the deep layer when the precipitation was low. Compared with the 4- and 8-year-old trees, the two low precipitation periods during the study period clearly resulted in more water use from deep layers for the 15- and 22-year-old trees. Overall, older jujube trees were more dependent on soil water from deeper layers compared to younger trees, resulting in extensive use of soil water from deeper layers which was difficult to be replenished by precipitation. This indicates that older trees are more vulnerable to future prolonged and extremely low precipitation, and measures are needed to improve water status, especially for deeper layers under older trees.