Site description
The current study was performed in the southern edge of the Gurbantunggut Desert near the Fukang Station of Desert Ecology, Chinese Academy of Science (44°17′N, 87°56′E). This region has a continental, arid climate with a dry, hot summer and cold winter. The minimum air temperature is -42.2°C in winter, and the maximum air temperature is 44.2°C in summer. The annual mean temperature and annual precipitation average are 6.6°C and 164 mm, respectively, with an annual pan evaporation of approximately 1,000 mm. The stable snow cover can last 100–150 d with depth of 25 cm in winter, which then quickly melts and recharges soil moisture. Therefore, plants in this area experience frequent water deficits in the later growing season (Xu, Yu, & Li, 2017).
From the southern edge of the Gurbantunggut Desert to the interior, topography defines a spatial gradient of depth to groundwater (Wu, Zheng, Yin, et al., 2019). Along this gradient, five sites with different groundwater depths were selected (Fig. 1). We used a hand-held laser rangefinder to determine the DGW every month (Deli Tools Co., Ltd , Ningbo, Zhejiang, China). All the sites have been protected from logging since their declaration as “Desert public welfare forests” in the 2000s. The location coordinates of the sampling site is shown in Table 1. The straight-line distance is 15 km from the outmost site to the innermost site. The soil in the desert has a sandy texture (1.3% clay, 13.7% loam, and 85% sand). In such a short distance, we assumed that precipitation patterns were the same. The contrasting hydrological conditions of groundwater were ideal for studying the influence of DGW on the hydraulic properties and growth of H. ammodendron .