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 .