4.1. SWC variability in different landcover types
In general, amounts of mean annual precipitation determine the type and
spatial pattern of dominant landcover in an ecosystem (Liu et al., 2020;
Sun et al., 2014); however, human activities shift the rate and the
magnitude of this relationship, particularly with large-scale programs
such as “Grain for Green” in China (Cao, 2008; Cao et al., 2011). New
landcover types change the water cycle from the land surface to the
atmosphere by abiotic evaporation and biotic transpiration; with a
complex hydraulic interaction between landcover and the environment, a
new trade-off forms gradually and this change produces irreversible
impacts to the local people’s livelihood (Liu et al., 2020).
SWC in this study decreased in the following order: natural forest,
brushland, grassland, planted forest, and mixed forest, with SWC in
natural forest differing significantly from that in the other vegetation
types (p<0.05) (Fig. 4). Furthermore, SWC in planted and mixed
forests years decreased with age in this study, likely leading to a
negative feedback to the water circulation processes in the ecosystem;
these results were consistent with those of previous studies (Wang et
al., 2011; Zhu et al., 2017), and may be illustrative of the repetitive
degradation of the planted area within the past 20 years (A et al.,
2019; He et al., 2007; Jia et al., 2017; Muneepeerakul et al., 2011; Sun
et al., 2014). In the early stages of plantation establishment, soil
moisture formed a new trade-off between the new vegetation cover and
precipitation, and then a relatively stable water cycle was gradually
established (Fu et al., 2003; Gómez-Plaza et al., 2001; Sun et al.,
2014). It appears that forestland in arid regions can accumulate soil
moisture when vegetation pattern is fully mature many years after
planting. These findings highlight ecological efficiency of
afforestation with hysteresis over long-time scales (Guan et al., 2020;
Jia et al., 2017).
Topography is another notable source of SWC variability (Famiglietti et
al., 1998; Fu et al., 2003). However, in this study spatial pattern of
SWC showed along slope position SWC did not exhibit a significant
declining trend from the top to the toe of slope in this study (plot #9
in Fig. 3 and Fig. 5). These characteristics could generally be
explained by -14.3% runoff depth (forests decreased net water yield)
and a mean annual precipitation of 374.1 mm in the Pailugou catchment
(adjacent to this study area) in the Qilian mountains (He et al., 2012)
indicating that interflow along the subsurface in this region cannot
determine a self-organized vegetation pattern, as reported in the loess
hilly region of China by Sun et al. (2014). The terrain-related
processes may be the main environmental factor affecting the spatial
pattern of vegetation during wet periods (Grayson et al., 1997; Western
et al., 1999).