4.2. Spatial pattern and heterogeneity of SWC across succession stages of a planted forest
As a spatial auto-correlation indicator, C/(C+C°) of > 0.75 for most plots indicated a consistent spatial pattern of SWC among the sampling plots (Table 3). Auto-correlation distance (A ) of SWC occurred with non-homogeneity in this study, in which, forest and mixed forest showed a broader range compared with grass and shrubland (A in plot #3 was 4.3 m) in Table 3. Additionally, A for planted or mixed forest was still larger than that for natural forest area; these results indicated that SWC was not as stable elsewhere as it was in natural land cover in the early few decades of growth of a planted forest (Jia et al., 2017; Jia et al., 2006; Zhang et al., 2016). Furthermore, RSS for the fitting models was close to 0, which indicated that a sound spatial dependence was found in the geostatistical analysis for our sampling plots; this was mainly related to the coincident transpiration and transformation of the soil moisture across land cover types (Qiu et al., 2001; Yang et al., 2017).
Interception maps of SWC for the survey plots outline the spatial pattern of SWC in a more intuitive way (Webster et al., 1990). The mixed plots (#6, #7 and #9 in Fig. 5) showed a strong auto-correlation; extremely similar continuity characteristics of SWC suggest that SWC in this region was mainly controlled by the frequency and magnitude of precipitation events (Fu et al., 2003; Qiu et al., 2001; Sun et al., 2016), and the landcover type played a secondary role (Grayson et al., 1997). Similar results were also demonstrated by Liu et al. (2020), who found that transpiration and evapotranspiration in boreal forests depend mainly on precipitation patterns.