2.2 Field data collection
We established a 40 m × 40 m sample plot at each study site. Each sample plot was divided into 16 equal-sized grid cells of 10 m × 10 m (Fig. 1). All trees and shrubs were investigated in the interval’s grid cell (Blue gird cell in Fig. 1), there are eight cells in each plot and total 32 cells in woodlands and 16 cells in scrublands. We then set 2 × 2 m quadrats at four corners of the selected grid cells, from which soil depth (SD), rock bare rate (proportion of exposed rock) (RBR) and elevation were recorded. The rock bare rate was estimated visually by three observers, elevation was measured using a hand-held GPS, and soil depth was measured by vertically inserting a single-headed, sharp steel rod at the center and four corners of each 2 × 2 m quadrat until the rock was reached.