Background Rocky desertification is the most serious eco-geologically environment problem in the karst area of Southwest China. The surface exposed by outcropped boulders is a special landscape feature of rocky desertification slopes in karst regions of Southwest China. At present, there are few studies on the morphology, outcropping state and spatial distribution characteristics of outcropped boulders on the slope. This work aims to explore the outcropping characteristics of the outcropped boulders, and then provide a basis for the related researches on rocky desertification.

Methods A typical rocky desertification slope in Zunyi city, Guizhou province was taken as the survey area. A full-slope carpet field survey was conducted, and a total of 375 exposed boulders were investigated. Firstly, We systematically investigated the spatial distribution, the direction of the outcropped boulders, and the heights of the outcrops as well as their influencing factors, such as slope gradient, altitude, slope aspect, and land use type. Then by using SPSS18.0 software (Statistical Product and Service Solutions 18.0), we explored the relationship between the exposed heights of boulders and the slope gradient.

Results 1) The number of exposed boulders was the most in the middle of the slope (183 boulders), 1.63 times and 2.29 times of that on upslope and downslope, respectively. The reason was that the middle part of the slope suffered from more severe erosion and the soil thickness was relatively thinner. 2) The average height of exposed boulders on the slope was (0.35±0.25)m. In contrast, the influence of slope gradient on the height of exposed boulder was greater than that of other factors. With the increase of slope gradient (0°-15°), the height of exposed boulders increased in the form of a power function. 3) The direction of most boulders was crossed with the slope aspect, and there were significant differences in exposed height between different directions, following a descending order of the vertical direction (0.43±0.23)m > the oblique cross direction (0.33±0.28)m > the parallel direction (0.26±0.20)m. The exposed boulders, striking vertically to the slope aspect, played an important role in retaining overland runoff and sediment. 4) The exposed height of boulders on agricultural land (0.32±0.26)m was smaller than that on secondary forest and grassland (0.37±0.18)m, confirming the land changing model of "native forest and grassland-farmland-rocky desertification land-secondary forest and grassland" under the influence of mankind in karst areas.

Conclusions The number and height of exposed boulders are closely related to the characteristics of the rocky slope. This study provides a useful reference for understanding the basic characteristics of outcropped boulders on rocky desertification slopes in karst area and carrying out relevant studies on soil erosion and earth surface processes more scientifically in Southwest China.