Fig. 1

Sampling location and soil geochemistry. a Location of three sampling sites and their abbreviations in parentheses. Locations of previously studied boulder fields are mapped and their references are shown in the legend. Top-right corner maps the arid and hyperarid climate regions across South America [11]. b Non-metric multidimensional scaling (NMDS) ordination (stress = 0.024) of anion and cation concentrations in each soil sample. Different colors represent different sampling sites (green = L, red = M, yellow = Y). Filled vs unfilled data points correspond to the sample type information (below (B) or beside boulder (C)). Blue vectors represent fitted ion species onto the ordination with adjusted p-value < 0.05. c An example drone image used to map individual boulders at high resolution; raw data collected by Sager et al. [33] and reanalyzed in this study. Image frame corresponds to 15 x 15 m. d Corresponding mapping of individual boulders imaged in c, same scale. e Satellite image distinguishing “densely” and “loosely” packed boulder accumulations