Materials and methods
Study site, geology and forest stand
The study area is a semiarid tropical forest located at the Southern extension of the SSMVC, in central Mexico (Fig. S1; 2350 m.a.s.l.; latitude 22.25° and longitude -101.12°). Climate is semiarid with the main precipitation period occurring between June and September and winter rains between December and February (5-18% of total annual rainfall). For the last 65 years, average annual rainfall was 408 mm (weather station “La Purisima”, 22° 5’ 22.4”, 101° 12’ 28.9” and records on site), where in 64% of the years rainfall was below and only in 12% above the average of 500 mm (Rodríguez-Robles et al., 2017). The topography is abrupt and irregular representing “complex terrain”. The landscape is characterized by slopes steeper than 30° and elevations ranging between 1900 and 2870 m.a.s.l. According to the classification system of the World Reference Base for Soil Resources (WRB), the extremely shallow soils and fractured rocks of this area correspond to lithic-paralithic Leptosols (LPlip) (FAO, 2006). The average depth of the organic soil horizon ranges from 10 to 25 cm; soil organic matter accumulates in crevices and soil pockets (Pérez, Arredondo, Huber, & Serna, 2014). The dominant native tree species are Pinus cembroidesZucc. (1832) and Quercus potosina Trel. (1924); they both form pure and mixed forest stands with little understory, while Quercus species exhibiting in general dimorphic root systems and Pinus species producing shallow superficial root systems (Cermak, Nadezhdina, Meiresonne, & Ceulemans, 2008; Kutschera & Lichtenegger, 2002). Tree roots anchor underneath weathered rocks, from where they obtain mineral resources. Lithological profiles show a high density of vertical roots distributed in rock fractures and soil pockets (Fig. S2). With geophysical prospection tools, we observed that this forest landscape composed of monospecific and mixed pine and oak stands developed over heterogeneous soil/rock profiles consisting of extremely shallow surface soils (< 25 cm) over granulated rock material (regolith; at 25 to 45 cm depth), impermeable volcanic rocks with pronounced fractures and fissures (from 35 to 75 cm depth), rock pockets (at 5 to 38 cm depth), and fresh rock (below 80 cm depth) (Fig.S2).