Implementation of geophysical methods for spatial niche description
In each of the 12 experimental plots, we determined the vertical extent of soil, regolith, and massive rock, the presence of rock fractures, as well as the horizontal and vertical distribution of different root diameters using a ground penetrating radar (GPR, MALÅ RAMAC X3M –ProEx system coupled to an inspection wheel and shielded antennas: 500 and 800 MHz). We also generated 12 profiles with the electric radar tomography (ERT, SYSCAL KID SWITCH-24, IRIS instruments, with a 24-multi-electrode switch box) (four for each stand type) on four dates (October 2013, December 2013, February 2014 and May 2014). In each ecohydrological period, we monitored the two-dimensional (20 x 5 m area) water distribution at 5 points separated at equal distances along a 25 meter transect considering the different substrate layers with ERT. The geophysical methods employed in this study were described in detail in Rodríguez-Robles et al. (2017). All roots detected in the radar profile along each 25 m transect were grouped into five root diameter classes (< 3.0, 3.0–4.0, 4.0–5.0, 5.0–6.0 and > 6.0 cm). Finally, for calibration purposes, individual roots (total of 76) were excavated to determine their depth and diameter in situ.