Fig. 1

Representative electrical resistivity (ER) tomograms (a, b, c) and corresponding radial distribution profiles (d, e, f) obtained for the 58 studied trees. For easier comparison, the 3 examples come from the same plot and have a very similar diameter. The zones of high electric resistivity (ER) values (≥400 Ωm) in 2D tomograms are indicated by red colors, while the zones of low ER values (≤200 Ωm) are indicated by blue colors. To optimize the visualization of the tomograms, the limits of the displayed ER ranges have been manually set between 150 and 500 Ωm; minimum/maximum ER values may exceed these limits. For each tomogram, ER values were extracted along a selected diameter of 2-cm wide (white dotted line) corresponding to a west/east orientation of the trunk. Values of the ER profiles are displayed according to their position (0 cm corresponds to the trunk center)