Geophysical imaging of tree root absorption and conduction zones under field conditions: a comparison of common geoelectrical methods

Aims Our objective was to identify the most accurate and simple non-destructive method for visualising a tree’s root system, based on the assumption that tree physiological processes affect subsurface physical properties. To investigate this, we tested four geoelectrical methods, i.e. electrical resistivity tomography (ERT), electromagnetic induction (EMI), modified earth impedance (MEI) and ground-penetrating radar (GPR), each providing geophysical maps representing the spatial distribution of physical quantities that allow for the identification of structural and functioning roots. Methods The four geoelectric methods were applied to a semi-solitary 13-year-old European ash ( Fraxinus excelsior ) ‘Atlas’ (diameter at breast height = 15.1 cm, height = 8.3 m) situated in a 14 × 14 m plot. Subsequently, we unearthed the roots using an air spade to visualise the actual root system. A 3D model and orthomosaic of the root system was then created from 177 photographs. Finally, root-zone maps from each technique were compared with the excavated root system to determine the spatial accuracy of each method. Results Our results showed that the spatial accuracy of each method used to detect root system structure (conduction zones) varied widely, ranging from 12.38% for MEI, to 44.59% for GPR, 74.54% for EMI and up to 92.66% for ERT. The results for functioning roots (absorption zones) also varied along the same gradient, ranging from 14.06% for MEI, 50.63% for GPR, 84.64% for EMI and up to 105% for ERT. Conclusions Based on our case study, ERT, followed by EMI, provided the most reliable reconstruction of a tree’s root system, with EMI successfully detecting many individual absorption zones.