Non-invasive methods for the investigation of trees’ root system in the urban environment

Construction activities, maintenance and expansion of below-ground grey infrastructures are known to often have precedence over tree preservation, and they have a high chance of damaging nearby trees’ roots. Damages to tree root systems are often due to the fact that is difficult to locate roots. Non-invasive techniques capable to assess the presence and the position of tree roots, may help guide the excavation activities in order to avoid damaging the primary roots of the affected tree and preserve fine roots. The objective of this paper is to review and discuss the existing knowledge about the main non-invasive methods to investigate tree roots in the urban environments. The ground penetration radar (GPR) is a geophysics technique based on the emission short pulses of electromagnetic waves by an antenna and the detection of the reflected waves by a receiver. GPR show its capacity to adequately locate the position and the diameter of coarse roots, with sufficient resolution, in the first 90-100 cm, even on paved sites, with no need of pavement displacement. The electric resistivity tomography (ERT) is a geoelectric technique that consists in applying electric current into the soil through electrodes and measuring the difference in electric potential (voltage) at a selected position. Positive correlation has been found between soil resistivity and coarse root biomass. Earth impedance method (EIM) also involve electricity enforcement into the soil-tree system but allow to quantify only absorbing root surfaces and ignores non-absorbing root surfaces. Sonic tomography applied to root system have shown good results in correctly locate coarse roots in the first 30 cm of depth.