Geophysical imaging of root-zone, trunk, and moisture heterogeneity

Abstract

The most significant biotic and abiotic stress agents of water extremity, salinity, and infection lead to wood decay and modifications of moisture and ion content, and density. This strongly influences the (di-)electrical and mechanical properties and justifies the application of geophysical imaging techniques. These are less invasive and have high resolution in contrast to classical methods of destructive, single-point measurements for inspecting stresses in trees and soils. This review presents some in situ and in vivo applications of electric, radar, and seismic methods for studying water status and movement in soils, roots, and tree trunks. The electrical properties of a root-zone are a consequence of their moisture content. Electrical imaging discriminates resistive, woody roots from conductive, soft roots. Both types are recognized by low radar velocities and high attenuation. Single roots can generate diffraction hyperbolas in radargrams. Pedophysical relationships of water content to electrical resistivity and radar velocity are established by diverse infiltration experiments in the field, laboratory, and in the full-scale 'GeoModel' at Kiel University. Subsurface moisture distributions are derived from geophysical attribute models. The ring electrode technique around trunks images the growth ring structure of concentric resistivity, which is inversely proportional to the fluid content. Healthy trees show a central high resistivity within the dry heartwood that strongly decreases towards the peripheral wet sapwood. Observed structural deviations are caused by infection, decay, shooting, or predominant light and/or wind directions. Seismic trunk tomography also differentiates between decayed and healthy woods.

Application of ground penetrating radar for coarse root detection and quantification: a review

“…1a) (al Hagrey 2007). Then a portion of energy will be reflected back to the surface and recorded by the receiving antenna, while the remainder continues to propagate deeper until it is attenuated thoroughly (Barton and Montagu 2004).…”

“…The strength of reflections at an interface between two materials depends on the reflection coefficient R (Conyers 2004;al Hagrey 2007):…”

Detection of buried tree root samples by using geoelectrical measurements: a laboratory experiment

“…However, attempting to describe large undisturbed underground root systems very precisely is difficult. Non-destructive techniques such as ground penetrating radar (Butnor et al 2001;Cermak et al 2000;Hruska et al 1999;Stokes et al 1999;Wielopolski et al 1999) and electrical tomography (Al-Hagrey 2007;Heeraman et al 1997;Petersen and Al-Hagrey 2009) or a combination of these different tools (Amato et al 2008;Nadezhdina and Cermak 2003;Zenone et al 2008) have been used. Radar is not adapted as clayey material strongly attenuates the electromagnetic signal and coarse fragments generate numerous echoes.…”

Advances in experimental methods for root system architecture and root development

“…Ground penetrating radar (GPR) should be useful to investigate root systems of forest trees on forest ecosystem level (Nadezhdina and Č ermák 2003;Butnor et al 2003;Barton and Montagu 2004;al Hagrey 2007). GPRs have not only the lowest wave length and resolution to detect small targets but also the highest wave attenuation that limits resolution and penetration in wet conducting media, thus they have been used to estimate single root segments or biomass with a relatively low precision (Butnor et al 2003;Barton and Montagu 2004;al Hagrey 2007). But, at present, the accuracy and resolution of GPRs are not enough for fine roots (root diameter\1 mm), and more advanced GPRs are needed.…”