DOI: 10.1093/jxb/erg154 REVIEW ARTICLE: FIELD TECHNIQUES Instrumental methods for studies of structure and

New methods using different physical principles have been successfully applied in studies of root systems of large trees. The ground-penetrating radar technique provides 3D images of coarse roots (starting with a diameter of about 20 mm) from the soil surface down to a depth of several metres. This can even be done under layers of undisturbed materials such as con-crete, asphalt and water. Fine roots cannot be visua-lized by this method, but the total rooted volume of soil can be determined. The differential electric con-ductance method has been used for fast measurement of conducting (absorbing) root surfaces. However, more testing is needed. Both these methods are non-invasive. The results can be veri®ed by an almost harmless excavation of whole root systems, including ®ne roots, using the ultrasonic air-stream (air-spade) method. This method is suitable for all studies, as well as practical operations on roots or objects in their vicinity, where a gentle approach is required. Sap ¯ow measurements on their own or in tandem with soil moisture monitoring play a leading role in studying root function and hydraulic redistribution of ¯ow in the soil. The water absorption function of roots can be studied by measuring sap ¯ow on indi-vidual root branches directly (as on crown branches) and also indirectly, by measuring the radial pattern of sap ¯ow in different sapwood depths at the base of a stem. Root zone architecture can also be estimated indirectly by studying its functionality. The heat ®eld deformation method with multi-point sensors has been found to be very convenient for this purpose. A combination of several such methods is recom-mended whenever possible, in order to obtain detailed information about the root systems of trees. Key words: Air-spade excavation, electric conductance, ground-penetrating radar, radial ¯ow pattern, sap ¯ow