Electrical measurement of the absorption surfaces of tree roots by the earth impedance method: 1. Theory.

Analysis of plant root systems is difficult under field conditions, especially root systems of adult trees, which are large and complex and include fine absorbing roots as well as conducting coarse roots. Although coarse roots can be visualized by several methods, there are technical difficulties with root quantification. The method presented here focuses on the quantification of absorbing root surfaces through an electrical (the modified earth impedance) method. It is based on the experimentally verified fact that an applied electric current flows from the roots to the soil (or vice versa) through the same interfacial areas and predominantly in the same way as water (water solution of minerals or nutrients) flows from the soil to the tree. Based on the different conductivities of tree tissues and soil, the interfacial area, which represents the absorbing root surfaces (or root absorption zones), can be calculated. Only the theoretical description of the method is presented in this paper: the experimental verification of the method under field conditions is presented in the accompanying paper.

Electrical measurement of tree root absorbing surfaces by the earth impedance method: 2. Verification based on allometric relationships and root severing experiments.

We validated, by means of allometric relationships and root severing experiments, the modified earth impedance method developed for measuring absorbing root surfaces. For the allometric studies, a series of 350 small and large trees of six broadleaf and coniferous species in several experimental sites was examined. We found a good linear ln-ln fit between absorbing root surface area and basal area (or stem cross-sectional area at the root collar in seedlings) over a range of stem diameters from 0.5-55 cm. The absorbing root surface area also changed consistently with crown projected area and the root-accessed area (territory) of the tree. At the whole-tree level, absorbing root surface area reached about 70 times that of basal area and 40% of crown projected area, or roughly 1/3 of the root-accessed area in Norway spruce (in this species, the ratio was relatively larger in small trees and smaller in large trees). The absorbing root surfaces of mechanically severed parts of Norway spruce root systems changed in about the same proportions as the geometrically determined parts of the severed root systems. These results are promising and support field applications of the method in biological and ecological studies.