Assessing the applicability of the earth impedance method for in situ studies of tree root systems

Several electrical methods have been introduced as non-invasive techniques to overcome the limited accessibility to root systems. Among them, the earth impedance method (EIM) represents the most recent development. Applying an electrical field between a cormus and the rooted soil, the EIM measures the absorptive root surface area (ARSA) from grounding resistance patterns. Allometric relationships suggested that this method was a valuable tool. Crucial assumptions for the applicability of the EIM, however, have not been tested experimentally. Focusing on tree root systems, the present study assesses the applicability of the EIM. Six hypotheses, deduced from the EIM approach, were tested in several experiments and the results were compared with conventional methods. None of the hypotheses could be verified and the results allow two major conclusions. First, in terms of an analogue electrical circuit, a tree-root–soil continuum appears as a serial circuit with xylem and soil resistance being the dominant components. Allometric variation in contact resistance, with the latter being the proxy for root surface area, are thus overruled by the spatial and seasonal variation of soil and xylem resistances. Second, in a tree-root–soil continuum, distal roots conduct only a negligible portion of the electric charge. Most of charge carriers leave the root system in the proximal parts of the root–soil interface

Root functions as influenced by different water supply

The present thesis covers three chapters, each focusing on a root function during different water supply. Firstly, soil oxygenation by Carex rostrata in flooded soil is studied. The study showed that this wetland species can increase significantly soil oxygen availability. The aeration effect, however, was largely influenced by general diffusion conditions with the latter being a function of soil water status. Secondly, fine root dynamics in mature Fagus sylvatica is studied during exceptional drought. Despite severe soil drying, an overall weak relationship between fine root formation and soil water status appeared and there was no significant relationship for fine root mortality. Thirdly, fine root dynamics of mature Fagus sylvatica and Picea abies are compared under mesic conditions. This study revealed large between-species differences with Fagus sylvatica acquiring soil borne resources with short lived fine roots. Unlike for Picea abies, fine root formation and mortality showed a seasonal pattern indicating accelerated fine root turnover during favourable seasons. It is suggested that patterns of fine root dynamics in temperate forests are largely endogenously controlled. The results thus showed that site conditions and species-inherent patterns must be considered in order to understand the relationship between the root functions studied and soil water status