Background and Aims Tree root system structure and development are difficult to assess and poorly understood in natural conditions because of soil heterogeneity and the difficulty in extracting mature tree root systems without damaging them. The purpose of this study was to understand root system development plasticity according to biological and physical parameters: species tree age and size, soil material, water availability, slope angle, and climate. Methods Two hundred and forty-three mature trees from twelve species were uprooted from homogeneous French dikes fills. Root system structure (root number and size, root system span, depth and volume) was compared between two contrasting soil materials: fine and coarse. Results Tree species had little influence on root system structure: all root system types and root size could be found for most of the species according to site conditions. Heart root systems were limited to fine material while mixed and tap root systems were found on coarse material. In coarse materials, trees developed few but rather large roots (> 5 cm in diameter and >4 m in length). In fine materials, root systems had three times more roots but they were 40% smaller and shorter. Roots were 20% more numerous and 65% larger on the downslope side due to water availability at dike or riverbank toe. Conclusion Root system structure was mainly influenced by soil material and water availability and far less by tree species. Tree root systems are opportunistic in developing in the direction where water and nutrients are plentiful: whatever the species, predicting its dimensions and structure requires a thorough investigation of soil and other environmental conditions. This study gives a new insight in root development: it will help predict tree root growth in various environments and particularly on dikes.

We added induced polarization to the concept of classical resistivity to map in-situ root systems in soil. 2. Both TDIP and SIP are applicable for identifying root polarization. 3. High resolution images showed a correlation between root location and complex resistivity anomalies under semi-controlled conditions. 4. Using a frequency band provided useful information for locating coarse roots.

List of fi gures 121 control of the fi ller materials and using materials that do not match the original construction materials. Details of the repair work tend to fade quickly from archived records, with the result that heterogeneities lay hidden within the body of the dike-potential points of weakness for the structure that must fi rst 1. Synonymous with: "fl ood protection dikes" that do not have to resist a permanent hydraulic head; these dikes are generally built above the normal water level of the river and are "dry" when the diagnostic survey is performed. 2 BACKGROUND TO THE CIVIL ENGINEERING DIAGNOSIS OF FLOOD PROTECTION DIKES 2.1 Nature, functions and composition of dikes This section is a summary of chapter 1 of the guide produced by Mériaux et al. (2001).

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