| Focus 2/2 | The importance of deep soil horizons

Rooting of 150-year-old oak trees in a thick soil

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Researchers from the Laboratory of Pedology of the University of Franche-Comté have studied the organization of the root system of pedunculate oak (Quercus robur) developed under non-restrictive edaphic conditions (colluvial leached brown soil). [1]

Root density, inclination, lateral and deep extension of roots were measured on 150-year-old pedunculate oaks established in colluvial leached brown soil. Soil materials, silty-clay and clay-silt… are accumulated 4 m thick above a karstified limestone rock. The current soil, as well as the underlying soil horizons, allow root development without physical or chemical constraints, down to the rock. Pedunculate oaks have a root system that can be divided into two parts: the surface system, which extends up to 60 cm deep, and the deep system, located below 60 cm.

At the level of the surface system and within a radius of 3 m around the tree, rooting is intensive and composed of roots of all diameters (less than 1 mm to more than 10 cm), with maximum root density. The roots are inclined at an angle of 80-85° to the vertical. Extensive rooting can extend up to a distance of 20 m from the tree. The deep root system, which develops within a radius of 2-2.5 m, is subdivided into 2 parts: from 60 to 120 cm (intensive deep system) and below 120 cm, up to more than 4 m (extensive deep system). It is composed of subvertical (tap) roots. An estimate of the volumes of soil prospected intensively and extensively gives 17 and 800 m3 respectively.

Notes et références

Cover image. Exposed roots of an ancient oak tree (Quercus robur) in Fox Hills Forest in Northrepps Parish, Norfolk, UK. [Source: Kolforn (Wikimedia), CC BY-SA 4.0, via Wikimedia Commons]

[1] Lucot, E. & Bruckert, S. (1992). Organisation du système racinaire du chêne pédonculé (Quercus robur) développé en conditions édaphiques non contraignantes (sol brun lessivé colluvial). Annales des sciences forestières, INRA/EDP Sciences, 49 (5), pp.465-479.  hal-00882815 – https://doi.org/10.1051/forest:19920503