Plasticity of Root Architecture of Pinus pinaster as a Function of Installation Techniques and Soil Preparation the Six First Years

Coarse root architecture in young trees largely determines anchorage of trees but little is known about the effect of soil preparation and installation techniques. Pinus pinaster is the main species used in planted forest of South-West Europe.

Four intensities of ploughing (0, 25, 50 and 100% of the surface) were used as soil preparation with three repetitions in a 3 ha sandy spodosol plot. Partial ploughing leaved a 25 cm deep ditch at 1 m from the tree. P. pinaster were established either by direct seeding or by planting 1-years-old seedling grown in turf plugs or in grilled, vertically grooved containers. Each year, the root system of 3 trees in each combination of treatments and block was precisely measured using a 3D digitizer. We then performed an architectural analysis of root systems, according to Danjon et al. (2013ab), computing characteristics by root types.

The tree size was proportional to ploughing intensity and the differences increased with age.Ploughing intensity did not influence any of the computed root architecture characteristics at any age, but local variability of the hard pan depth had an increasing effect. Container grown trees showed much larger deformations of their shallow roots than seeded trees, part of them grew as sinkers. As a consequence, they exhibited large circular sectors without shallow roots and had a larger proportion of oblique and vertical root volume at the expense of shallow roots, even at 6 years. Turf plug root systems had intermediate characteristics for shallow roots, but 17% of their taproot were not vertical vs. 9% for container trees.

In sandy soil, the main effect of ploughing is to destroy the understorey, and not to loose the soil for a better root growth. Planting can more or less modify the architectural model of the species, altering the tropism of shallow root.