Pinus pinaster root architecture 2 to 5 years after container rearing and outplanting: tropism loss, root clustering and resilience

Planting is a widespread afforestation technique. We characterised the modifications of the root system architecture and anchorage properties that occasionally result from container rearing and outplanting.

We set up an original method to assess these root architecture modifications in 3D-digitised root systems of 1-m tall saplings. We applied this method to 236 three- to five-year-old outplanted Pinus pinaster saplings initially reared in containers from 15 commercial planted stands, and to 48 saplings from a seeded-in-place stand.

Among the outplanted trees, the gravitropism of the first-order root was mainly modified just below the plug where a non-vertical positioning resulted in 13% of non-vertical taproots. Two-thirds of the laterals displayed large deformations within 6 cm radial distance. Nineteen percent of the roots branching from stump lost their plagiogravitropism after non-horizontal positioning in their first six centimetres. This root type change (toward oblique or sinker roots) and their directional clustering explain why container-grown trees can present a wide circular sector devoid of shallow roots.

The persistence of tropism was better for the taproot than for the shallow roots. The apexes of roots essential for anchorage should not be in a position at odds with their natural tropism after outplanting. In most of the observed stands, planted trees generally regenerated a root system following the architectural scheme of the species. However, the percentage of future crop trees was too low in a few stands.