Abstract
This chapter is a review on anchorage and root systems of forest trees, with a focus on Pinus pinaster growing in the sandy spodosols of the Landes forest, south-west France. The reliability of the anchorage of a tree depends upon the intrinsic characteristics of the root system of the species (architecture and wood properties), of the way the seedling develop its root system and on soil characteristics. Root systems adapt largely and selectively to mechanical stimuli induced by wind forces. Each root element has a specific role on tree anchorage, which depends also from its position relative to the dominant wind F. Danjon et T. Fourcaud and/or slope. Pinus pinaster saplings have generally a taproot system. Older P. pinaster trees grow a multiple taproots system, which are the most stable root system types in sandy soils. From the mechanical point of view, a taproot system is similar to a pole guyed by the shallow lateral roots. The multiple taproots system can be seen as a cage trapping a large mass of soil and guyed by the distal part of surface roots. This cage can weight several tons and is a major component of tree anchorage, especially for large individuals. P. pinaster saplings are more stable if they have a vertical, deep and thick taproot and lateral roots homogeneously distributed within the shallow soil layer. Usually, P. pinaster develops a very small amount of oblique and horizontal roots at intermediate depth. Roots do not fork spontaneously, and new roots do not appear on older parts of the root system. As a consequence, when a 10-years-old tree has a heterogeneous circular distribution of shallow horizontal roots, it is not able to generate sinkers in sectors without shallow roots. Consequently, the cage of the adult tree can be incomplete, which decreases its anchorage efficiency. P. pinaster adapts largely and efficiently to the available soil depth and to the dominant wind. The latter adaptation improves wind firmness, because in Atlantique weather regimes, the direction of storms is generally the same than the direction of dominant winds (O'Cinnéide 1975). P. pinaster provenances from the French Atlantic coast or from Corsica have the potential to develop a root system architecture providing a good anchorage in sandy spodosols. It may be a natural selection for stability. Conversely, the root architecture of Spanish and north-African provenances seems less efficient for anchorage. P. pinaster can potentially develop a root system providing a good relative stability in all soils of the Landes forest. However, the anchorage of this species can be heavily and durably altered by all factors which can perturb its development, particularly all regeneration procedures. And it bears needles in winter.