Retrospective analysis of radial growth in a Pinus pinaster thinning experiment in the Landes de Gascogne. Relationship with the water balance.

A dendrochronological analysis was carried out in 1998, in a Pinus pinaster thinning experiment in SW France. This experiment was established in 1967 (at 19 years old). It consists on 5 thinning intensities with 5 repetitions. Radial growth pattern in each thinning modality has been compared and annual interactions between radial growth and water balance - from Thornthwaite ETP formula - have been studied in the Landes soils and climate environment. At high thinning intensity, trees display higher water stress resistance. Nevertheless, during the years with very low rainfall, radial growth is much reduced in high thinning intensity plots, which is a mesophytic trait. During this thinning experiment, with tree age ranking from 20 to 35, thinning effect duration was no longer than 4-5 years. On a sylvicultural view-point, these results point out the interest of frequent thinning practices (every 4 or 5 years) with an economical equilibrium between high thinning intensity and stand density.