Influence of reafforestation techniques on architectural development of the root system and on toppling in Pinus pinaster saplings

In the French Landes de Gascogne forest, the last 2009 storm damaged many young stands of Pinus pinaster. The susceptibility of the young trees to wind damage mainly depends on the mechanical stability of their root system. An optimal development of the root system allows a better anchorage at the adult age. In sandy soil, tillage could be a part of the key in the shallow root development because it can add artificial microtopography. To assess the incidence of reafforestation techniques on root system development and to find out which tree structure features are associated to toppling, we selected three neighboring 13-years old planted stands with different tillage (full ploughing, strip ploughing and rotary ploughing). An inventory of stem leaning and direction of leaning, height and DBH was made. Three trees per toppling level (undamaged, leaning and toppled trees) were sampled in each plot. The root system architecture was measured by 3D digitizing and microtopography was measured in a radius of 3 m around the trees.The percent of straight trees was significantly more important (37%) in the full ploughing compare to the other stands (23 %) and the toppled trees were the tallest. After thirteen years, the strip ploughing plot had still a strong microtopography. The circular distribution of the root volume was studied as a proportion in three wind-oriented sectors. Undamaged trees showed a large reinforcement of root volume, length and number in the leeward quarter only, except in the strip plough plot. Larger trees which uproot with a root soil plate show generally a windward and a leeward reinforcement. In these smaller trees, which toppled, leeward roots could not move due to low plasticity of sand, whereas leeward roots were lifted easily in these low cohesion soil. To prevent toppling, reafforestation techniques should not hamper the root development leeward