EFFECTS OF MINERAL-NUTRITION AND WATER SI TE CONDITIONS ON RADIAL GROWTH OF CORSICAN PINE IN WESTERN FRANCE

The importance of mineral nutrition and soil water regime (water soil reserve index and lateral water supply) for the radial growth of 1 808 mature Corsican pines (Pinus nigra Arnold ssp laricio Poiret var cors icana) (average age = 30 years) sampled in 183 sites in western France (Pays de la Loire region) was studied. Foliage and chemical soil anal yses indicate that growth depends exclusively on phosphorus nutrition. The optimal P foliar concentration is 0.15% (N/P foliage ratio = 10), the critical value is 0.12% (N/P = 14) and the deficiency level is 0. 10% (N/P = 18). Needle content of other nutrients is sufficient for tr ee requirements and does not differ between trees with different growt h levels. The average concentrations are 1.59%, 0.19%, 0.12% and 0.59% for N, Ca, Mg, K, respectively. The response of trees to phosphorus i s greater on moist sites than on dry sites, but considerably decreases during drought periods. Soil water regime, which is found to predispo se trees to climatic variations, may locally accentuate the effects of exceptionally dry years and hence induce durable radial growth decrea se and subsequently forest decline. However, low competition between t rees seems to be able to decrease the intensity of dryness by minimizi ng water competition within stands. In order to improve the radial gro wth of Corsican pine plantations, the best advice is to avoid this spe cies on sites clearly unfavourable for mineral nutrition and/or for wa ter supply, and to apply an adequate silviculture (regular thinnings) in order to minimize water competition between trees, especially durin g climate crisis.