Dendroecologie du pin laricio de corse dans l'ouest de la France. Evolution du potentiel de croissance au cours des dernieres decennies

Abstract

A total of 1 808 Corsican pines (Pinus nigra Arnold ssp laricio Poiret var corsicana) were sampled in western France (Pays-de-la Loire region). The evolution of the mean growth of each tree and the relationship between the radial growth and the present crown conditions were studied. For each ring, earlywood and latewood widths were measured separately. The analysis of the mean radial growth in relation to the cambial age showed that the young rings (≤ 10 years) were mainly made up of earlywood (70 to 80%). It was only from 40 years onwards that the percentages of each ring compartment become equal (50%). The radial growth mean curves from 1935 to 1991 showed i) strong annual variations, in relation to annual climatic conditions; ii) several crises of varying severity and length, on the decade time scale, during which growth declined because of sustained climatic stresses (1952, 1962 and since 1981); iii) an increasing general trend of the growth indices +50, +60 and +35% for, respectively, the total ring, the earlywood and the latewood. These results confirm the long-term growth trends observed in several French regions for various species, which may be interpreted as a result of global climatic changes (as a consequence of the increase in atmospheric CO2), atmospheric anthropogenic deposits, especially of nitrogen compounds, and improvement of silviculture (selection of seedlings, thinning, density of plantation). By stratification of the whole sample according to crown condition, divergent curves were obtained. The loss of vitality of the now declining trees occurred from the end of the 1960s onwards in relation to the succession of years characterized by a pronounced rainfall deficit (1967-1978). The impact of these dry years on the declining trees has been particularly amplified by locally unfavourable water site conditions. The importance of crown condition in assessing the current vigour of trees is also discussed.