EFFET DE LA VARIABILITÉ CLIMATIQUE ET DES ÉVÉNEMENTS EXTRÊMES SUR LA CROISSANCE D'ABIES ALBA, PICEA ABIES ET PINUS SYLVESTRIS EN CLIMAT TEMPÉRÉ FRANÇAIS

The objectives of this article are to highlight how extreme climatic events and climate variability influence radial growth of conifers (silver fir, Norway spruce, Scots pine) and how elevation and soil water capacity (SWC) modulate the sensitivity to climate. We sampled 33 stands in France in western lowlands and in mountain forests (400-1700 m). Soil water capacity ranged from 50 to 190 mm. We established stand chronologies for total ring width, earlywood and latewood width from 985 trees. Response to climate was analysed by using pointer years and bootstrapped response functions. Principal component analysis was applied on pointer years and response function coefficients to exhibit the ecological structure of the forests studied. Extreme winter frosts caused greater growth reductions for silver fir than for Norway spruce, especially with increasing altitude. Scots pine was the least frost sensitive species. Exceptional spring droughts caused notable growth decrease especially when local conditions were dry (altitude < 800 m and SWC < 100 mm for silver fir, western lowlands for Scots pine). Earlywood of silver fir depended on previous September, November and current February temperature after which current June and July water supply influenced latewood. Earlywood of Norway spruce was influenced by previous September temperature after which current spring and summer droughts influenced both ring components. In Scots pine, earlywood and latewood depended on the current summer water balance. Local conditions mainly modulated latewood formation. If climate becomes drier, low-elevation dry stands or trees growing in western lowlands may face problems as their growth highly depends on soil moisture availability.