TREE-RING RESPONSE OF POPULUS-TREMULA L AND QUERCUS-ROBUR L TO RECENTSPRING FLOODS OF THE SAONE RIVER, FRANCE

The effects of floods on the radial growth of Populus tremula L. and Q uercus robur L. in past decades were studied on a forested island in t he Saone River (France). The study considered two stands including bot h tree species, located above and below the altitude of 169 m, which c orresponds to the altitude reached by major river floods. Only the Pop ulus trees closest to water showed a significant growth reduction (nar row rings) in response to recent floods. The reaction of Quercus to pr olonged flooding was reflected in the size and distribution of vessels in annual rings. Rings formed in years when trees were flooded have a larger porous zone, including homogeneous small vessels forming a wid e earlywood zone. Since 1980, the Saone River floods have shifted from late-winter - early-spring to late-spring flood events (March-April a nd even early June). Late floods of large magnitude and duration, as i n 1983, 1986 and 1989, had precedents only in the 19th century (1856). We hypothesize that such major floods were produced by minor changes in regional atmospheric circulation which favor a higher frequency of rain events of Atlantic influence. Tree-ring signatures of flooded tre es are therefore narrow rings in Populus and anomalous structure of po rous earlywood in Quercus.