Higher growth synchrony and responsiveness to drought in managed-young than in unmanaged-old sessile oak stands during a shift to hotter summers

Historical management legacies may condition recent responses of forests to climate warming. In Eastern Europe, unique and well-preserved old-growth sessile oak forests remain. They represent a perfect setting for comparing their radial-growth responses to climate variability, particularly drought stress, with respect to younger managed forests in a region experiencing ongoing aridification. We assessed radial-growth variability and responses to climate variables in three stands dominated by sessile oak (Quercus petraea (Matt.) Liebl.) located in western Romania: a very old unmanaged stand, an old managed stand from seed origin and a young managed stand from sprout origin. There was a shift towards warmer and drier conditions from the 1980s onwards which could have contributed to reduce temporal growth variability, but it did not increase growth coherence among coexisting trees. Very old and old stands showed a higher growth variability among conspecifics comparing with the young stand, which presented a higher growth synchrony. Warm and dry summers and elevated vapor pressure deficit reduced sessile oak growth. The highest responsiveness of growth indices was found for 12-month long droughts ending in summer in the case of young and old managed stands, whereas the very old unmanaged stand showed the lowest responsiveness. Improving tree growth of younger and even old managed stands may make them more responsive to summer soil and atmospheric drought, thus increasing their vulnerability to further water shortage. A better knowledge on how oak growth variability and responsiveness to drought depend on root-to-shoot ratios is needed to manage oak forests and make them more resilient to aridification.