Population dynamics and long-term growth depressions in European bog oaks as indicators of climate changes in the Holocene

The dendrochronological data set of absolutely dated sub-fossil oak trunks from Irish, Dutch and German bogs consists of some 2600 series. They cover the period from 6000 BC to AD 1700. The distribution of the trees in time shows distinct changes in the frequency, germination and dying-off. One way to graphically represent germination and dying-off phases is to calculate the ‘mean age’ of all trees at every calendar year. Where trees are uniformly ageing the mean age chronology rises; recruitment of juvenile trees and dying-off of old trees causes the chronology to drop. The GDO-events (sudden drop) coincide with growth depressions in the regional ring-width chronologies. Regional mean-age chronologies of the bog oaks contain similar elements, sometimes over long periods. This observation indicates common large-scale climate forcing.