New insights for the interpretation of ancient bog oak chronologies? Reactions of oak (Quercus robur L.) to a sudden peatland rewetting

Subfossil bog oak material is an important palaeo-climate and hydrology proxy for the Holocene. A correct interpretation of the subfossil material in terms of growth and population dynamics requires understanding of the underlying processes. The catastrophic rewetting after dyke-failure of a drained peatland forest in NE-Germany provides a unique natural laboratory to calibrate bog oak growth pattern with a known event. Growth differences among groups of currently vital, damaged and dead oaks in the years before the flooding were used to estimate the adaptation potential in dependence on age, tree-size, competition class and groundwater table. Dendrochronological analysis of growth patterns and wood-anatomical parameters revealed changes in the wood in reaction to sharp hydrological shifts (drainage and rewetting). Group specific chronologies were contrasted against instrumental data to evaluate the influence of climate and hydrology on radial growth of oak. Groundwater table proved to be the overriding factor influencing oak growth at our site, already during times of intensive drainage. Results show an influence of micro-site conditions (elevation above soil-water level) along with a higher chance for slower growing and possibly former suppressed trees to survive the catastrophic rewetting. These trees display a typical ring-pattern with four to five years of depressed growth and a subsequent recovery, characterised by a significant increase in ring-width, especially in the earlywood. Trees that died or were damaged following the rewetting typically show strongly reduced latewood-width before they gradually die-off. Our findings are discussed with regard to the interpretation of subfossil bog oak material and a modified version of release detection analysis is proposed as a tool to sharpen the identification of hydrological shifts in bog oak material. The results of the study can contribute to a better understanding of ecophysiological mechanisms underlying growth reactions (and adaptation potential) of oaks to prevailing high water levels.