Population cycles and hydroclimatic forcing in a wetland oak population: A 1.800-year tree-ring record from the Rhine Delta

Abstract

Subfossil bog oaks provide high-resolution archives of hydroecological variability in fluvial wetlands. In this study we analyse an exceptionally large dendrochronological dataset of 500 oak trunks recovered from peat deposits in the Angstel–Vecht region of the western Netherlands. The dated tree-ring record spans 1371 BCE to 488 CE and is integrated with new geological and paleobotanical evidence from the Weesp-Bloemendalerpolder area to reconstruct woodland development within an alder carr wetland system. Oak growth in this environment was strongly constrained by hydrological conditions related to Rhine discharge and groundwater levels. The oak population shows marked multi-decadal to centennial variability in recruitment and mortality. Mortality episodes occur approximately every 173 (±22) years. In contrast, no periodicity was found in recruitment or for a consistent lag between mortality and consequent recruitment. Hydrological stress years, and years of reduced growth reconstructed from ring-width anomalies, show no consistent direct relationship with mortality. They yield however substantially fewer germinations than normal years. This suggests that oak population dynamics was governed by a complex interaction of hydrological stress, age structure and internal rhythms. These internal dynamics were modulated by longer-term hydroclimatic variability associated with the North Atlantic atmospheric circulation. Five phases of woodland development are distinguished between c. 1371 BCE and 488 CE, reflecting changing interactions between hydrology, recruitment and mortality cycles and human exploitation.