Conference Agenda

Wood, the most abundant biological tissue in terrestrial ecosystems, has played a pivotal role throughout human history. Ancient timbers, shaped and crafted by our ancestors, are now part of our cultural heritage. The study of wooden cultural artifacts, including archaeological wooden remains and timber from historic buildings, allows us to document and answer questions about our cultural past, the organisation, and technological craftsmanship of (pre)historical societies and past forest management practices.

Dendrochronology, the science of analyzing tree-ring patterns, enables precise chronometric dating of wooden objects and constructions, offering a deeper understanding of historical contexts. When combined with archaeological and historical research, dendroarchaeology utilizes exact tree-ring dates of (pre-)historical wood to uncover details of past societies and their environments. But the same timbers also provide an archive of precisely dated wood tissues at an annual resolution, extending back millennia. The close relation between stable oxygen isotope ratios in tree-ring cellulose and hydroclimate, provides an invaluable tool for reconstructing past precipitation patterns.

Yet, this robust climate-signal in tree-ring cellulose has also opened new avenues for precise chronometric dating. Stable oxygen isotope dendrochronology now allows dating timbers that cannot be resolved through conventional ring-width dendrochronology. This advancement is particularly relevant in midlatitude regions with temperate climate, where moisture stress is typically low, what allows trees to grow fast, produce wide and sometimes invariant tree-ring series that challenge conventional dendrochronological approaches.

This keynote will explore how stable isotope dendrochronology contributes to the field of dendroarchaeology and cultural heritage studies, highlighting the contributions these fields make to one another.

Knowing past climatic patterns is essential for a better understanding of our history, and improves predictions of future conditions in a changing climate. Tree rings serve as natural archives of environmental conditions, providing annually resolved climate data. While tree-ring widths are widely used for climate reconstructions, they are less effective in temperate regions, where multiple factors influence growth. Instead, the stable oxygen isotopic composition in tree rings provides a more robust proxy, as it is strongly interlinked to a combination of temperature, precipitation and drought.

This study aims to establish a high-resolution hydroclimate reconstruction for Flanders (1100 to 1600 CE) using oxygen isotope ratios (δ¹⁸O) in oak tree rings from old timber. For calibration, increment cores are collected from living trees (Quercus sp.) along a transect stretching from the Belgian North Sea coast to southern Germany. The sampling locations represent diverse growing conditions, related to differences in soil type and elevation.

Up to 200-year-long time series of δ¹⁸O in individual tree ring cellulose were processed. From these δ¹⁸O time series, a composite chronology was developed. This chronology was then calibrated against instrumental climate data, linking it to key climatic variables such as temperature, precipitation and drought.

Preliminary results demonstrate strong inter- and intra-regional correlations between the δ¹⁸O series. Initial calibrations reveal a significant positive correlation with temperature (r = 0.52) and a negative correlation with summer precipitation and the Standardized Precipitation Evapotranspiration Index (SPEI) (r = -0.53), highlighting the potential of this approach for reconstructing past climates.

Ultimately, this research aims to reconstruct summer climate conditions during the late Middle Ages (1100-1600), providing new insights into hydroclimate variability in Flanders. Historical oak samples will therefore be obtained from medieval timber constructions, which are abundant in Flanders’ architectural heritage. However, the isotopic potential of these timbers for climate reconstructions remains unexplored.

Hydrology plays a fundamental role in the development and function of ombrotrophic raised bogs, which rely exclusively on precipitation for water and nutrients. These peatland ecosystems are highly sensitive to climatic fluctuations, with changes in water table levels influencing the balance between carbon accumulation, and decomposition of organic matter. Due to a near constant rate of deposition peatlands are ideal archives of past environmental conditions, providing a record for understanding long-term hydrological and ecological responses to climate change. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in peat deposits serve as valuable proxies for reconstructing past environmental conditions, offering insights into the balance between organic matter input and decomposition. Here we present isotopic records from three ombrotrophic raised bogs in Northern Ireland to evaluate how variations in hydrology and vegetation composition have influenced carbon and nitrogen dynamics over the period 6000-5000 BP. By integrating isotopic data with testate amoebae-based water table reconstructions and plant macrofossil analyses, we explore the interactions between shifts in bog hydrology and ecological responses with variations stable carbon and nitrogen isotopes. Our findings reveal variations in δ¹³C and δ¹⁵N, corresponding to phases of decreasing bog surface wetness and vegetation shifts. This highlights how hydrological and ecological changes influence rates of carbon accumulation and decomposition providing insight into how future changes may alter carbon emissions from peatlands.

Hydroclimate reconstructions from periods of rapid change, like the Younger Dryas (12.9 – 11.7 ka), improve our dynamical understanding of how precipitation patterns vary with temperature. In central Europe, paleoclimate records from the Younger Dryas often display conflicting signals due to the region's complex alpine topography and variable moisture pathways, underscoring the need for high spatiotemporal resolution in climate reconstructions. Small lakes are widely distributed across the region and can provide continuous, high-resolution sedimentary archives. Within lake sediments, the hydrogen isotope ratios of plant waxes (δ²H_wax) have been used as proxies of precipitation hydrogen isotope ratios (δ²H_precip) and thus hydroclimate. However, δ²H_wax values in small catchments are also influenced by vegetation changes, complicating their interpretation.

We deconvolved the influence of vegetation and δ²H_precip values on sedimentary δ²H_wax values in a sediment core spanning the past 13 kyr from Rotsee, a small lake in central Switzerland. We used molecular distributions and δ¹³C_wax values to calculate the fraction of sedimentary plant waxes from woody plants and grasses over time. Using a Monte Carlo simulation, we then estimated past δ²H_precip values, based on ²H/¹H fractionation factors from each plant group.

We show that δ²H_precip values decreased to -75 ‰ around Rotsee during the cold Younger Dryas. δ²H_precip values rapidly increased to -55 ‰ in the early Holocene and were relatively constant around -60 ‰ throughout the Holocene. Importantly, our vegetation-corrected reconstructed δ²H_precip values are stable through the widespread forest clearing caused by the ancient Romans ~2 ka, when the relative abundance of grasses increased. These results demonstrate how vegetation corrections may be required from small lake systems to reliably interpret δ²H_wax values as hydroclimate proxies. With appropriate corrections, such records can be used to map past δ²H_precip values across Europe through periods like the Younger Dryas, when both climate and vegetation changed dramatically.