Tree rings reveal signs of Europe's sustainable forest management long before the first historical evidence

Wood has been a crucial resource for human societies, fueling a constant demand for construction timber and fuelwood. To meet this demand, particularly in central Europe during medieval times, sustainable silvicultural systems were developed, notably the Coppice-with-Standards (CWS) system, also known as "taillis sous futaie" (France), "Mittelwald" (Germany), and "monte medio" (Spain). CWS involves a two-storey forest structure: an understory of coppice species harvested regularly for fuelwood and an uneven-aged overstory of standard trees grown for high-quality timber. While historical records of CWS in Germany date back to the 13th century, its actual implementation likely extended much further into the past. This study aims to use dendrochronological methods to investigate the growth patterns of oaks in currently managed CWS forests and compare them to those found in historical timbers to determine when CWS practices began.

Previous research on CWS forest management has focused on its historical and geographical distribution across medieval and post-medieval Europe, its persistence until the 19th century (when fossil fuels replaced wood as the primary fuel source), and its recent resurgence due to increasing interest in biodiversity and sustainable resource management. Much of the past research has examined coppice systems more than CWS systems, with dendrochronological studies mostly limited to abandoned stands. This study seeks to address this gap by analyzing actively managed CWS forests and their historical counterparts.

The study used tree-ring width (TRW) data from two actively managed CWS forests in northern Bavaria (Weigenheim and Welbhausen), containing a total of 161 oak standards. Tree-ring width chronologies were created, and a growth averaging (GA) analysis was used to identify release events (periods of increased growth) in individual trees, associated with understory harvesting. The average interval between release events in the modern CWS dataset was determined. This information was then used to develop a methodology for identifying CWS-related growth patterns in historical timber. The researchers then analyzed a large dataset of 2120 oak timbers from historical buildings and archaeological excavations in Bavaria and northeastern France, dating between 300 and 2015 CE. A modern reference group of oak samples from sawmills and a prehistoric reference group (before 5000 BCE) were included for comparison to control for potentially confounding factors. The GA method, which calculates the percentage of growth change (%GC) between consecutive 10-year TRW means, was employed to detect release events in individual trees. Strict criteria were applied to identify CWS signals in the historical samples: three or more consecutive release events with an average interval of 26–36 years and a standard deviation of less than 5 years. Statistical analyses were performed to assess the significance of findings, and comparisons were made between modern and historical datasets.

All 161 samples from the modern CWS dataset exhibited characteristic release events, with an average interval of 31.2 years between events. The intensity of these events decreased with tree age. Analysis of the historical datasets revealed that 12 (1.2%) of 995 samples from Bavaria and 20 (1.8%) of 1125 samples from northeastern France exhibited growth patterns consistent with CWS management. Strikingly, several of these samples dated to the first millennium CE, preceding the earliest historical mentions of CWS by centuries. The modern reference group showed only a 0.9% probability of exhibiting a CWS signal, providing support for the finding that the historical examples weren't due to random chance. None of the 115 samples from the prehistoric reference group displayed cyclical release events, confirming the method's reliability in distinguishing between natural growth patterns and those induced by human management. Positive correlations (r = 0.42, p > 0.001) existed between the Bavarian and French historical datasets, indicating the comparability of the results from both regions.

The findings suggest that CWS forest management in central Europe was practiced far earlier than previously documented. The tree-level approach allowed the detection of individual trees likely originating from CWS forests, even without detailed stand information. The identified CWS growth patterns in historical timbers provide strong evidence of the long history of sustainable forest management in Europe. The low percentage of CWS-managed oaks in the historical datasets may reflect the limitations of the approach in detecting CWS in all cases, or perhaps the localized practice of CWS. The results highlight the importance of incorporating dendrochronological analyses in future studies of historical forest management to gain a more comprehensive understanding of past practices.

This study demonstrates the potential of tree-ring analysis to identify historical CWS management systems, providing a novel method for investigating past forest practices. The growth patterns observed in modern and historical oak samples offer valuable insights into resource management and the organization of rural societies. The finding of CWS evidence predating written documentation underscores the need for further research incorporating dendrochronological analysis into broader historical and archaeological studies. Future work could focus on refining the detection method, expanding the geographical scope of the analysis, and examining the potential impact of CWS management on paleoclimate reconstructions.

The study's focus on oak standards inherently involves a selection bias, as builders preferentially selected larger, dominant trees. This could potentially affect the interpretation of the results. The sampling height in the modern CWS stands differed from standard dendroecological practices, although previous research indicates this has minimal effect on TRW ratios. The criteria for identifying CWS signals in historical samples were strict, potentially leading to an underestimation of the actual prevalence of CWS forests. Finally, while the study accounts for many possible confounding factors, the complexity of ecological interactions and the limitations of the data might still obscure certain aspects of the past forest management practices.