Tree rings reveal dry conditions during Charlemagne’s Fossa Carolina construction in 793 CE

The first navigable canal across the main European watershed in southern Germany was the most ambitious and visionary hydro-engineering project of the Early Middle Ages, known as Charlemagne’s Fossa Carolina. Hydrological conditions would have been crucial for both the construction process and subsequent navigability. Written sources specify that heavy rain was a crucial factor for the abandonment of the project in late 793 CE, a few months after digging began. Our understanding of past climate variability in this region is, however, limited by the lack of high-resolution proxy archives. Here, we use annually resolved and absolutely dated ring-width measurements of 2469 drought sensitive oaks (Quercus sp.) from Bavaria, Germany, to reconstruct regional changes in hydroclimate back to 500 CE. Periods of anomalously dry summers occurred from ∼600 to 800, ∼900–1000, ∼1200–1400 and ∼1700–1900 CE, whereas increased humidity characterized the 9th century, High Middle Ages (11th–12th century) and the 17th century CE. Relatively dry conditions during the construction period favoured the building progress of the Fossa Carolina. However, the heavy rain which stopped the project in 793 CE is not reflected in radial oak growth. This is likely related to i) the historically recorded event happened outside the growing season and ii) compared to dry extremes, wet extremes are generally less pronounced in tree-ring width. Nevertheless, our results provide the first local hydroclimate reconstruction covering the entire Early Middle Ages and thus, placing the Fossa Carolina in a long-term high-resolution environmental perspective.