Tree rings reveal dry conditions during Charlemagne’s Fossa Carolina construction in 793 CE
Introduction
Charlemagne, King of the Franks from 768 to 814 CE and crowned Holy Roman Emperor in 800 CE, planned the largest and most ambitious hydro-engineering project of the Early Middle Ages in Central Europe (Werther et al., 2018), seemingly anticipating the idea of the modern Rhine–Main–Danube Canal which was completed in 1992 (Gürtler and Urban, 2013). In 792/793 CE, according to recent multidisciplinary studies (Werther et al., 2019a, 2019b), Charlemagne initiated the construction of a canal to bridge the main European watershed. By joining the tributaries of the rivers Main and Danube, the so-called Fossa Carolina was foreseen to combine North Sea and Black Sea drainage basins (Zielhofer et al., 2014; Werther et al., 2015) (Fig. 1A). The selection of the optimal construction site must have required extensive topographical knowledge by means of surveys and careful planning, reflected in the ideal course (Schmidt et al., 2018) and position of the Fossa Carolina (Fig. 1B), directly at the shortest distance between the two rivers, Altmühl (Danube catchment area) and Swabian Rezat (Main-Rhine catchment area). Since these two rivers are separated by only 13m elevation (Leitholdt et al., 2012; Zielhofer et al., 2014; Werther et al., 2015), this site was particularly suitable for bridging the European watershed. At the same time, it must be acknowledged that the construction of the Fossa Carolina could have served many purposes. It may have been intended to facilitate commerce, communication and travel, as some have proposed, or to aid the movement of soldiers and military supplies for operations against the Avar confederation in the Pannonian Basin. It may also have been meant, perhaps like other large early medieval earthworks, as a “mark of authority”. Indeed, it has been proposed that the fossa had little practical application and was designed as a physical manifestation of Carolingian authority, an “assertion of territoriality”, a warning even, in a borderland (Squatriti, 2002; McKitterick 2004, 2008; Werther et al., 2019a).
It is not surprising that this major project, whatever its purpose, with direct contribution from the king is described in several contemporary written sources (Hack, 2014). The revised version of the Royal Frankish Annals (Annales qui dicuntur Einhardi, 1895) – a text written around 814–17 CE – specifies that heavy rain was one of the main reasons that the project had to be abandoned (McKitterick, 2008: 27; Hack, 2014; Nelson, 2015). Thus, if the account of the project and its abandonment encountered in the revised Royal Frankish Annals is to be believed matter-of-factly, an unexpected surplus of water apparently became a serious problem in late 793 CE (Werther et al., 2015). Nevertheless, the reviser’s account has to be treated critically (Squatriti, 2002; Hack, 2014). On the other hand, high water tables had been a precondition for navigation on the canal and the adjacent waterways. The Royal Frankish Annals and other contemporary written sources specifically describe that King Charlemagne travelled by boat to the construction site. Therefore, the small rivers North and South of the canal, which are no longer navigable today, must have had enough water to carry boats (Rau, 1993; Hack, 2014; Nelson, 2015; Werther and Kröger, 2017). Hence, local hydrological conditions were crucial not only for the construction process and the abandonment of the project, but also for the navigability of the entire fluvial network. The only way to provide thorough understanding of the local hydroclimate in Early Medieval times, unbiased by human interference (e.g. from propaganda), is an independent reconstruction based on tree rings as annually resolved climate proxy data.
Archaeological excavations in 2013 and 2016 focused on this extraordinary example of early medieval hydro-engineering. Three sections of the canal have been excavated, each 2m wide. Therefore, 6m of the total length of 2.9 km have been fully documented and provided several well-preserved wooden structures (Fig. 2) (Herzig and Werther, 2014; Werther et al., 2015; Werther, 2016). Although the relatively high number and quality of oak (Quercus sp.) timbers enabled dendrochronological dating and chronological comparison to the written sources, a careful dendro-climatological assessment of the excavated wood samples had not yet been performed. This study includes the construction timber of the Fossa Carolina into a combined local dataset of living and historical oaks and provides new insights into the local hydroclimate before, during and after the historical construction.
Variation in tree-ring width (TRW) of oaks from lower elevation sites is mainly driven by hydroclimatic spring – early-summer conditions (e.g. Büntgen et al., 2010). Aside from precipitation, temperature is an important factor contributing to hydroclimatic conditions. Combined drought indices like the Standardised Precipitation-Evapotranspiration Index (SPEI) (Vicente-Serrano et al., 2010) and the self-calibrating Palmer Drought Severity Index (scPDSI) (Van der Schrier et al., 2006) take this into account. With the exception of a 2000-year precipitation reconstruction for the Main region (Land et al., 2019) and the spatially explicit Old World Drought Atlas (OWDA), for much of the Common Era (CE) (Cook et al., 2015), none of the existing regional TRW-based hydroclimatic reconstructions from Central Europe (e.g. Oberhuber and Kofler, 2002; Wilson et al., 2005; Čufar et al., 2008; Büntgen et al., 2010; Dobrovolný et al., 2018) covers the Carolingian period (8th – 10th century CE).
Here, we provide a hydroclimatic reconstruction for the area surrounding the Fossa Carolina. We present the first regional oak TRW chronology for the Franconian Keuper and Jura region in northern Bavaria, which continuously spans the past 1500 years. Our new record is compared with other hydroclimate reconstructions for Central Europe (Wilson et al., 2005; Büntgen et al., 2010, 2011; Dobrovolný et al., 2018; Land et al., 2019) including the OWDA (Cook et al., 2015). Special consideration is given to the hydroclimatic conditions at the time of construction of the Fossa Carolina in order to unravel the extensively discussed reasons for its abandonment (recapitulatory Werther et al., 2015). In so doing, our study provides new impetus to the ongoing discussion on Charlemagne’s hydro-engineering project and sheds new light on the perception of the event in historical sources. Furthermore, it stimulates innovative regional studies and allows further steps in historical, archaeological and paleoenvironmental research.
Section snippets
Regional setting
The archaeological site of Fossa Carolina is located approximately 3 km north-east of the town of Treuchtlingen in northern Bavaria (48.99°N, 10.93°E and 411m above sea level (asl)). This rural area of Southern Germany belongs to the eastern part of the geological region of the South German Scarplands. The area is characterized by a temperate oceanic climate (Cfb-climate according to the Köppen-Geiger climate classification system, cf. Peel et al., 2007). The annual mean temperature is 8.2 °C
Tree-ring data
For this study we compiled a dataset of 2469 dated oak TRW series and developed a chronology covering 1633 years for the period 383–2015 CE. The dataset is composed exclusively from regional oak (Quercus sp.) samples (Fig. 1C) including 28 excavated timbers from the Fossa Carolina construction. A total of 117 living trees are used to update the chronology into the present whereas the majority of the samples (n = 2352) were derived from historic buildings, archaeological and subfossil material
Regional oak chronology
We developed a 1516-year long regional chronology for oak covering the period 500–2015 CE for the two forest ecoregions Franconian Keuper and Jura (Fig. 3C). The average growth rates are similar for the living and historical subsets whereas the mean segment length differs considerably justifying the split detrending approach. The recently sampled living trees show a radial growth of 1.57 mm per year on average thereby reaching mean ages of 135 years. In contrast, the historical material has an
TRW chronology characteristics
The regional TRW chronology covers the last ∼1500 years with sufficient replication except for two periods in the late 9th century and during 1750 and 1850 CE at the transition between historical and recent oak material (Fig. 3E). The lower sample depths lead to a decrease in EPS shortly below the threshold of 0.85. For the latter period, material from living old trees in combination with timber from modern period constructions might be accessible. For the late 9th century, wooden remains from
Conclusions
Hydroclimatic conditions would have been crucial for Charlemagne’s Fossa Carolina, which effectively would have bridged the European watershed in southern Germany at the end of the 8th century. This study presents a new regional oak TRW chronology for the Franconian Keuper and Jura, providing proxy data for hydroclimatic variations back to 500 CE. The hydroclimate reconstruction produced for the area surrounding the Fossa Carolina reflects dry and thus, favourable conditions for the
Acknowledgements
Financial support was provided by the German Research Foundation (DFG) for AS (SE 2802/1-1) and WT (DFG, TE 613/3-2). UB received funding from the project “SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions” (CZ.02.1.01/0.0/0.0/16_019/0000797). LW, JS, CZ were supported in the scope of DFG priority programme 1630 (ZI 721/10-2, ET 20/7-2).
The authors thank Emily Lippert, Chair of Forest History, University of Freiburg, for
References (91)
- et al.
Tree-ring indicators of German summer drought over the last millennium
Quat. Sci. Rev.
(2010) - et al.
Provenancing Baltic timber from art historical objects: success and limitations
J. Archaeol. Sci.
(2005) - et al.
Assessing seasonal drought variations and trends over Central Europe
Adv. Water Resour.
(2019) - et al.
Towards the extremes: a critical analysis of pointer year detection methods
Dendrochronologia
(2019) - et al.
Climate-growth analysis using long-term daily-resolved station records with focus on the effect of heavy precipitation events
Dendrochronologia
(2017) - et al.
Linking European building activity with plague history
J. Archaeol. Sci.
(2018) Baltic timber in Western Europe – an exciting dendrochronological question
Dendrochronologia
(2002)- et al.
A new instrumental precipitation dataset for the greater Alpine region for the period 1800–2002
Int. J. Climatol.
(2005)
Historical hydrology for studying flood risk in Europe
Hydrological Sciences–Journal–des Sciences Hydrologiques
Reconstructing summer temperatures in northern Fennoscandinavia back to AD 1700 using tree–ring data from Scots pine
Arctic Antarct. Alpine Res.
Is there memory in precipitation?
Nat. Clim. Chang.
The little Ice age in scientific perspective: cold spells and caveats
J. Interdiscip. Hist.
Cooling and societal change during the late Antique little Ice age from 536 to around 660 AD
Nat. Geosci.
2500 Years of European climate variability and human susceptibility
Science
Methods of Dendrochronology—Applications in the Environmental Sciences
Calculating unbiased tree-ring indices for the study of climatic and environmental change
Holocene
Spatial regression methods in dendroclimatology: a review and comparison of two techniques
Int. J. Climatol.
The ‘segment length curse’ in long tree-ring chronology development for palaeoclimatic studies
Holocene
Program ARSTAN; Version 48d2, Autoregressive Tree-Ring Standardization Program
Old world megadroughts and pluvials during the Common Era
Sci. Adv.
Reconstructing dry and wet summers in SE Slovenia from oak tree-ring series
Int. J. Biometeorol.
Das Zeitalter der Karolinger
Spatial and temporal characteristics of climate in medieval times revisited
Bull. Am. Meteorol. Soc.
A tree-ring perspective on temporal changes in the frequency and intensity of hydroclimatic extremes in the territory of the Czech Republic since 761 AD
Clim. Past
May–July precipitation reconstruction from oak tree-rings for Bohemia (Czech Republic) since AD 1040
Int. J. Climatol.
The Maunder minimum
Science
Tests of the RCS method for preserving low-frequency variability in long tree-ring chronologies
Tree-Ring Res.
Effect of scaling and regression on reconstructed temperature amplitude for the past millennium
Geophys. Res. Lett.
Complex climate controls on 20th century oak growth in central-west Germany
Tree Physiol.
Regensburg, Wandalgarius and the novi denarii: charlemagne’s monetary reform revisited
Early Mediev. Eur.
Waldökologische Naturräume Deutschlands – Forstliche Wuchsgebiete und Wuchsbezirke – mit Karte 1:1.000.000, Mitteilungen des Vereins für Forstliche Standortskunde und Forstpflanzenzüchtung 43
Der Rhein-Main-Donau-Kanal – Idee, Geschichte und Technik
Der Bau des Karlsgrabens nach den Schriftquellen
CRU TS4.01: Climatic Research Unit (CRU) Time-Series (TS) Version 4.01 of High-Resolution Gridded Data of Month-By-Month Variation in Climate (Jan. 1901- Dec. 2016)
Stand der dendrochronologischen Auswertungen Greding-ICE-Trasse Oktober 2004
Beiträge zur Archäologie in Mittelfranken
Dendroarchäologie: Mensch und Umwelt – eine Wechselwirkung, eingraviert in Holz
Berichte der Bayerischen Bodendenkmalpflege
Der Karlsgraben im Fokus der Dendroarchäologie
Mitteleuropäische Eichenchronologie. Trierer dendrochronologische Forschungen zur Archäologie und Kunstgeschichte. (= Trierer Grabungen und Forschungen 11)
Mainz: von Zabern.
Computer-assisted quality control in tree-ring dating and measurement
Tree-Ring Bull.
Enhanced growth after extreme wetness compensates for post-drought carbon loss in dry forests
Nat. Commun.
Fachwörterbuch der Flößerei. Bemerhaven: Deutsches Schiffahrtsmuseum
Langzeitverhalten der Lufttemperatur in Baden-Württemberg und Bayern. KLIWA-Projekt A 1.2.3 “Erarbeitung und Bereitstellung von langen Reihen interpolierter Gitterpunktwerte (Tageswerte) und Analyse des Langzeitverhaltens von Gebietsmittelwerten der Lufttemperatur in Baden-Württemberg und Bayern”. KLIWA-Berichte 5
A 1286-year hydro-climate reconstruction for the Balkan Peninsula
Boreas
Cited by (7)
Climatic signatures in early modern European grain harvest yields
2023, Climate of the PastHistorical Forest Management Practices Influence Tree-Ring Based Climate Reconstructions
2021, Frontiers in Ecology and EvolutionClimate and society in European history
2021, Wiley Interdisciplinary Reviews: Climate ChangeSediment budgeting of short-term backfilling processes: The erosional collapse of a Carolingian canal construction
2020, Earth Surface Processes and Landforms