The impact of the volcanic double event in AD 536 and AD 539/540 on tree-ring growth and felling activity in Danish oak trees

Tree-ring chronologies around the Northern Hemisphere have revealed the formation of extremely narrow
growth rings during the mid-sixth century due to drastic climate changes caused by two or more large volcanic
eruptions in AD 536 and AD 539/540. We explored the influence of these climate changes on tree-ring growth
and felling activity of Danish oak trees (Quercus sp.) between AD 300 and AD 800. In total, 654 wooden samples
from archaeological excavations were used to create a solid tree-ring chronology on a national and regional level
(eastern and western Denmark). Tree-ring data from The International Tree-Ring Data Bank were used for
comparison of growth patterns. Furthermore, felling dates from all samples were pooled into ten-year intervals to
reveal changes in felling activity over time; a factor which reflects human activity in relation to climate changes.
The national tree-ring chronology revealed a pronounced decrease in growth in AD 536 and AD 539 (33% and
53% growth reduction relative to the level of AD 535). Regionally, the same results were found for western
Denmark, while trees from eastern Denmark showed the largest growth reduction in AD 540. Furthermore, treering
chronologies from four sample sites did not show a conspicuous growth decrease. This regional and local
variation may be due to differences in soil characteristics between eastern and western Denmark and/or local
differences in climate change impacts. Other tree-ring chronologies from the Northern Hemisphere showed
growth reduction in AD 536 and the years between AD 539 and AD 543 indicating regional differences in growth
and climate conditions as well.
A gradual decrease in felling activity was found from AD 430 and between AD 470 and AD 550 it was almost
absent. The low amount dendrochronological material from the fifth century very likely affected our results and
no effect of climate changes on felling activity i.e. changes in human activity could be linked to this event.