The annual growth rings of oak trees provide researchers with information on summer droughts from late medieval times to the present
The severe epidemic of plague known as the "Black Death" caused the death of a third of the European population in the 14th century. It is probable that the climatic conditions of the time were a contributory factor towards the disaster. "The late Middle Ages were unique from the point of view of climate," explains Dr. Ulf Büntgen of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) in Birmensdorf, Switzerland. "Significantly, there were distinct phases in which summers were wetter than they are today." What exactly took place at the time can be reconstructed today by studying the annual growth rings of old oak trees. "Annual growth rings provide us with an accurate indication of summer droughts for each individual year, dating back to late medieval times," adds Professor Dr. Jan Esper of the Johannes Gutenberg University Mainz. Together with colleagues at the universities of Bonn, Gießen, and Göttingen, Büntgen and Esser managed, with the aid of the information provided by tree growth rings, to identify for the first time the summer drought periods over extensive areas of Germany in the last 1000 years. Their results have been published in the leading specialist journal Quaternary Science Reviews.
Using dendrochronology, the researchers have been able to demonstrate, for example, that a ridge beam in an old timber-framed house in the city of Kassel must have come from a tree felled in 1439. In this technique, the pattern of annual growth rings is compared with those in already dated wood samples. "We can thus determine the exact age of every beam," says Büntgen, describing the process. The ridge beam can also provide information on whether past summers in Kassel were wet or dry. "If a summer tended to be wet, the trees generally grew faster, thus resulting in wider growth rings," Esper explains. However, the information available from one beam is not enough to allow reliable conclusions about the climate in Kassel in 1439 to be reached. A large number of wood samples are required.
For their survey the researchers analyzed 953 different pieces of oak. To obtain information on the more recent past, they took wood from living trees. They also took samples from wooden construction elements of old timber-framed houses, castles, and churches, thus roughly covering the period of the last 1000 years. All construction wood samples were obtained in the north of the German state of Hesse and the south of Lower Saxony, while the living wood came from the region of the Kellerwald-Edersee National Park. "Oak trees in this area are particularly sensitive to climate change," states Büntgen, explaining why these sites were selected. The oldest wood sample used in this survey dates back to the year 996 A.D., a time when the Holy Roman Empire was just coming into being. A total of 135,000 individual growth rings were measured to obtain a detailed overview of the history of rainfall in Germany, covering major eras ranging from the optimal Medieval climate (warm and humid) through the Little Ice Age (dry and cold) to that of the Industrial Climate Change (dry and warm).
The late Middle Ages were characterized by two distinct wet periods in Central Europe in the 13th and 14th centuries, separated by dry summer weather between 1300 and 1340. "The increase in summer rainfall between 1350 and 1370 is remarkable and occurred exactly at the time when the plague broke out and spread across the entire European continent," Büntgen specifies. This was followed by a generally drier phase from the late 15th century to the early 18th century. More wet summers occured at the beginning and at the end of the 18th century, while a trend towards a drier climate has developed over the last 200 years.
"We think that our results will also be useful for historians, as it may possible to associate droughts with famines and perhaps even large-scale migration events," is the view shared by the climate researchers Büntgen and Esper. The researchers hope that collaboration between the natural and social sciences in interdisciplinary research projects will, in future, provide more information on the links between climatic and social processes of change. They themselves will be continuing their research into the Medieval plague epidemic, the Black Death.