Dendroclimatic investigations and cross-dating in the 1700s: the tree-ring investigations of Johan Leche (1704–1764) in southwestern Finland

Cross-dating is considered one of the most important principles of dendrochronology. The first known cross-dating attempts, conducted between the years 1737 and 1783 by independent researchers in France, Germany, and Sweden, were related to the identification of growth rings believed to have formed during the harsh winter of 1708–1709. In this paper, we present a novel perspective on the birth of dendrochronology and cross-dating in the 1700s. We focus on the ideas and experimental studies of one of the lesser known pioneers, Johan Leche (1704–1764), the Professor of Medicine at the Royal Academy (Turku, Finland). Instead of winter harshness, Leche was fascinated by the drought that killed and damaged several trees and tree species in the summer of 1757. Leche conducted several dendrochronological and field experiments described only in his unpublished manuscript filed in the National Archives in Sweden. Reviewing Leche’s measurements of ring-width series of multiple radii and his comparisons between the radii, it seems only fair to include him in the long list of natural historians who made important contributions to the field of dendrochronology. His manuscript was never published, which is why Leche’s dendrochronological experiments and recommendations never reached a wider audience