Discovery of the 5.7-year Douglass cycle: A pioneer's quest for solar cycles in tree-ring records

The astronomer A.E. Douglass is generally recognized as the founding father of dendrochronology. He studied tree rings in the search for evidence that solar variation (as seen in sunspots) is reflected in climate variation. He was convinced that his quest was successful. Analysis of some of his early data using Fourier decomposition and comparison of tree-ring periodograms with those based on known solar cycles suggests that the cycles he found may not exist or may not be of pure solar origin. The findings here reported suggest a much stronger influence of tides on the tree-ring records than commonly considered. Douglass’s great merit as the pioneer of tree-ring dating in archeology and tree ring- based climatology remains unaffected by the findings here presented.