Evidence of an 11-year periodicity in tree-ring series from formosa related to the sunspot cycle
It has been pointed out previously that tree-ring series from Formosa show an 11-year periodicity related to the sunspot cycle. However, significance tests of this finding have not been made and therefore the statistical reliability has not been known. Spectra of tree-ring series from other sites reported before show no evidence that the 11-year sunspot cycle appears in tree-ring series. Spectral analyses have been made of published tree-ring data from Formosa and previous studies have been re-examined. Tree-ring series from two different sites in Formosa show an 11-year spectral peak at the level of confidence higher than 90%. Cross spectral analysis between the tree-ring series and the Zurich annual sunspot number shows that the 11-year fluctuations in the tree-ring series are related to the sunspot cycle with a time lag of 1/4 phase at the 99% confidence level. Using a band-pass filter technique, it is found that the 11-year fluctuations in tree-ring series correspond to almost each cycle of the sunspot number except for periods of low solar activity. Significant 11 -year fluctuations in the tree-ring series appear during most of the periods of an ‘active’ sun as estimated by carbon 14 abundance in tree-rings.