Evidence of Solar Activity and El Niño signals in tree rings of Araucaria in Argentina, Brazil and Chile

Temporal shifts in the life cycle of trees are seen as indicators of global warming with potentially severe impacts on ecosystem functioning. Tree rings record past climatic variations through years and the effect of solar activity on tree growth, because of the impact on climate conditions. To investigate the role of solar cycle on climatic variability and analyze the relationships between the biosphere and solar activity we used tree ring chronologies of Araucaria angustifolia and Araucaria araucana. Tree cores and sections were collected at Irati and Curitiba in Brazil, Caviahue in Chile, and Tolhuaca in Argentina. We obtained an average tree ring chronology of approximately 289, 117, 439, and 849 years for these areas, respectively. The tree ring chronologies of 439 and 849 years included the period of the Maunder and Dalton minima. The time series were analysed using spectral, wavelet and cross wavelet techniques to identify periodicities and trends contained in tree growth. Analysis based on the Multitaper method of annual growth rate identified two cycles with periodicities of 11 (Schwebe cycle) and 5.5 years (2nd harmonic of Schwebe cycle). Good agreement between the time series of tree rings of Chile and Argentina and the 11-year solar cycle was found during the periods of maximum solar activity. Our results also showed variation with periods of 2-7 years. We interpreted this as a response of tree radial growth to local environmental condition, and a possible influence of El-Niño events. The Morlet complex wavelet analysis was used to study the most important variability factors on temporal scales, and their stability in time, as shown in all time series studies. We also applied the cross-wavelet spectral analysis to evaluate time lags between tree ring and sunspot number time series, tree ring and Southern oscillation Index and temperature/precipitation. Chile and Argentina showed more significant responses of fluctuations in tree ring time series to variations of short and long periodicities in comparison with Brazil. These results provided new evidence on the solar activity-climate pattern-tree ring connections over centuries.