Tree-ring growth patterns and temperature reconstruction from Nothofagus pumilio (Fagaceae) forests at the upper tree line of southern Chilean Patagonia

Abstract

Nothofagus pumilio (Poepp. et Endl. Krasser) is a deciduous species that dominates the upper tree line of the Chilean and Argentinean Andes between 35° 36' and 55° 31' S. In this study we describe the development of 21 tree-ring width chronologies of N. pumilio across its southernmost range of distribution (51-55° S) in Chile, and analyze the relationship between N. pumilio tree-growth and climate. Ten of the tree-ring chronologies in our study, show an increasing trend and/or above average tree-ring growth since ca. 1960 to 1996, which coincides with the increase in temperatures shown by instrumental records from southern Patagonia. Another dominant pattern in tree-growth is a remarkable ca. 7-year cycle present in three of the study sites from Isla Navarino, the southernmost of our study areas (ca. 55° S). These two dominant tree-growth patterns, represent the main modes of variation of the N. pumilio tree-ring records in the southern Chilean Patagonia, accounting for 14 and 28 % of its total variance, respectively. Based on the positive correlation between tree growth and temperature in several of the study sites analyzed in this work, we developed a reconstruction of the Punta Arenas minimum annual temperatures from the seven tree-ring records with the highest correlation with temperature, covering the 1829-1996 period. The reconstruction shows that during most of the 19th century, minimum annual temperatures remained below-average and increased to values fluctuating around the mean during the 1900-1960 period, followed by a clear trend with above-average values after 1963.