Frost record in tree rings linked to atmospheric circulation in northern Patagonia

Abstract

Cold air incursions over subtropical South America are the precursor conditions for frost, a recognized extreme thermal weather event affecting plant growth damaging agricultural production over the whole Argentinian territory. Given that sub-freezing temperatures occurring during the active growing season may harm the cambium tissues and their daughter cells, frost injury can be recorded in annual tree rings in the form of anatomical anomalies. For this study, six forest sites of Araucaria araucana, a multi-centennial tree species, from NW Patagonia were considered. From 321 trees, 1374 frost injuries in the growth rings, named frost rings, were identified and their calendar dating allowed the development of a regional chronology of frost rings covering the period AD 1256 to AD 1993 (738 yrs.). This represents the longest record at present of extreme low thermal events for Patagonia. Frost injuries were mostly restricted to the middle section of the growth rings, suggesting the incidence of late spring frosts. Moreover, frost rings were observed mainly in the juvenile portions (<50 yrs. of age and <6 cm in stem diameter) of the tree stem, indicating that young trees display a greater sensitivity to frost events. Large-scale geographical freezing events were recorded in the years AD 1889, 1916, 1941 and 1948. The atmospheric genesis of these events was sustained by a trough at 500 hPa approaching from the west toward the continent, along with an approach of the Pacific anticyclone at 1000 hPa which invaded the southern part of South America on the day of the frost event. At the hemispheric scale, it was found that atmospheric circulation patterns related to La Niña were associated with the regional frost ring record in A. araucana. The regional frost ring record from A. araucana is proposed as a regional proxy of sub-freezing temperatures in paleoclimate reconstruction and as a background for models about the future behavior of the climate in scenarios of change.