Removal of a non-climatically induced seven-year cycle from Nothofagus pumilio tree-ring width chronologies from Tierra del Fuego, Argentina for their use in climate reconstructions

Abstract

A pronounced seven-year cycle is one of the most prominent features of many Fuegian and southern Patagonian Nothofagus pumilio tree-ring width chronologies. However, the source of this variability is still unclear. Here we provide multiple lines of evidence that suggest this phenomenon comes from non-climatically induced ecological factors, most probably triggered by insect outbreaks. To test this hypothesis we removed the seven-year cycle, by means of Singular Spectrum Analysis, from a network of 44 tree-ring width chronologies, from the eastern Tierra del Fuego Island in Argentina. The chronologies that were filtered this way showed a stronger climatic response, and were more successfully modeled using the Vaganov-Shashkin-Lite (VS-Lite) process-based forward model of tree growth. We conclude that the removed periodicity does not have a climatic source. Additional evidence from direct observation of affected and non-affected trees during defoliation, due to Ormiscodes sp. outbreak in 2010, is provided. We also tested the hypothesis of probable massing effects on the growth of N. pumilio at a site with a 12-yr record of seed production, but this hypothesis was not confirmed. Taking into account the non-climatic nature of this signal, we explore the utility of corrected chronologies for reconstruction of climatic parameters in Tierra del Fuego, especially for summer temperature and frost day frequency. This study is the first systematic analysis of the pronounced high-frequency cyclicity in N. pumilio tree growth in southernmost South America.

A pronounced seven-year cycle is one of the most prominent features of many Fuegian and southern Patagonian Nothofagus pumilio tree-ring width chronologies. However, the source of this variability is still unclear. Here we provide multiple lines of evidence that suggest this phenomenon comes from non-climatically induced ecological factors, most probably triggered by insect outbreaks. To test this hypothesis we removed the seven-year cycle, by means of Singular Spectrum Analysis, from a network of 44 tree-ring width chronologies, from the eastern Tierra del Fuego Island in Argentina. The chronologies that were filtered this way showed a stronger climatic response, and were more successfully modeled using the Vaganov-Shashkin-Lite (VS-Lite) process-based forward model of tree growth. We conclude that the removed periodicity does not have a climatic source. Additional evidence from direct observation of affected and non-affected trees during defoliation, due to Ormiscodes sp. outbreak in 2010, is provided. We also tested the hypothesis of probable massing effects on the growth of N. pumilio at a site with a 12-yr record of seed production, but this hypothesis was not confirmed. Taking into account the non-climatic nature of this signal, we explore the utility of corrected chronologies for reconstruction of climatic parameters in Tierra del Fuego, especially for summer temperature and frost day frequency. This study is the first systematic analysis of the pronounced high-frequency cyclicity in N. pumilio tree growth in southernmost South America.