Assessing dendroecological methods to reconstruct defoliator outbreaks on Nothofagus pumilio in northwestern Patagonia, Argentina

Abstract

We examined the use of dendroecological techniques for detecting past defoliations caused by Ormiscodes amphimone Fabricius (Saturniidae) in Nothofagus pumilio (Poepp. et Endl.) Krasser forests in northwestern Patagonia. We evaluated the suitability of the conifer Austrocedrus chilensis (D. Don) Pic. Serm. et Bizarri as a nonhost climate control for reconstructing Ormiscodes outbreaks on N. pumilio. Additionally, we assessed the effectiveness of three alternative procedures to detect past outbreaks: the use of a regional host chronology (instead of the nonhost chronology), the detection of pointer years (i.e., extremely narrow tree rings caused by defoliation), and the use of a morphological tree-ring signature associated with defoliation. Although differences in tree-ring growth response to climate between N. pumilio and A. chilensis did not support the use of the latter species as a reliable climatic control in most of our study area, the alternative procedures were effective in detecting past defoliation events. Based on the performance of the methods assessed here, we designed and tested a protocol for reconstructing past Ormiscodes defoliations on N. pumilio stands. Our results reinforce the need to conduct explicit comparisons of growth responses to climatic variability for host and potential nonhost species on a site-specific basis as well as the advantages of using multiple independent methods to more accurately detect past insect outbreaks.