Examining the potential of Austrocedrus chilensis tree rings as indicators of past late-spring frost events in central Chile

Abstract

Austrocedrus chilensis is a South American conifer broadly distributed across the subtropical and extratropical Andes that is widely utilized in tree-ring studies. This species has clear annual growth rings that are sensitive to the moisture supply and has been extensively used to reconstruct the past hydroclimate during the last millennium. Despite a great number of dendrochronological studies based on tree-ring width, little is known about the potential of the species to record intra-annual anomalies and particularly frost rings. In this study, the main traits of A. chilensis frost rings were studied and the ability of this endemic Cupressaceae to record spring frosts at five sites across a latitudinal gradient between the Mediterranean and Northern Patagonian Andes was evaluated. The average ages of trees in the study sites varied from 168 to 343 years, with minimum and maximum ages of 33 and 919 years. The results indicated that 85% of the frost rings occurred at the beginning of the earlywood and 15% showed a mid intra-ring position. Regarding the portion of the ring circumference affected by frost damage in cross sections, 59% of the injuries partially affected the entire ring, 30% affected the complete ring circumference, and 11% resulted in a ring fracture. Freezing temperatures that generated frost rings in A. chilensis from the upper treeline coincided with events below 0 °C recorded in the agricultural Central Valley of Chile. We estimated the potential time window of the formation of A. chilensis frost rings over a two and a half month period from the end of September to mid-November (early spring). Our results indicated that tree age was a determinant factor affecting the ability of trees to record frost rings. The maximum frequency of frost rings occurred at 12 years and the maximum age at which 95% of the total frost injuries occurred within our network was about 120 years. Both the exceptional longevity and the excellent state of preservation of relict wood demonstrates that A. chilensis frost rings provide a reliable proxy for monitoring and reconstructing late-spring frost events in central Chile.