A new method for dating tree-rings in trees with faint, indeterminate ring boundaries using the Itrax core scanner

Eastern Australia is known to experience multi-decadal periods of flood and drought. Subtropical Southeast Queensland is one region where these devastating extreme events occur regularly yet a full understanding of their frequency and magnitude cannot be determined from the short duration (<100 years) climate data available for the region. Tree-rings are a potential source of long-term (>100 years) proxy rainfall information but locating suitable forest stands is difficult due to extensive land clearing by European settlers. Another factor deterring the use of trees as proxy data sources is that longer-lived species frequently contain anomalous rings, particularly faint rings, hindering their use for paleoclimate study. Here we present a method which overcomes the problems of identifying faint ring boundaries in trees by using X-radiographs and density patterns developed on the Itrax core scanner. We analysed 39 tree cores from 20 trees at a site in D'Aguilar National Park located just north of Brisbane city in Queensland, Australia. Each core had a 2 mm lath cut perpendicular to its rings which was then passed through an Itrax core scanner. The tree-ring boundaries were identified on the image by both the visual features in the radiograph and the change in density observed between rings. From this information we developed a tree-ring chronology. The chronology was checked using bomb-pulse radiocarbon dating on five trees to confirm the annual nature of the rings, and to correct dating errors in the chronology due to false rings which are common in this species. Climate response function analysis showed Austral annual rainfall (June-May) was the dominant environmental variable driving tree growth. Finally, a 69-year statistically significant reconstruction of Brisbane precipitation was produced showing that this non-destructive Itrax ring identification technique together with age validation by bomb-pulse radiocarbon dating is useful for dendroclimatological studies of trees with faint ring boundaries.