Validating putatively cross-dated Callitris tree-ring chronologies using bomb-pulse radiocarbon analysis

Nuclear weapons testing during the 1950s generated an atmospheric pulse of the carbon isotope, 14C. Worldwide, trees growing during that period and in subsequent decades assimilated 14C-enriched CO2, leaving a distinct isotopic signature that can be used to precisely date tree rings. Thirty single-ring samples were extracted for AMS 14C analysis from cores taken from living trees of five different Callitris species [C. endlicheri (Parl.) F.M. Bailey, C. glaucophylla Joy Thomps. & L.A.S. Johnson, C. intratropica Benth., C. preissii Miq., and C. rhomboidea R.Br. ex Rich. & A. Rich] at 13 sites. The ages of individual tree rings were determined by both 14C bomb-pulse dating and cross-dating (based on 20–30 cores from the same site) in order to (1) provide independent verification of tree-ring dates, (2) detect false or missing rings from sites with otherwise good chronologies, and (3) test whether growth rings were annual for cores from sites where cross-dating was not possible. Our approach confirmed dates on chronologies from monsoon tropical sites, provided checked chronologies in subtropical and temperate sites, and improved dating control on arid-zone ring counts. It was found that Callitris are more likely to form regular annual rings when growing in seasonally dry environments than in more arid sites with highly variable precipitation patterns.