Determining the age and growth rate of Acacia harpophylla in central Queensland, Australia, using radiocarbon in trees lacking annual growth rings

Context Determining the age of culturally, historically or ecologically significant trees in Australia is hampered by a lack of annual tree growth rings in many species. In central Queensland, land clearing has decimated primary brigalow (Acacia harpophylla F.Muell. ex Benth.) forests and very little information about the age or growth rates of old brigalow trees in remnant forests is available to guide conservation and management. Aims Determine the first quantitative estimates of age and growth rates of mature brigalow trees. Methods We used radiocarbon dating of sequentially collected wood from the bark to the pith of four sampled trees. Key results It is likely that three of the four standing-dead trees (~25 cm diameter at breast height) sampled began growing in the last quarter of the 19th century (1841–1881 Common Era (CE), ~150 years old), with one tree likely to have begun growth in the mid-20th century (1938 CE). All died in the early 21st century. The trees showed consistent growth rates of 1.3 mm year−1, and two trees also displayed short periods of rapid growth (4–8 mm year−1), which are likely to be associated with canopy openings. Conclusion Brigalow trees take up to 150 years to mature. In-series radiocarbon dates are critical in dating trees such as these, because their true age is likely to be before the bomb-spike but within the radiocarbon plateau from ~1650 to 1950 CE. Implications The lag time between tree establishment and maturity (~100 years) favours restoration that is based around assisting naturally regenerating brigalow through management rather than undertaking tree planting activities because of the inherent risk of failure owing to climate.