Estimating the accuracy of tree ageing methods in mature Eucalyptus obliqua forest, Tasmania

Abstract

Estimates of tree age are important for making management decisions on the tree hollow resource because hollows suitable for fauna occur with greater frequency in older trees. The methods used to age trees vary with the practicalities of obtaining wood samples and the quality of the wood samples available. Ring counting is commonly used on smaller sample sizes when complete wood samples are available. When wood samples are incomplete, a combination of ring counting and extrapolation is often used. When no wood samples are obtained, growth models are generally used to estimate tree age. This paper uses all three methods, including three different growth models, to examine the accuracy of ageing trees. Simple regressions between tree age and diameter at breast height (over bark, dbh) provided the most accurate growth models at a site. Age estimates obtained from such models, however, had unacceptably large errors when few trees were used or when variable growth rates occurred. Under these circumstances, smaller errors margins were obtained from using a model based on tree dbh and site attributes than when averaging growth rates across sites. The estimated error in tree age estimates when using ring counting and extrapolation was about 10% of the tree age. The error of extrapolation increased with the amount of wood sample that was missing. Error margins were large for the oldest trees (average ±42.4 y for trees > 350 y old) but less than ± 15 y for most (73%) of the trees estimated to be 100-300 y of age. These middle-aged trees are often the most useful to study when examining the rate of hollow production in eucalypts. Therefore, age estimates acquired in this way are generally accurate enough to be useful for making management decisions regarding the tree-hollow resource in production forests.