Two models for predicting the number of annual rings in cross-sections of tree stems

Abstract

The number of annual rings is an important parameter in many models for predicting stem wood and fibre properties. To develop applicable tools for characterization of properties, two models (1 and 2) for predicting the number of annual rings in cross-sections from arbitrary heights in stems were evaluated and adapted to fit trees of Norway spruce and Scots pine in Sweden. Model (1) was a new model based on a diameter measurements (over bark) at an arbitrary stem cross-section, related to diameter at breast height (over bark) and tree age at breast height (1.3 m) as explanatory variables. To establish a reference and possible alternative for application it was compared with a previously published model (2) aimed at predicting the number of annual rings at log ends, based on tree height, log end height, tree age and stem taper. Both models were fitted using data relating to 248 stems from 42 stands of Norway spruce and 116 stems from 20 stands of Scots pine. Tree age at breast height varied from 16 to 207 years for spruce, and from 18 to 129 years for pine. Both models showed great flexibility when the parameters were fitted to measurements for each individual tree. Standard prediction errors for cross-sections were estimated. For spruce, the error was 5.9 rings for model (1) and 5.8 rings for model (2). For pine, the error was 7.8 rings for model (1) and 8.4 rings for model (2). The models provide comparable results and are both useful for operative predictions of the number of annual rings. Model (1) may have advantages as it operates with tree age and two diameter measurements only (cross-section of interest and breast height) and therefore is simple to use and, for example, implement into a harvester measurement system.