Calibrating fixed- and mixed-effects taper equations

Abstract

Accurate and affordable measurements of upper-stem diameters are now possible thanks to recent advances in laser technology. Measurement of the midpoint upper-stem diameter can be employed to improve the accuracy of diameter predictions along the tree bole. Felled-tree data from a loblolly pine ( Pinus taeda L.) plantation was used to evaluate two approaches: (1) calibrating a segmented taper equation by constraining a parameter, and (2) localizing the taper model by predicting the random effects for each tree. The calibration technique is much simpler and produced less-biased prediction of diameters and is therefore recommended. Calibration results were similar for both fixed- and mixed-effects taper models, even though a slight gain in accuracy and precision was attained with the mixed-effects model.