Effects of Measurement Error in Total Tree Height and Upper-Stem Diameter on Stem Volume Prediction

Abstract

In addition to dbh, total tree height and upper-stem diameter have been commonly used to improve tree stem volume estimation. However, measurement error in these variables has not been accounted for when realized gains on estimating stem volume are assessed. The aim of this study was to evaluate whether height and upper-stem diameter actually improve accuracy of volume estimation when measurement error is considered and its effect on prediction. dbh was considered to be observed without measurement error. For this analysis, a volume equation and three stem taper functions (with two algebraic constraining alternatives) were fitted and applied over four data sets of stem measurements from loblolly pine stands. Several levels of variability of measurement errors were assessed using a Monte Carlo simulation approach. These conditional predictor variables improved volume prediction when no measurement error was considered, as expected. However, as measurement error increased, the accuracy of volume estimation decreased. When the three variables were considered for volume estimation, measurement error in height caused a positive bias, whereas that corresponding to upper-stem diameter yielded a negative bias. Finally, in terms of volume prediction, inclusion of height clearly justifies the effort needed to measure it, whereas inclusion of upper-stem diameter is only recommended if its measurement error is low.