Factors influencing the accuracy of ground-based tree-height measurements for major European tree species.

Tree height is one of the most important forest characteristics and is one of the crucial measurements taken for either practical or scientific reasons. However, the accuracy of a tree-height measurement may vary in relation to many factors. The work described here thus sought to evaluate the accuracy of ground-based tree-height measurements for major forest-forming tree species of the temperate and boreal zones. The focus was on the importance of factors affecting accuracy of the measurements in question at larger geographical scales. In line with the above research goals, data were gathered from 299 stands throughout Poland and heights of 2388 sample trees of eight species, growing in different stands and site conditions, were measured; heights were then compared with measured lengths of felled trees as a reference. In total, 10 variables to determine factors that may influence ground-based tree-height measurement accuracy were used. We merged them into 4 groups: measurements, topography, stand and biometric-related factors. Results showed that biometric and topographic factors had the greatest relative influence on the accuracy of measurements of tree height. Tree length and species, followed by the slope of the terrain, tree age, and height above sea level were the most important factors found to affect accuracy. In most of the cases studied the terrestrial tree-height measurements were underestimated when set against definitive measurements of length. This was true for all species studied except oak, for which height measurements were typically overestimated. Notwithstanding the broad geographical scope of the work, the particular device used and the team factor were only found to have a marginal influence on measurement accuracy.