Assessing Precision in Conventional Field Measurements of Individual Tree Attributes

Ville Luoma,Juha Hyyppä,Joanne White,Ninni Saarinen,Markus Holopainen,Mikko Vastaranta,Michael Wulder

doi:10.3390/f8020038

Abstract

Forest resource information has a hierarchical structure: individual tree attributes are summed at the plot level and then in turn, plot-level estimates are used to derive stand or large-area estimates of forest resources. Due to this hierarchy, it is imperative that individual tree attributes are measured with accuracy and precision. With the widespread use of different measurement tools, it is also important to understand the expected degree of precision associated with these measurements. The most prevalent tree attributes measured in the field are tree species, stem diameter-at-breast-height (dbh), and tree height. For dbh and height, the most commonly used measuring devices are calipers and clinometers, respectively. The aim of our study was to characterize the precision of individual tree dbh and height measurements in boreal forest conditions when using calipers and clinometers. The data consisted of 319 sample trees at a study area in Evo, southern Finland. The sample trees were measured independently by four trained mensurationists. The standard deviation in tree dbh and height measurements was 0.3 cm (1.5%) and 0.5 m (2.9%), respectively. Precision was also assessed by tree species and tree size classes; however, there were no statistically significant differences between the mensurationists for dbh or height measurements. Our study offers insights into the expected precision of tree dbh and height as measured with the most commonly used devices. These results are important when using sample plot data in forest inventory applications, especially now, at a time when new tree attribute measurement techniques based on remote sensing are being developed and compared to the conventional caliper and clinometer measurements.

Highlights

Tree species, diameter-at-breast-height, and height are the most common individual tree attributes measured or determined in the field [1,2]
Four individual mensurationists each measured the same 319 trees in different plots using calipers and clinometers in order to characterize the precision of traditional field measurements of dbh and height
It was possible to obtain greater precision in dbh than height measurements, while Norway spruce was measured with greater precision than Scots pine or birch

Summary

Introduction

Diameter-at-breast-height (dbh), and height are the most common individual tree attributes measured or determined in the field [1,2]. Several studies have proved that exact circularity is rare and the form of a cross-section is more like a closed convex [4,5,6,7]. The most common methods for the field measurements of dbh are the use of calipers or diameter tapes. Calipers are often used and preferred for dbh measurements in temporary plots or when measuring a large number of trees, as they are quick and efficient to use [9]. The dbh is measured by placing the two arms of the caliper on perpendicular sides of the tree at the height of 1.3 m. When the two arms are pressed together against the tree trunk, a 90◦ angle must be formed with the scale beam.

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