Repeatability estimates for oleoresin yield measurements in three species of the southern pines
Repeatability was estimated for constitutive oleoresin yield measurements in 10 stands of three species of pines native to southeastern United States. Trees of these species that discharge large quantities of oleoresin upon wounding are considered to be most resistant to attack by southern pine beetle (Dendroctonus frontalis Zimmermann). Oleoresin yield is usually measured from one or more small surface wounds cut into the bole at breast height. Because multiple measurements per tree are possible, phenotypic variation in oleoresin yield includes among- and within-tree components. Thus it is of interest to determine the repeatability or relative contribution of variation among trees to phenotypic variation in oleoresin yield observed in populations. In the sample of stands studied, repeatability estimates were moderate to moderately high (r = 0.43–0.77), with no stands displaying low repeatability. For most stands estimates were greater than 0.5, implying that variation among trees is greater than variability within trees. These results suggest that for most purposes, two measurements per tree are adequate for estimating tree resin yields at breast height. Because initial attacks of D. frontalis are believed to most frequently occur at bole heights of 3–5 m, measurements taken on trees in two loblolly pine stands were used to develop equations for predicting oleoresin yields at a height of 4.5 m from assessments made at 1.5 m. To obtain reasonably good predictions it was necessary to develop a unique equation for each site. In addition, a new expression for approximating the standard error of repeatability estimates is introduced.