Testing a Novel Method to Approximate Wood Specific Gravity of Trees

Abstract

Wood specific gravity (SG) has long been used by foresters as an index for wood properties. More recently, SG has been widely used by ecologists as a plant functional trait and as a key variable in estimates of biomass. However, sampling wood to determine SG can be problematic; at present, the most common method is sampling with an increment borer to extract a bark-to-pith core—a difficult task requiring considerable physical effort and sometimes repeated borings to hit the pith, with potential to damage the trunk of small trees. Here, we test a novel sampling method that reduces the effort to obtain an adequate increment core and results in less wood extracted from the tree. The Wiemann approximation predicts the point along a radius at which the wood SG equals the disk cross-sectional SG. When SG changes linearly across the radius, the point of approximation is at two-thirds of the radial distance. As a test, we compare SG at various points along the radius with the true SG of the disk. At all points except the point of approximation (i.e., at two-thirds of the radial distance), wood SG differed significantly from the true cross-sectional mean SG, whereas at two-thirds of the radial distance, the wood SG did not differ significantly from the tree mean SG for a group of 128 tropical trees and 25 temperate trees that had been sampled bark to pith. Overall, the method shows promise in that wood SG of a tree may be estimated accurately and without bias by boring the tree only to the point of approximation, that is, one-sixth of the trunk diameter. However, boring to one-sixth of the diameter provides an unbiased estimate of SG only if radial variation in SG is a linear function of radial distance. A limitation of the method is that an initial subsample of trees must be bored to determine the pattern of radial variation in a species before the approximation can be applied to other individuals.