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Describing the variability of the specific gravity value of wood in trees and forests
Top height, cross-sectional area or basal area and timber volume are measures of the first, second and third dimension of trees. The wood specific gravity, ρw, value advances our understanding on the fourth dimension of trees and forests. ρw is therefore a primary variable in biomass evaluations. ρw is measured and reported as oven-dry weight divided by wet volume. The wood specific gravity is conventionally calculated from wood core samples with dimensions of 5 x 5 x 15 cm usually taken at 1.30 m and at 1/3rd of the stem cross sectional area. Non-conventional technologies such as the resistograph and near-infrared spectroscopy technologies provide also compatible wood specific gravity values for trees. However, ρw varies along the stem cross section, during the lifespan of the tree, between trees of the same species and between trees of different species. In addition to tree genetics, several environmental factors play important roles when explaining differences in wood specific gravity. This summary reports a compilation of studies reporting the variation of ρw along the tree stem and how an average value can be derived from these variations in order to quantify more precisely the different dimensions of trees and forests. Nearly 50 different studies were collected from the scientific literature and most of them did not report statistical association between ρw and the dimensions of trees. The assessment of an average ρw value shows that it differs in contrast when it is measured from the recommended place in the stem. This deviation overestimates total biomass by approximately between 5 and 10% of the total tree or forest under study. Therefore, new evaluations of ρw must be physically derived in order to evaluate the good and services provided by trees and forests correctly.