Wood density is the most extensively studied and quantified wood property as it is relatively easy to measure and correlates with most of the commercially important properties made from plantation forests. Together with pulp yield, basic density determines the mass of pulp available for paper production from a given volume of wood. Basic density is defined as oven-dry mass divided by green volume. Often basic density is referred to incorrectly as specific gravity, the latter being a dimensionless value relative to the density of water. As a relatively cheap and easy-to-measure indicator of wood quality, basic density has often been used in evaluations of the forest resource. However, extensive density analyses can still be expensive and often involve destructive sampling. Consequently there has been an increasing demand for non-destructive methods for obtaining whole-tree estimates of wood density.
Density variation occurs as a function of growth rate, climate, silviculture and breeding, and also varies within the tree. Density usually tends to increase from the centre of the tree to the bark, and from the base of the stem to the top, although this is not consistently the case. Whole-tree density is a volume-weighted average of the density at the various points within the stem. Whole-tree estimates from a single sampling point may be made if the patterns of within tree variation are known and quantified.
Variation arises from variation in the components that comprise wood. Therefore, density can vary because of changes in vessel size and frequency (hardwoods), fibre dimensions (wall thickness and diameter), percentage of parenchyma (e.g. ray) and wood chemistry. Two samples of similar density may have markedly different fibre properties. The pattern of variation of the components defining density may be different from that of density itself. For example, points within a stem where fibres have a small diameter and thin walls may have similar density to other points where fibres have a larger diameter and thicker walls. Thus two wood samples with similar densities can differ markedly at the cellular level. End users may be able to make more effective utilisation decisions if detailed knowledge of the component variables (e.g. fibre wall thickness and perimeter) is available.
For pulp and paper manufacture, wood in the basic density range of 400–600 kg/m3 is preferred. Densities above this range indicate fibres with thick walls relative to their diameter that are stiff and resistant to collapse, resulting in poor interfibre bonding and unacceptably weak paper. However, within the preferred density range, high-density wood has some advantages, such as improved digestor productivity. The chip digestor is a major capital cost in the pulp mill, and its volume often limits the rate of pulp production. At the same pulp yield, higher wood density means more weight of pulp produced per unit time.
Papers made from high-density wood often tend to be bulky, with an open structure, which is porous and more compressible, giving better printability and opacity. However, more refining is needed and paper tensile strength and smoothness can suffer. Woods at the low end of the density range have fibres that collapse more readily, forming ribbon-like shapes with greater surface area. These fibres, therefore, bond well together. Papers made from these fibres are smoother, have high tensile and bursting strengths, but low opacity, and are best suited to packaging products.