Three scales of variation in fiber geometry

Variation of Norway Spruce (Picea abies) fiber geometry is studied in three different scales, the geometry being described with two dimensionless variables. Oblongness is a measure of fiber slenderness, contributing to the mechanical behavior of a fiber network. Compactness is a measure of cross-sectional geometry, contributing to the flexibility and collapsibility of the fiber.

Within individual fibers, compactness is invariant along fiber length. Cross-sectional cell-wall area varies significantly along the length of the fiber. The change rate of wall area is greatest in the vicinity of the edges, but no plateau is found in the middle of the fiber. Within small wood specimens, consisting of a few annual rings, fiber compactness varies greatly, being lowest in earlywood and highest in latewood. The variation of oblongness is less pronounced. Within a tree, oblongness increases from the pith of a tree towards the bark. Fiber compactness mainly depends on growth rate: the higher the growth rate, the lower the compactness. The total variation of fiber compactness is dominated by the variation within an annual ring. Thus, it can be most effectively manipulated through hydrodynamic fractionation. Oblongness can be controlled by wood classification, by screening, or by combining these techniques.