The Structures of Softwoods and Hardwoods and their effect on Wood Drying
The Structure of Wood
The structure of wood influences:
influences moisture movement rates via the permeability.
Wood is anisotropic and hence the permeability is anisotropic (different in different directions - i.e. the ease of flow of liquids (“permeability”) is greatest longitudinally (up and down a tree), less tangentially (tangential to the growth rings) and much less radially (centre of trunk to bark). Longitudinal permeability can be in some tree species be up to 10000 times the transverse permeability (this is the reason why the ends of drying timber stock or logs often suffer from “star shake” – the ends have dried and shrunk too rapidly).
Pit “aspiration”. Bordered pits (in softwoods) can become closed ("aspirated") as a water meniscus moves through the pit and the torus deflects to touch and become stuck to the pit border (see diagram below). This happens more in earlywood than latewood – hence on drying wood earlywood permeability is reduced more than latewood permeability (see below).
The Structure of Softwoods
The internal structures of Softwoods are simpler and much less varied than those in hardwoods. Also more is known about softwood structure than hardwood structure because a great deal of research has been undertaken on softwoods because of their importance to the pulp and paper industries.
In softwoods flow of fluids (Note: for our purposes sap can be considered to be mainly water) is:
along tracheid cells.
cell to cell via bordered pits (visualise wood being made up rather like a bundle of drinking straws (with their ends sealed) - the straws representing the tracheid cells - each straw/ tracheid cell being connected to its neighbour by a large number of bordered pits on the tangential sides of the cells.
dependant on the condition of the bordered pits – they might be open for flow or closed for flow (called “aspirated”).
Diagram of a bordered pit on the tangential walls of a softwood tracheid (cross section)
Key:
1 - flow of a liquid from one tracheid cell to its neighbour, passing via the pit entry, through the permeable margo and leaving via the border to the pit in the next tracheid
2 - pit border
3 - pit torus - importantly when the margo and torus are deflected this can seal against the inside of the pit border, stopping movement of liquid
4 - permeable (and flexible) margo
5 - showing the central torus and permeable margo of a bordered pit
Diagrammatic representation of flow of fluid through the tracheid cells via bordered pits in softwood
In sapwood:
Green condition:
Torus is central - pits open for flow
Earlywood flow is greater than latewood flow
During drying:
The air-water meniscus moves through bordered pits. This may cause aspiration.
Earlywood bordered pits have more flexible margos and less deep “chambers” and are thus more easily aspirated.
Latewood bordered pits have more rigid margos and deep “chambers” and are thus less easily aspirated.
Thus drying reduces the permeability of earlywood more than it does latewood.
In heartwood:
Sapwood dries as it becomes heartwood.
many pits (esp. earlywood) aspirate.
margos become encrusted with extractives.
Thus the permeability of heartwood is greatly reduced.
The Structure of Hardwoods
Hardwoods have complex and varied structures.
Flow is:
Longitudinally: high in sapwood because of the occurrence of long vessels
Radially: via rays
Tangentially: complex - via vessels, pits, fibres and vertical parenchyma. Thus flow tangentially is much less than tangential flow in softwoods).
Flow through hardwoods is much less dependent on pits interconnecting the cells and so drying has less effect on hardwood permeability than on softwoods.
Flow in heartwood
Gums, resins and tyloses (remains of dead cell contents) reduce the permeability of the wood.
