Terms of ‘Softwood’ and ‘Hardwood’ – from Forest and Wood Products Research and Development Corporat

Topics Covered

Hardwoods and Softwoods

Growth

Growth Rings

Production of Wood in a Tree

Sapwood and Heartwood

Hardwoods and Softwoods

The terms ‘softwood’ and ‘hardwood’ do not indicate softness or hardness of particular timbers. In fact, some hardwoods are softer and lighter than softwoods. The main differences between hardwoods and softwoods are botanical, and relate to the way the tree grows and the timber is laid down:

•        leaves – Hardwoods have broad leaves, while softwoods are conifers and have more needle-like leaves. 

•        colour – Hardwoods often have darker coloured wood, while softwoods are invariably light in colour.  (Note that there are a number of species of hardwoods with light coloured woods.)

•        density – Most hardwoods have thicker cell walls than softwoods. Hardwoods often have higher densities than softwoods.  Again this is not a definitive test, but it does reflect most of the Australian species.

•        microstructure –The essential difference between the wood from hardwoods and softwoods is the presence of vessels in hardwoods. These are continuous pipes running the length of the tree and serve as conduits for water and nutrients in the outer layers of wood in a growing tree. In hardwoods, the cells are closed and cannot function as conduits. In softwoods, the cells have openings to other cells. This means the cells are the nutrient conduits. The actual cells in the softwood species have the same function as the vessels in hardwoods. The open cell structure of softwoods makes them generally more receptive than hardwoods to preservative treatments to enhance durability.

Softwood species - In most parts of Australia, Australian grown pine will be readily available. Also some imported species such as spruce, imported pine, douglas fir or the spruce pine fir (SPF) mix will probably also be available in a limited number of sizes.

Australian pine (seasoned and locally produced), radiata, pinaster, slash, Caribbean pines

Radiata pine (seasoned or unseasoned and imported)

Douglas fir (unseasoned and imported)

SPF (an unseasoned imported spruce/pine/fir mix)

Hem/fir (an unseasoned imported hemlock/fir mix)

Hoop pine (Queensland grown, mainly unseasoned)

Cypress (mainly grown in Queensland or NSW)

Hardwood species - The species and sizes in which hardwoods are available has much more variation with location than the species and sizes of softwood available across the nation.  The range carried in a given area will depend on what local mills are able to produce. As hardwood forestry is quite dynamic, it is very likely that availability of structural hardwoods will continue to change over time. It is imperative to check on availability before specifying any structural hardwood. Larger cross-section hardwood members (bridge timber, poles and piles) can still be obtained in some parts of Australia, but availability of specific species and grades can be restricted and long lead times often occur.

Unseasoned hardwoods, mixed species such as blackbutt, and ash-type eucalypts (Vic, Tas, SA).  Other species such as spotted gum, flooded gum, tallowwood, stringy bark (Queensland, NSW); karri (WA) messmate (Vic)

Seasoned hardwoods (mixed species) blackbutt (Queensland, NSW), and other species such as jarrah (WA).  More hardwoods are becoming available as seasoned product as saw mills move towards value added products.

Seasoned mountain ash (most readily available in Victoria, Tasmania and NSW, but also available elsewhere) may also be marketed as Tasmanian Oak, or Victorian Ash

There is a huge variety of Australian seasoned hardwood species available as flooring timber, and lining boards. The range varies from place to place but will often include Tasmanian Oak (light colour), Brush box (various shades of brown), Jarrah (brown to red) and blackbutt (honey coloured to brown).

Some seasoned hardwood is imported in joinery sizes. There is a range of Asian species that are commonly available including: Meranti, Kapur

Growth

The growth of the tree trunk happens upwards and outwards, and in general, growth is directed to maximise the light on the leaves. In a dense forest, the top will push straight upward to get to the light through the forest canopy.  Branches lower in the tree will not become established as there will not be enough light for them to flourish for long as the tree grows upwards and shades the leaves.  This gives rise to a long straight trunk with few lower branches.  The only large branches will be at the top, once the top of the tree has found its way through the canopy. Under these growth conditions, they tend to have straight trunks with few low branches.

The same tree in an open paddock will be quite a different shape. Where trees are grown in isolation, they tend to have large branches over the length of the trunk and to have substantial bends or curvature in the trunk. As the lower branches are not shaded and will receive plenty of light, they will flourish and become very large. More of the tree’s growth will be directed towards the branches, and the trunk will not need to be so tall. The tree shape will be wide, with very large branches, often the same size as the main trunk. Forest-grown timber is therefore much more suitable for use as a structural material.

The tip of the tree grows upwards, but elsewhere the only growth in the trunk is outwards.  A nail hammered into the outside of a tree will stay at the same height, even though the top of the tree will grow upwards over the years. The wood growth cells are on the very outside of the wood, so the most recently laid down wood cells are on the outside. The trunk becomes thicker by laying down new cells on the outside of the trunk. A nail in the trunk will eventually be covered by new cells and be buried in the wood of the trunk. The oldest wood at any level in a trunk is at the centre of the trunk.

Growth Rings

As a tree grows, the wood is laid down in concentric rings. Where a definite growing season exists (temperate climates) the wood laid down early in the growing season ie. spring and early summer has vigorous growth leading to larger cells with thinner walls and hence a lighter colour. The wood laid down later in the growing season when the leaves are older and sunlight or water less plentiful (latewood) has slower growth, smaller cells, thicker cell walls and, hence, a darker colour.

The growth pattern for each season gives light coloured wood closer to the centre of the tree and darker coloured wood nearer the outside. The disparity between these two types of wood gives a ring for each growing season. At least one such ring is laid down each year. In some years, sudden changes in seasons may cause the tree to believe that there are two growing seasons, and two rings will be laid down. Severe trauma (such as a bushfire) may remove most of the leaves, and the tree will grow new leaves and start a new ring.  In these cases, there will be two narrow rings laid down in one year.

Growth rings are characteristic of timber grown in temperate regions. Trees grown in tropical regions with excellent conditions for growth all through the year do not show pronounced growth rings.

Production of Wood in a Tree

A very young sapling is a small twig. The growth centre is wrapped around the outside of it and lays down new cells, one layer at a time. Once the tree has filled out, the original twig is locked inside the rest of the wood. It is quite dark in colour and is known as the pith.

The wood growth cells are in a thin layer between the bark and the sapwood. This thin layer is called the cambium. It lays down new bark on the outside, and new wood cells on the inside. As the tree grows, each layer of wood cells remains in its original position except the cambium. It is continually moving outwards.

The bark offers protection for the cambium and for the new wood.  However as the tree’s girth is continually increasing, the bark must grow to accommodate it. It sheds continually under the pressure from the new bark being created underneath.

Sapwood is the newest wood in a tree. When the wood cells are first laid down, they have very thin walls and large hollow voids in the middle. The outermost wood carry sap from the leaves to the cambium where it is used to manufacture new cells. The sap is very high in starch, and remains in the sapwood even after the wood has been milled, dried and processed. The starch makes sapwood very attractive to fungal and insect attack.

If the growing tree is injured or if insects can get to the sapwood, the tree reacts by floating the area with gum and laying down much more wood and bark to heal the damage.

As the sapwood sees service over a number of seasons, the cell walls thicken up, making them stronger, but for softwoods, less useful as conduits. Eventually they are blocked completely and in all species, the remaining void is used to store waste products from the tree growth. These waste products originate in the leaves as by-products of photosynthesis or in the cambium as by-products of cell formation. Once the cells are blocked off, they are free of starch and the waste products they now contain are known as extractives.

The mature cells blocked, and carrying extractives no longer grow in wall thickness and are now known as “heartwood” or “truewood” as this material is the wood we generally associate with timber. It has the colour of the mature wood because of the extractives. As cells are converted from sapwood to heartwood, the boundary between sapwood and heartwood is steadily moving away from the centre of the tree at roughly the same rate as new cells are being formed.

The streaked appearance of timber (sometimes called grain) is due to the growth rings.

Sapwood and Heartwood

When looking at a cross section of a log, the central wood portion is usually considerably darker than the part adjacent to the bark. The light coloured wood is called sapwood, and the remainder is the heartwood.

Darker wood is heartwood, lighter wood is sapwood

As described in Production of wood in a tree sapwood is composed of new cells that form vertical conduits for water and nutrients from the roots to the leaves. (In hardwoods they form the conduits by creating vessels with cells as a pipe wall, and in softwoods, the cells are the conduits themselves.) The walls of the hollow cells are still growing inwards, and are laden with starches for their own growth.

Heartwood is composed of cells that have stopped thickening, and the remaining spaces form receptacles for waste products (extractives). This wood is older, is more durable than sapwood, and often harder, although it is not necessarily stronger.

Heartwood appears to form the main structural support for trees and it is generally thought that the inclusion of extractives are the main reason why the natural durability of heartwood is enhanced. The amount of sapwood present in a trunk is species dependent, varying from a narrow band of 10 mm for some hardwoods to a large proportion of the cross-section for some softwoods. There are numerous theories as to why sapwood is converted into heartwood, but it seems that as the tree gets larger and matures, parts of the cross-section conducting sap and storing food reserves become redundant. The redundant sapwood is converted into heartwood that is physiologically inert, and a balance is reached whereby the starches previously stored in these sapwood cells are released to promote the production of more wood. Heartwood therefore has a much lower starch content, and the hollow cells are filled with extractives that are sometimes toxic to fungi and insects giving it much higher durability than sapwood.

Sapwood is much less durable than heartwood and often has marginally lower structural properties. The high starch content in sapwood makes it very attractive to fungi (rot-producing organisms) and insects. Some insects such as lyctus generally lay their eggs in the hollow cells. In heartwood where the cells are already full of extractives, the eggs cannot be laid and the wood-consuming larvae cannot be produced. Most heartwood is, therefore, not susceptible to lyctus attack.

Source: The Forest and Wood Products Research and Development Corporation

For more information on this source please visit The Forest and Wood Products Research and Development Corporation

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