WoodSolutions Teaching Resource Growth of Timber

Timber the Material Teaching Resource Timber, the Material teaching resource package provides an introduction to timber. This resource covers a variety of topics and is suitable for inclusion in Civil Engineering, Architecture and Building courses at university level, and a large number of courses in the Building and construction streams at Trade Training level. The series of PowerPoint files can be downloaded and used in your classes. Go to www.education.WoodSolutions.com.au for up to date teaching resources.

GROWTH OF TIMBER Timber Grows on Trees! Stores carbon Sustainable Planting to meet demand Recyclable Selection of product Species of Timber Type of Processing Seasoning Treatment Radiata pine plantation Photo: PTAA The module introduces the concept that growing trees have characteristics that affect the type and quality of the timber that comes from them. Likewise the removal of timber from a forest has to happen one day, and the use of it in buildings can prove beneficial to the environment. Photo: Geoff Boughton

GROWTH OF TIMBER Hardwoods and Softwoods broad leaf generally higher densities often dark in colour Softwoods needle-like leaves generally lower densities often light in colour We make a distinction between hardwoods and softwoods. From the point of view of the user of the timber, little distinction is required. 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 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 Australian species of hardwoods with light coloured woods.) Density – Hardwoods often have higher densities than softwoods. Again this is not a definitive test, but it does reflect most of the Australian species. Microstructure – Hardwoods have separate vessels that act as conduits to take nutrients through the tree. Softwood cells are open and carry the nutrients through the tree. Australian hardwoods include: Mountain Ash Brown Box Spotted Gum Jarrah Australian softwoods include: Radiata Pine Cypress Pine Hoop Pine Photo: Geoff Boughton Photo: Geoff Boughton

GROWTH OF TIMBER Tree Growth New wood on outside of tree oldest wood on inside of tree youngest wood on outside diameter largest at base one ring (layer) per year Trees in forests grow towards light trunk straight lower branches die leaving small knots in wood Bark protects wood from damage tree sheds bark each year Tree Growth The growth of the tree trunk happens upwards and outwards. The tip of the tree grows upwards but elsewhere the only growth is outwards. A nail hammered into the outside of a tree will stay at the same height, even though 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 oldest wood at any level in a trunk is at the centre of the trunk. Tree growth is oriented towards light: 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. 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. The same tree in an open paddock will be quite a different shape. As the lower branches 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 will be wide, with very large branches, often the same size as the main trunk. The forest tree will be quite appropriate for timber. The tree in open country will not be, because it will have a short trunk with many large knots.

GROWTH OF TIMBER Production of Wood Sapling - fibrous juvenile wood becomes pith Production of wood 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.

GROWTH OF TIMBER Production of Wood Pith - the start of growth in the tree the original sapling Cambium - growth cells wood cells created on inside bark cells created on outside Bark - protects wood new bark made continually bark falls off each year Wood new wood cells on outside newest wood takes nutrients to leaves The tree starts laying down wood on the outside of the pith through the cambium (shown in green on the slide) and the new wood (sapwood) (shown in light colour.) At the same time a layer of bark appears on the outside of the cambium. (shown as very dark) The wood growth cells are in a thin layer between the bark and the sapwood. This thin layer is called the Cambium (shown as bright green on the slide). 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 That is moving outwards continually The bark of a tree is being laid down on the outside of the cambium while wood is being laid down on the inside. 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.

GROWTH OF TIMBER Production of Wood Cambium - growth cells wood cells created on inside bark cells created on outside Sapwood - newest wood on outside of tree (~ 1-3 cm) takes nutrients from root to leaves Heartwood - older wood cells closed – can’t pass nutrients storage for waste - extractives Production of wood This slide shows the next step in the development of the tree. The tree is growing bigger. The diameter has increased, by new wood being laid down on the outside. The cambium is still on the outside of the tree, and the first wood that was laid down has now been converted to darker heartwood. 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 cells carry sap from the leaves to the cambium where it is used to manufacture new cells. The sap is very high in starch. This makes it very attractive to fungal and insect attack. If the 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.

GROWTH OF TIMBER Production of Wood Sapwood - newest wood New wood has thin cell walls Most effective on outside Heartwood - older wood Oldest sapwood becomes clogged New heartwood added on outside Production of wood This slide shows another step in the growth of the tree. More wood has been laid down on the outside and the previous sapwood is now heartwood. The tree now has more heartwood than sapwood. The bark continually peels and is replaced by new bark on the outside. The text describing pith, cambium remains unchanged, text for sapwood and heartwood changes,) As the sapwood sees service over a number of seasons, the cell walls thicken up, making them stronger, but also less useful as conduits. Eventually they are blocked completely and 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. These cells 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. The boundary between sapwood and heartwood is steadily moving away from the centre of the tree at the same rate as new cells are being formed. Cells are converted from sapwood to heartwood.

GROWTH OF TIMBER Growth Rings Early wood rapid growth at beginning of growing season thin, large cells lighter colour Late wood slower growth often limited by lack of water small thick cells darker colour gives the growth ring Growth rings The streaked appearance of timber (sometimes called grain) is due to the growth rings. The slide has a number of pictures of tree cross-sections showing growth rings. They 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. The growth rings are due to differences in the way the tree lays down wood cells early in the growing season when the leaves are new, compared with later in the growing season when the leaves are older. Photos: Geoff Boughton

GROWTH OF TIMBER Growth Rings Generally one ring per year some climates may have more than one growth season per year fires or disease may produce an extra ring give texture and figure to wood Growth rings Each year, the growth is most vigorous near the start of the growing season (ie spring and early summer), and slower at the end of the season (late summer and autumn). The cells are larger at the beginning of the season and smaller at the end of the season. The larger cells give lighter coloured wood, and smaller cells give darker coloured wood. The growth pattern gives a growth ring with light coloured wood closer to the centre of the tree and darker coloured wood nearer the outside. 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. Photo: Geoff Boughton

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