Hardwoods And Softwoods

Commercial timbers are defined as hardwoods or softwoods according to their botanical classification rather than their physical strength. Hardwoods (an-giosperms) are from broad-leafed trees, which in temperate climates are deciduous, losing their leaves in autumn, although in tropical climates, where there is little seasonal variation, old leaves are constantly being replaced by new. Softwoods (gymnosperms) are from conifers, characteristically with needle-shaped leaves, and growing predominantly in the northern temperate zone. Mostly they are evergreen, with the notable exception of the European Larch (Larix decidua) and they include the Californian redwood (Sequoia sempervirens), the world's largest tree with a height of over 100 metres.

Although the terms hardwood and softwood arose from the physical strength of the timbers, paradoxically balsa (Ochroma lagopus), used for model-making, is botanically a hardwood, whilst yew (Taxus baccata), a strong and durable material, is defined botanically as a softwood. Under microscopic investigation, softwoods show only one type of cell, which varies in size between the rapid growth of spring and early summer (early

Fig. 4.5 Cell structure of timber (after Desch, H.E. 1981: Timber: its structure properties and utilisation, 6th edition. Macmillan Education -Crown Copyright)

wood) and the slow growth of the late summer and autumn (late wood). These cells, or tracheids, perform the food and water conducting functions and give strength to the tree. Hardwoods, however, have a more complex cell structure with large cells or vessels for the conducting functions and smaller cells or wood fibres which provide the mechanical support. According to the size and distribution of the vessels, hardwoods are divided into two distinct groups. Diffuse-porous hardwoods, which include beech (Fagus sylvatica), birch (Betula pendula) and most tropical hardwoods, have vessels of a similar diameter distributed approximately evenly throughout the timber. Ring-porous hardwoods, however, including oak (Quercus robur), ash (Fraxinus excelsior) and elm (Ulmus procera), have large vessels concentrated in the earlywood, with only small vessels in the latewood (Fig. 4.6). The Jerwood Library of Trinity Hall, Cambridge (Fig. 4.7) illustrates the visual quality of limed oak as an architectural feature within the context of a sensitive built environment.

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