Forest resources and products

10.1.1 Timber production and trade: loss of natural habitat

Forestry not only comprises the art and science of initiating, regenerating and cultivating woodlands and forests (silviculture, see Section 10.3.1), but also all the practical procedures involved including road construction and maintenance, felling and selling timber, control of deer and other animal populations. The main product of commercial forestry is wood, which is employed as a fuel, a most important sustainable raw material, the basis of the paper industry, and for at least 10 000 other uses (Sutton, 1999). The economics of plantation forestry in particular are complex; huge amounts of capital are involved in the ownership of the ground, and the costs of establishing a particular crop are not recovered for many years. Whereas a bakery will sell its products within a day or two, it is only in the case of the very fast-growing introduced tree species that a forester has a chance of cropping the trees he plants within his working life. The impact of the compound interest of establishment and maintenance costs on the industry is very great indeed, while the value of particular timbers varies widely with time. In the past huge areas of natural forest have been virtually plundered; exploited and cut down without regard for the long-term consequences, a process often called 'mining', that is treating forests as a non-renewable resource. Half of the world's forest growing 8000 years ago has already been lost. On top of this, however, there has been a loss of quality of much of what remains. Over half of what we have left is badly degraded and fragmented and only 20% of the original forest cover remains intact and in large tracts (Bryant et al., 1997). The area of truly natural (= old-growth) forest in the world is now substantially less than that; indeed virtually no entirely natural woodland survives in many industrialized countries. That which remains is highly valued; it represents one of the world's most important ecosystems, has great biodiversity, while its stored carbon may slow the development of global warming (Section 11.3). By far the greatest remaining natural forest area is that of Amazonia where 80% of logging is at present illegal and the total area of the forest is, after a period of reasonable stability, again shrinking at a considerable rate with much of the land being used for rearing cattle or growing crops. These losses are a tragedy in that the plundered areas are unlikely to recover in the foreseeable future, whereas the 20% of legal logging, in which selected mature trees are immediately replaced by appropriate young plantings, allows the forest to maintain itself. The area lost in 2003 was over 2.3 million ha (see Fig. 11.1); much of it was subsequently used to grow non-GM soya beans for the European market.

In some areas natural forests are simply plundered for their finest and tallest trees, often taking out a very small percentage of the trees present, but leaving an impoverished ecosystem that may never fully recover. Studies of the effects of such selective logging in Amazonia, Africa and South-East Asia, have compared animal populations in undisturbed primary forest with those in which only the valuable species have been removed. Despite differences in the flora and fauna across such vast distances, results of these studies are remarkably similar (Terborgh, 1992). Disruption of the forest canopy caused a decline in numbers of frugivorous (fruit-eating) birds and primates, particularly the larger ones. This probably results from the loss of important food resources. Numbers of leaf-eaters (foliovores) are often unaffected and may even increase when the regenerating vegetation provides more young leaves. On occasions when commercially valueless fig trees were poisoned an essential part of the diet of many rain-forest birds and mammals was lost.

When a particular West Malaysian dipterocarp forest was harvested only 3% of the trees were extracted, yet Johns (1988) found that 51% of the trees were destroyed as 5% were pushed aside in road building and 43% injured or killed during felling and timber dragging. Canopy height was lowered and much debris remained. While the harvested dipterocarps were not important in primate nutrition, loss of other species reduced fruit production although young foliage continued to be abundant. In other cases the harvested trees have previously provided food for primates.

Former long-distance trade in timber was much greater than is often realized. Recent dendrochronological investigations of the roof timbers used in the construction of Salisbury cathedral in southern England, whose foundation stone was laid in 1220, show that many beams came from trees which were more than 300 years old when felled in the area around Dublin, Ireland. Timbers were used green and some still have their bark on, so that both the year and season of felling can be determined; the earliest date from the spring of 1222. The cathedral was built with extraordinary speed, being completed in 1258, and when the spire was added it was at 404 feet the tallest medieval structure in the world.

At the present time wood is often transported from one side of the world to the other and much of the trade is now sustainable (Section 10.6). The profitability of any timber crop is determined by five factors, of which two of the most important are average price per unit volume and the length of rotation required to reach a given volume. The three others are the quantity which can be sold; the costs of production including tree stocks, planting, thinning, pruning and logging; and the risks which involve management, marketing, disease and climate (Maclaren, 1996). Length of rotation is particularly important when paying compound interest on money borrowed to pay for planting and preparation costs. Nevertheless slow-growing trees, for example, in northern Sweden can still be profitable providing the value of timber is not too low.

10.1.2 Timber quality and seasoning

Timber quality varies with speed of growth and the conditions under which trees are grown; its nature varies in the different plant groups. Timbers are usually classified as softwoods coming from conifers (gymnosperms) and hardwoods from broadleaved trees such as oak and beech (angiosperms). It should be noted that these are commercial timber expressions since 'softwoods' range in density from 380 kg m~3 in western red cedar Thujaplicata to 672 kg m~3 in yew Taxus baccata, and in 'hardwoods' from 140kgm~3 in balsa Ochroma pyramidale to 1280-1370 kg m~3 in lignum vitae Guaiacum officinale. (Water has a density of 1000 kg m~3 or a specific gravity of 1.) The strength of timbers depends upon their structure and the speed of their growth. As noted in Chapter 1 most timbers contain concentric rings which in seasonal climates outside the tropics are produced annually. These annual rings are composed of earlywood grown in the spring and latewood grown in the summer. In the softwoods water conduction up through the wood is by means of thin-walled tracheids, hollow cells with valves called pits. The earlywood tends to be low density (wide tracheids with thin walls) and the latewood denser (narrow tubes with thick walls). Conduction in the hardwoods of angiosperm trees is through pipe-like collections of hollow vessels. These vessels are organized within the annual rings in two patterns. In ring-porous hardwoods such as that of the oak there is every year a conspicuous line of very large earlywood vessels which contrast with the numerous small vessels found in the latewood zone. Diffuse-porous hardwoods such as the beech have numerous small vessels distributed relatively evenly. Rapidly grown softwoods such as pine have a proportionately wider low-density earlywood so to get strong timber from pine it must

FORESTERS'

IMPROVE SITE

Cultivation Drainage

Fertilizer application

OPTIONS

OPTIONS

MANIPULATE CROP

Planting distance Planting pattern Respacing Pruning Thinning Rotation age

Actions which influence

GROWTH OF CROWN AND ROOTS

which determine

Stem size/size class distribution Stem form

Ring structure/wood density etc. Uniformity of growth Branch/knot size and number Sapwood thickness Amount of juvenile wood Compression wood development

Figure 10.1 Ways in which foresters can influence tree growth and timber development. (From Brazier, 1979. Information Paper IP 12/79. Building Research Establishment. Reproduced by permission of BRE.)

be grown slowly. Conversely, and counter-intuitively, rapidly grown ring-porous timber such as oak has proportionately wider dense latewood and so provides strong hard timber. The major ways in which foresters can influence the growth of trees and the timber they contain are illustrated in Fig. 10.1.

Turning trees into well-seasoned strong and stable timber is a complex and painstaking process well described by Sherrill (2003), who points out the potential dangers faced by anyone who is trimming or felling trees, particularly if these are old and diseased as is not uncommon in urban parklands. Once the trunk and any major branches are converted into logs of convenient length, the wood is transported to the mill and converted into planks, beams or even veneer. Milling for maximum usable volume is a science in itself, while Fig. 10.2 demonstrates how the distortion during shrinkage associated with seasoning is related to the fundamental structure of the tree. In the sequence C-B-A the end-grain of the boards becomes increasing perpendicular to their faces with the result that the board is less likely to cup. Board C cups because it is more flat sawn on the bark side than on the centre side. Board A is perfectly quarter sawn, the grain is perpendicular and it should remain flat throughout a very long life. Though E is mainly quarter sawn, it contains the heart and may well warp and split right through the pith in the centre. G is a rift-sawn beam

Grain Equilibrium Moisture Content

likely changes in shape following drying out. Distortion continues until the boards come to EMC (equilibrium moisture content). This does not remain completely constant but varies with the moisture content of the surrounding environment being higher in rainy and humid periods. (From Forest Products Laboratory, 1999. Wood Handbook, Wood as an Engineering Material,General Technical Report, FPL-GTR-113, US Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, pp. 3-8.)

likely changes in shape following drying out. Distortion continues until the boards come to EMC (equilibrium moisture content). This does not remain completely constant but varies with the moisture content of the surrounding environment being higher in rainy and humid periods. (From Forest Products Laboratory, 1999. Wood Handbook, Wood as an Engineering Material,General Technical Report, FPL-GTR-113, US Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, pp. 3-8.)

which takes on a slight diamond shape, while F shows how a spindle or dowel changes shape as it dries.

Trees, logs and sawn timber are all influenced by stain fungi. Sap stain is also known as blue stain, though its colour may vary from blue to brown or black depending on the species of wood-inhabiting fungus and the type of tree in which it is growing. Sap stain is generally caused by three groups of fungi (Breuil et al, 2005). These are ophiostomatoids, mainly Ceratocystis, Ophiostoma and Leptographium, black yeasts such as Hormonema dematiodes and Phialodophora spp. and dark moulds including Alternaria alternata and Cladosporium spp. Wood products stained by them are substantially devalued because of their cosmetic appearance. Customers may also assume that such timber is also infected with moulds, white or brown rot. More seriously, several sap stain fungi are pathogenic and if dispersed by bark beetles or wood-borers contribute to the death of standing trees by disrupting the flow of water to the tree crown. The dead trees may then be attacked by decay fungi or form the initial fuel of forest fires. The colour patterning caused by these fungi, known as spalting, is sometimes shown to advantage by wood turners and furniture makers.

An enormous number of tree species around the world are used for timber. Their uses are prodigious and often highly specialized. For example, some 200 tree species, of which more than 70 are endangered, are used in making musical instruments. This special case of wood use is so important that a special group, known as SoundWood, an international conservation programme of Fauna & Flora International (FFI) has been established to ensure the conservation of the trees involved (May, 2003; plus see www.soundwood.org/). The timber of many of these endangered trees is very valuable, that of the African Blackwood Dalbergia melanoxylon sells for up to $20 000 a cubic metre. Seedlings of this tree, called mpingo in Swahili, are now being cultivated and existing adults safeguarded wherever possible. The mahoganies, the larger timber-yielding ebonies, rosewoods and the Brazilian pernambuco Guilandina echinata used to make violin bows are all at risk. In North America tight-grained, and thus usually old-growth and slow-grown, wood of Sitka and Engelman spruce (Picea sitchensis and P. engelmanii) is used for the soundboards of instruments from guitars to pianos. Recent investigations of the violins made by Stradivarius (1644-1737) at Cremona, northern Italy, are of interest in this respect. The wood he used probably came from the Alpine region north of the city, in an area still known as 'The Forest of Violins'. The very dense wood of his 'Messiah' violin was recently found to have about 220 annual rings within a zone of Norway spruce Picea abies 15 cm wide, compared with 90-150 rings in a modern violin. Annual rings formed during the Little Ice Age (see Section 9.1.3) were closer than those of more recent years, and Stradivarius lived during its very coldest period, known as the Maunder Minimum after E. W. Maunder who recorded a drop in solar activity at this point.

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