The principal raw material yielded by trees is wood, which can be used in many ways and for many purposes and which, along with coal — the product of trees of earlier geological periods — made possible the smelting of ores and the development of all branches of industry.
Trees grown for their wood are cultivated in forests. Unlike other raw materials where the supplies are exhausted after a time, if felling is wisely planned, the supply of wood in a forest remains constant and the felled wood is replaced by new wood in the form of new growth. In the mild climate of central and western Europe, the annual increment per hectare, depending on the site, type of soil and prevailing species, averages from three to ten cubic metres of wood that can be cut without decreasing the supply and production of further wood. With systematic care and fertilization this increment can be increased even further, as is shown by the plantations of cultivated hybrid poplars with an annual increment of fifteen to twenty-five cubic metres of merchantable wood.
Felled wood is processed in two ways: mechanically, whereby only the shape is changed, and chemically, whereby not only the shape is changed but also the substance. Wood processed by the first method is used mainly in the building industry, the joinery trade, furniture making, wheel and waggon making, in mines, on railway lines, etc. The favoured wood of the building industry is that of conifers: spruce, pine, fir and larch. It is long, light, well suited for beams, columns, sawn wood, doors, window-boxes and in building ships and transport vehicles. In the furniture industry, it is mainly the wood of broad-leaved trees such as oak, elm, walnut, ash, beech, cherry and certain tropical exotics that is used to make the finer, more delicate pieces. In recent decades, only thin layers, called veneers, of these costly woods are being used; they are glued onto the main structure made of soft wood, thus making the final product not only lighter but also less expensive.
In chemical processing, the wood is either burned by the process of dry distillation, with no air present, or else is decomposed by various chemical agents. Originally, it was burned by the primitive method of the charcoal-stack, which yielded only charcoal used in the melting of ores and iron working. All other valuable products escaped either into the ground or the atmosphere. Today, it is distilled by modern methods, and it is the by-products, such as wood alcohol, vinegar, acetone and pitch oils — and not charcoal — that are of prime importance. It is mainly the hardwoods such as beech, oak, birch, maple and hornbeam which are used for industrial processing.
The chief product of the chemical decomposition of wood is cellulose, used in the manufacture of paper, textiles, guncotton and other products. The wood of spruce, fir, various pines, and poplar and, to a lesser degree, that of beech and birch is used mostly for this purpose. The proportion of wood processed by chemical means is growing rapidly. The papermills of the developed countries, for example, consume large areas of forest every month.
Other raw materials yielded by trees are essential oils, rosin and turpentine. These are obtained from live trees by boring holes in the outer layer of wood or bark, and catching the oils in containers placed beneath them. The best species of trees for this purpose are various pines, larches, and, in sub-tropical regions, members of the genera Agathis, Shorea and Canarium. Similar methods are used in the case of certain tropical, broad-leaved trees to obtain caoutchouc and latex, which are of importance in the rubber, textile and food industries — trees of the genera Hevea, Castilloa, Mimusops, Achras, etc.
The leather industry could not function without tannins, another product of many woody plants. In some trees, these are obtained from the bark (spruce, oak), in others from the wood and bark (chestnut, false acacia), and in still others from the leaves (staghorn sumach) or fruits (sapan). Before the day of synthetic dyes, trees were also an important source of natural dyes.
The pharmaceutical industry is another that looks to trees for a number of important substances, though as yet it is only beginning to make a thorough study of the possibilities they offer. Nevertheless, the lives of tens of millions of people have been saved by quinine, for example, extracted from the bark of the cinchona, a tree growing in the sub-tropics.
Trees are also a source of various other raw materials, e.g. cork from the cork oak and the Amur cork tree, and cotton and stuffing material (kapok) from various species of Ceiba, the kapok and silk cotton trees.
The fruits and leaves of trees are used to prepare various beverages such as coffee, cocoa, tea, and Coca-Cola, and there are many trees that provide us with edible fruits, either fleshy fruits, such as lemons, oranges, figs, dates, apples, pears, plums and cherries, or oily ones, such as walnuts, hazelnuts, almonds and chestnuts. Today, all these fruits are a good and welcome supplement to our daily fare, but in former times they played a vital role in man’s diet. In some undeveloped tropical regions they are the chief source of food to this day.