The genus Quercus with more than two hundred separate species produces the true oaks. Most of these are found in the northern hemisphere where, in temperate regions they may form pure stands, or may be dominant species in mixed wood lands, while in warmer countries they tend to occupy the mountain areas. Most of the true oaks are trees but some are shrubs. The trees, on the basis of wood structure, fall into three groups; the red oaks, the white oaks, and the evergreen oaks or live oaks; the red and white oaks are deciduous.
Descriptions of the species that occur within Europe are as follows. European oak
Quercus petraea Liebl. (Q. sessiliflora Salisb.) and Q. robur L. (Q. pedunculata Ehrh.) known also as English, French, Polish, Slavonian, etc oak, according to origin.
There is no essential difference in the appearance of the wood of either species. The sapwood is 25mm to 50mm wide and lighter in colour than the heartwood which is yellowish-brown. Quarter-sawn surfaces show a distinct silver-grain figure due to the broad rays. The annual rings are clearly marked by alternating zones of early-wood consisting of large pores, and dense late-wood. Conditions of growth accordingly govern the character of the wood to a great extent; for example, in slowly grown wood the proportion of dense late-wood is reduced in each annual growth-ring, thus tending to make the wood soft and light in weight. The growth conditions in the various countries which export oak, vary considerably.
Baltic countries, including northern Poland, produce oak which is generally hard and tough, but further south in Poland the growth conditions become more favourable to the production of milder, more uniformly-grown oak, the rich black soil of south-east Poland producing the famous Volhynian oak, the character of this type of wood changing but little in countries in Central Europe such as Czechoslovakia and Hungary, but being generally a little milder in character in Yugoslavia, from whence Slavonian oak is shipped. The weight of oak varies according to type; that from the Baltic area, western Europe, and Great Britain being about 720 kg/m³ and that from Central Europe about 672 kg/m³ on average after drying.
So-called brown oak is the result of fungus attack in the growing tree. The fungus, Fistulina hepatica, causes the wood first to assume a yellow colour, then a richer brown or reddish-brown. A yellow-coloured streak sometimes appearing in oak is the result of another fungus, Polyporous dryadeus, but since very few tree diseases persist after the tree is felled, dried timber is no different from normal coloured wood, indeed, brown oak is often preferred for its decorative appeal.