The oak is a dominant forest tree of the northern hemisphere, an ecosystem all of its own and a keystone species for a wide variety of habitats; a tree of proverbial national strength and character. There are more than 600 oak species, with the richest variety found in China, Mexico and North America. In Europe the deciduous Quercus robur (English or pedunculate oak) and Quercus petraea (sessile oak) reign supreme, but cork oaks (Quercus suber: see Chapter 1) and holm oaks (Quercus ilex) form sometimes evergreen forests further south in Mediterranean climates.
Oaks grow to a great age and size – specimens of over 1000 years old can be found in countries across Europe. They support large numbers of insects and birds, form partnerships with other trees and ground flora and are known to be able to communicate using chemical warning signals of vaporised tannin released into the air during severe insect infestations. The timber is highly valued for its great strength and resilience; traditionally it was felled and sawn or split while still green and seasoning was completed within the structure of a building, and the wood became harder with age. Palaces, halls and ships of great size and sophistication relied heavily on oak for their construction. Oak barrels used in fermenting wine, whisky and beer spawned a large, widespread industry from the medieval period. Wasp galls forming on its twigs were processed to make a permanent black ink used in manuscripts like the Lindisfarne gospels. Oaks were often allowed to mature to about 100-150 years to produce fine timber; but large area of oak woodland came under coppice management to allow regular cropping, every 20 years or so, of oak poles for construction. Many place names including Old English Ac- reflect once prominent individual trees or woods. It is hard to imagine our landscapes without them.
Acorns, usually collected by birds – especially jays – are buried in large numbers every autumn and germinate readily, although there is concern about a decline in oak regrowth in some areas. Acorns, like beech nuts, were traditionally eaten by pigs to fatten them for winter slaughter. Most are unpalatable to humans unless washed clean of their toxins; but the holm oak yields edible acorns. Oak bark is very highly prized for its rich tannins, used in curing leather. Oaks are genetically diverse and hybridise readily; even so, a number of diseases and pests, notably Acute Oak Decline (AOD) in the UK, threaten populations in many areas.