In most cultures, the sky god was the most powerful deity. So it was natural to link the ruler of the gods with the oak, king of the forest, known for its great age, size, strength and endurance. Another reason for linking tree and god is that, due to their height, oaks have often been blasted by lightning. So Shakespeare in King Lear talks of "oak-cleaving thunderbolts". 

The vault of heaven cracked

The sight of lightning striking an oak is unforgettable. Tolstoy's novel Anna Kerenina includes this vivid description, probably based on personal observation:

"Suddenly there was a glare of light, the whole earth seemed on fire and the vault of heaven cracked overhead. Opening his blinded eyes, to his horror the first thing Levin saw... was the uncannily altered position of the green crest of the familiar oak in the middle of the copse. 'Can it have been struck?' The thought had barely time to cross his mind when, gathering speed, the oak disappeared behind the other trees, and he heard the crash of the great tree falling on the others."

To our ancestors, lightning was a fire from heaven, sent by a god to show his anger. So the oak tree came to be seen as a channel chosen by the sky god to communicate with people on earth.

Oracle of Zeus

The Ancient Greek king of the gods was Zeus ''the Thunderer'' (known to the Romans as Jupiter), who was crowned with a wreath of oak leaves. At Dodona, in the mountains of northern Greece, Zeus had a famous oracle - a place where he was believed to speak through the medium of a sacred oak tree. For more than 1,000 years, from around 800 BC, pilgrims travelled from all over Greece to Dodona, to ask Zeus for advice. The god's answers, interpreted by priestesses, came in the rustling of leaves, the cooing of doves in the tree, or the clanging of bronze pots hanging from its branches.

The oracle at Dodona was destroyed in AD 391, when the sacred tree was cut down on the orders of the Roman Emperor Theodosius - a Christian determined to stamp out pagan worship. Archaeologists have now planted a new oak tree at the site, and visitors today can sit beneath its branches, listing to the wind rustling in its leaves for the voice of the ancient god.

Druids and mistletoe

For the Celtic druids of western Europe, the oak was sacred to Taranis, another thunder god. The sacredness of Taranis's tree extended to a plant which grew from it, mistletoe. The Roman writer, Pliny the Elder, described the significance of mistletoe to the druids of Gaul (France):

"The druids... hold nothing more sacred than the mistletoe and the tree that bears it, always supposing that tree to be an oak. They chose groves formed of oaks for the sake of the tree alone, and they never perform any of their rites except in the presence of it. In fact, they think that everything that grows upon it has been sent from heaven and is a proof that the tree was chosen by the god himself."

Donar and Thunor

The Germanic counterpart of Zeus and Taranis was Donar or Thunor, whose worship was brought to England by the Anglo-Saxons. Thundersley in Essex and Thursley in Surrey are just two places where Thunor was once worshipped in a sacred oak grove. Both names mean "Grove of Thunor".

During the Dark Ages, a common method of converting pagans to Christianity was to chop down their sacred oak trees. Willibald's Life Of St Boniface, Anglo-Saxon missionary to the German pagans, describes his miraculous felling of a tree sacred to Donar, at Gaismar in AD 723:

"Taking his courage in his hands (for a great crowd of pagans stood by watching and bitterly cursing in their hearts the enemy of the gods), he cut the first notch. But when he had made a superficial cut, suddenly, the oak's vast bulk, shaken by a mighty blast of wind from above crashed to the ground, shivering its topmost branches into fragments in its fall. As if by the express will of God... the oak burst asunder into four parts, each part having a trunk of equal length. At the sight of this extraordinary spectacle the heathens who had been cursing ceased to revile and began, on the contrary, to believe and bless the Lord. "

Boniface used the oak timbers to build a chapel, which he dedicated to St Peter.

Thor

The Viking name for Thunor was Thor. He was a fierce, red-bearded god with blazing eyes, who rode across the sky in an oak chariot, pulled by two goats, and whose weapon was a magic hammer, Mjollnir (the destroyer), also made from oak. Like Thunor, Thor was worshipped in sacred groves, the most famous standing at Uppsala in Sweden. According to an 11th century Christian writer, Adam of Bremen, Uppsala was a place of bloody sacrifices:

"The bodies are hung in a grove near the temple, a sanctuary so holy that each tree is regarded as itself divine, in consequence of the death and decay of the victims. Dogs and horses hang there beside human beings, and a Christian has told me that he has seen as many as seventy-two carcasses hanging there side by side."

Scandinavian temples to Thor held tall oak pillars, carved to represent the god. When the Vikings settled Iceland, they had to bring oak pillars with them, for the tree does not grow there. According to the Icelandic Sagas (which were written between the 12th and 13th centuries), when the settlers first saw land, they threw the pillars over the side of their ship, so that the god himself could guide them to their new home.