Throughout the Aryan world, the oak tree was considered sacred to the Thunder God. Anything that has been struck by lightning was set apart as holy unto Him and regarded as such by our ancestors. People who were struck by lightning and survived according to J.T. Sibley in her remarkable book The Divine Thunderbolt. Missile of the Gods:
"were usually believed to have gained special powers from the sky god, especially the gift of divination or an ability to communicate directly with the gods, and especially with the sky/thunder god."
No doubt such people went onto be the shamans of their tribes. Apparently those people who died after being struck by lightning were not allowed to be cremated (the natural rite of our ancestors) as this would be considered a sacrilege to the Thunder God. Instead they were probably buried.
Trees are particularly vunerable to lightning strikes, most especially the oak tree. According to the author wood from a lightning struck tree is much sought after (and indeed still is) and was used for the "construction of temples, altars, tools of worship, amulets and charms, idols of deities or spirits, in folk medicine, and so forth." It would be a good idea for my readers to seek out such wood and use it as part of their rites and for the construction of amulets of protection. As an aside I am awaiting the delivery of a special Thor's Hammer specially hand forged for me from old iron taken from a lightning struck bridge. Such an item should be considered thrice holy, being of iron (sacred to Thunor), blessed by lightning and in the form of a Hammer. I can think of little which is more sacred than that!
The oak is 60 times more likely to be struck by lightning than for instance a beech tree. As I have demonstrated before the oak in etymologically linked to the name of the Aryan Thunder God, *Perkunos or *Perkwunos, *perk being the Indo-European root for oak, from which we derive the Latin Quercus. Some scholars including the author of the aforementioned book consider the central European Hercynian Forest (which includes the Harz of northern Germany) to be derived from this root via the Proto-Celtic *erquu(n)s. No doubt this great vast forest endured many such lightning strikes and in the course of time the Teutons, Celts, Balts and Slavs came to strongly associate the oak with the supreme Sky and Thunder God. This is particularly the case with the Baltic and Slavic names of the deity which more closely resemble the original name, ie Russian and Czech Perun, Latvian Perkons, Lithuanian Perkunas, the Prussian Perkonis but also the Estonian Pikker, Old Indian Parjanja and the Germanic Fjorgyn.
According to Miss Sibley Odin's spear Gungnir was "modeled on Tyr's spear", a conclusion that I also came to independently some time ago. Tyr was the original Sky God and His presence can be detected back into the mists of pre-history. In turn Tyr's spear was based upon the oak-shafted spear of Zeus or Jupiter, oak of course being also sacred to this latter God.
In the Baltic lands bronze idols of the Thunder God would be placed under oak trees, a custom worthy for us to emulate today, providing of course that the tree is in a secluded place, out of the way of prying eyes and hands. Donar's Oak in Hesse was felled by the xtian missionary and cultural vandal Boniface in the 8th century CE. Wood taken from the oak was then used for the construction of a church.
Oak is represented in the Northumbrian/Anglo-Saxon rune Ac. The Celtic Ogham also has a few (stave) related to the oak-Duir, etymologically linked to Druid via *dru-wid, "knower of oak-trees." (The Book of Ogham, Edred Thorsson).
Symbolically the oak has long been associated with the German people and featured on the Iron Cross. It is of course also associated with England. However the following countries also celebrate the oak as their national tree: Serbia, Cyprus, Estonia, France, Moldova, Romania, Latvia, Lithuania, Wales, Galicia and Bulgaria.