To Teutons all trees were sacred, but the most sacred were the trees, which were struck by lightning. Those trees were seen as touched by Mjöllnir, the hammer of Thor/Donar, God of Thunder and lightning..
There where the treebark was damaged they burned or carved in the Kotouka signs: perthro and kenaz
Out of the branches they made amulets in which they burned or carved the same Kotouka sign. Knocking or touching the amulet should give luck and an enlightened fate.