Dendromancy
Dendromancy (Greek dendron, tree) is divination by observing trees: the way leaves fall, mistletoe forms, sounds of branches in the wind. Practice central in Celtic and Germanic Druidism. Sacred trees: oak, ash, hazelnut, mistletoe-bearing trees.
Celtic ogham
The Celtic ogham alphabet (4th-7th centuries AD) associates 20 letters with 20 sacred trees. Used for inscriptions and divination — like Celtic runes.