oaks are soul trees because nymphs, diviners, and priestesses lived in or by them and could express the oaks’ foreknowledge and understanding of events in hints and sayings.
Hillman identifies the Dodonan oak as the paradigmatic ‘soul tree,’ arguing that its capacity for prophetic speech — mediated by nymphs and priestesses — grounds the acorn theory of innate daimonic knowledge.
, The Soul’s Code: In Search of Character and Calling, 1996thesis