taru-adjective, adverb taking the 'to' particle
lofty; towering; tall and imposing
Literary adjective describing a tree that stands tall and majestic, often with a sense of solitary grandeur. Used with たる or と in classical or formal writing.
その松は亭々とそびえていた。
The pine tree stood lofty and towering.
高々 means 'at most' or 'merely' in modern usage, and can also describe something tall, but 亭々 specifically evokes a majestic, tree-like loftiness in literary contexts.
The exact derivation is uncertain; the kanji 亭 suggests a pavilion or a tall structure, and the reduplication emphasizes the lofty quality.