In a manner relating to or characteristic of trees; in the way or manner of tree-dwelling creatures.
From arboreal (relating to trees) plus the adverb suffix -ly (in the manner of). This transforms the adjective into an adverb describing how something happens or is done.
The -ly suffix transforms almost any adjective into an adverb: quick becomes quickly, slow becomes slowly, arboreal becomes arboreally. It's probably the most productive suffix in English for creating adverbs, and we use it so automatically we barely notice it.
Complete word intelligence in one call. Free tier — 50 lookups/day.