。
The leaves fell to the ground.
Literal
Tree-[possessive-の]-leaves [subject-が] ground-surface [on-に] fell.
地上 ('ground surface,' literally 'earth + upper') is a slightly more formal equivalent of 地面. に here marks the destination — where the leaves ended up. Standard intransitive 落ちる for 'fall' (contrast with transitive 落とす 'to drop').