。
The trees have started to leaf out.
Literal
Trees [subject-が] leaves [object-を] put-out began.
が here makes the trees the new-information subject — perhaps in answer to 'what's happening outside?' — where a topic-marked 木々は would frame them as a known topic of conversation. 出す is the transitive partner of 出る; the trees are actively putting out their leaves. 始める attaches to the verb stem for 'begin to X.'