。
The tree will soon lose its leaves.
Literal
Tree [topic-は] soon leaves [subject-が] fall [probably-だろう].
Another double-subject frame (は + が): the tree is the topic, the leaves are the subject of 落ちる. まもなく is a written/formal 'soon, shortly' — more neutral than すぐ ('immediately'). だろう is the plain tentative copula 'probably / I imagine,' the casual counterpart of でしょう.