。
The trees have already shed their leaves.
Literal
Trees [topic-は] already leaves [object-を] is-having-dropped.
落としていた is the past resultative of 落とす ('to drop'), the transitive partner of 落ちる ('to fall') — trees doing the dropping. すでに ('already,' written-style equivalent of もう) is a temporal adverb that pairs naturally with the resultative ~ている to give 'have already X-ed.' The combination has the same effect as English present perfect: an action that has happened and whose result is the current state.