。
The leaves of the trees are falling in the wind.
Literal
Trees [possessive-の] leaves [subject-が] wind [by-で] is-falling.
で here is causal/instrumental — 'by means of, due to' — marking what causes the falling. 落ちている is the intransitive 落ちる ('to fall') in ~ている form. With an activity verb, ~ている reads as the ongoing 'is falling' rather than a resulting state. The transitive partner 落とす ('to drop') would be used if a deliberate agent were doing the dropping.