This is a classic philosophical question regarding observation and existence. If no one is there to see, smell, or in any way detect an event, how do we know if it occurred, or in this case, does the tree even exist at all?
It raises several questions like "is sound only a sound if a person hears it?" From a scientific stance, it is thought to have occurred. If it can at some point be observed to be in a fallen state, it is believed to have caused sound waves to penetrate through the air.
But from a philosophical standpoint, this is essentially an unanswerable question.
An interesting related topic is that at the subatomic scale, this question of whether something exists if it is not observed is less straightforward. Particles act differently when they are directly observed than when they are not. This is known as the "observer effect". Particles seem to act as theoretical versions of themselves until they observed. It's above my full level of understanding, but here's a video about it that will blow your mind:
